Understanding how crop diseases and local weather change have interaction is necessary

In the past 150 years, the concentration of carbon dioxide in the atmosphere has risen from 280 portions consistent with million (ppm) to 410 ppm. For farmers this is mixed news. Any trade in familiar weather patterns brought about by way of the atmospheric warming this rise is bringing is sure to be disruptive. But extra carbon dioxide manner extra gasoline for photosynthesis and therefore enhanced enlargement—now and again by as much as 40%.

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And for those in temperate zones, rising temperatures may carry milder weather and an extended growing season. (In the tropics the consequences don’t seem to be so more likely to be benign.) What is not transparent, although, and now not a lot investigated, is how rising CO2 ranges will affect the relation between vegetation and the illnesses that have an effect on them.

History suggests that is an oversight. Devastating crop illnesses do all of sudden emerge from obscurity—frequently changing into epidemic a long way from their place of birth. In the 1840s, for example, a hitherto difficult to understand fungus from Mexico devastated the Irish potato crop for several years, bringing a few famine that killed a million folks. It would not be in any respect unexpected if a changing climate resulted in stipulations that brought about identical epidemics.

Saskia Van Wees, a botanist at Utrecht University, within the Netherlands, is considering this question and, with the assistance of a team of affiliates, arrange an experiment to take a better have a look at how plants respond to changing carbon-dioxide levels. She placed seedlings of Arabidopsis thaliana, a member of the mustard circle of relatives that serves as a guinea pig of the botanical international, in chambers containing other ranges of CO2. Some were at 450 ppm, the level within the laboratory; some smartly under this (150 ppm); and some well above (800 ppm), a focus that, if present traits proceed, is expected to be reached around 2090. After four weeks, when the crops had been established and healthy, Dr Van Wees let unfastened a mix of commonplace pathogens. These included those of 2 leaf diseases (Botrytis cinerea, recognized to laymen as grey mold, and Pseudomonas syringae), and two root illnesses (Rhizoctonia solani and Fusarium oxysporum). After a set number of days, appropriate to each disease, she studied the severity of the infections that resulted.
As she reports within the European Journal of Plant Pathology, top CO2 ranges had no affect on the root diseases. The workforce had anticipated this because carbon dioxide’s ranges within the soil aren’t a lot suffering from its levels within the air. The leaf illnesses have been, however, affected relatively so much. Specifically, the severity of B. cinerea an infection was substantially weakened by high levels of the gas and that of P. syringae used to be dramatically reinforced. The reasons for these adjustments seemed to be shifts in the ways by which the vegetation defended themselves.

Plants depend closely on two elements, salicylic acid and jasmonic acid, to power away agents of disease. In general, vegetation increase manufacturing of salicylic acid when faced with pathogens that feed on dwelling tissue and do likewise with jasmonic acid when warding off agents that kill the tissue first, ahead of feeding on it. Since B. cinerea consumes dead tissue and P. syringae living tissue, Dr Van Wees speculated that top CO2levels had been riding the plants to supply jasmonic acid more readily, to stay B. cinerea at bay, and to tone down salicylic-acid manufacturing, allowing P. syringae to flourish. Chemical analysis of the vegetation proved this concept proper.

Dr Van Wees’s experiments are the most recent of dozens of trials which make it transparent that plant biology is altered considerably through a variety of environmental components. This makes it tricky to expect what impact a changing local weather could have on explicit bits of agriculture. Carbon dioxide is a working example. It enhances growth of many crops but, as Dr Van Wees displays, it also shifts the defences to favour some sorts of disease over others.

To make matters even more sophisticated, proof is mounting that adjustments in temperature and water availability additionally shift plant immune responses. André Velásquez and Sheng Yang He, at Michigan State University, wrote an in depth overview at the battle between crops and diseases in Current Biology final year. They noted that although some valuable vegetation, equivalent to potatoes and rice, enjoy much less disease as moisture levels building up, this is not the case for many plants. High humidity, in general, favours the spread of botanical sicknesses. The same can be said for temperature—with some sicknesses, like papaya ringspot virus, thriving in emerging temperatures whilst others, for instance potato cyst, are weakened.

There is also the trouble of adjusting insect behaviour. Many disease-causing viruses, bacteria and fungi trip from one plant to another through hitching lifts on bugs. Few researchers doubt that numerous insect species’ behaviour will change because the local weather does, but exactly what’s going to happen is an open question.

The problems are daunting, then, but there is a manner to check out to resolve them. First, all necessary crops wish to have their responses to various illnesses studied beneath a spread of anticipated local weather stipulations. Second, genes which grant resistance to illnesses that may grow to be serious sooner or later want to be tracked down. Modern crops have been streamlined by means of synthetic variety to be superb at rising as of late. This way that they’ve the genes they need to flourish when confronted with the demanding situations anticipated from current prerequisites, but nothing extra. Such plants are thus susceptible to adjustments in their atmosphere.

A herbal variety
One strategy to in finding genes that may regulate this scenario is to appear to vegetation’ wild kinfolk. Uncossetted via farmers, these crops must live to tell the tale illness by themselves—and have been fitted out via evolution with genes to take action. Borrowing their dna is sensible. But that implies amassing and cataloguing them. This is being completed, but not speedy sufficient. The International Centre for Tropical Agriculture, a charity which works in the space, reckons that about 30% of the wild kin of modern crops are unrepresented in gene banks, and almost all the leisure are underrepresented.

Plant gathering is not, most likely, probably the most trendy of sciences. The very word “botany” is redolent of vascula-carrying Victorian parsons on bracing nation walks. More plant creditors (with salaried educational positions for them to occupy) would definitely assist. But there’s a second factor, redolent of a second form of Victorian plant collector—the pith-helmeted explorer searching international lands for interesting specimens—to address.

These days, attitudes to this type of gathering have changed. Most international locations are, rightly, protective in their genetic patrimony. If cash is to be made by incorporating genes from their plants into vegetation, they need to have a percentage of it. It is subsequently incumbent on rich countries to abide via rules that permit deficient ones to take part in seed gathering with out shedding out financially. Poor, plant-rich international locations are after all the ones whose farmers are in all probability to be harm through global warming. It would be ironic if that were made worse as a result of genes from the ones international locations’ plants were unavailable to future-proof the sector’s plants.

This article gave the impression within the Science and technology phase of the print edition under the headline”Blocking the road to rusty demise”

Effect of viruses on photosynthesis

The most obvious symptom of systemic virus infection is the mosaic pattern of the leaves. Yellowing, chlorosis is also frequent and characteristic sign of the altered photosynthetic activity. Virus infection effects photosynthesis in a complex manner, depending on the particular host-virus combination. The symptoms are basically different in the incompatible or the compatible host-virus interaction. 
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Hafiz Muhammad Rizwan Mazhar

                  Department of Plant Pathology, University of Agriculture, Faisalabad-Pakistan

 

                                             (E-mail: rizwanmazhar0

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Photosynthetic changes in incompatible host-virus interaction Virus infection often causes local lesions in resistant plants. The development of local lesions in hypersensitive reaction (HR) was described in details by electron micros- copy, based on the time course of the appearance of symptoms. The ultrastructure of the lesion and the surrounding cell layers and the role of the active zone in the localisation of the infection was highlighted (Israel and Ross, 1967).

In tobacco mosaic tobamovirus (TMV) infected N. glutinosa leaves the size and number of the starch grains increased in the chloroplasts following the infection (Weintraub and Ragetli, 1964). One day after the infection plasmolysis occurred, and first the cytoplasm and later the chloroplast membranes completely disappeared.

 In the remains of the chloroplasts large vesicles were formed, the number of mitochondria increased and their structure changed. Finally the whole cell structure collapsed and its content disin-tegrated. All these alterations were consequences of the loss of osmotic regulation, and have been related to the so-called ‘starch-lesions’ described by Holmes (1931).

Photosynthetic changes in compatible host-virus interaction For the synthesis of virions the parasite uses the metabolites and energy of the host plant, which are produced in the course of photosynthesis. It is evident that the phy- siological processes of the virus and the plant are linked in complex ways, the structure and the function of the chloroplasts are disturbed (Zaitlin and Hull, 1987).

Changes in the ultrastructure of chloroplasts

The chlorosis of leaf tissues in the compatible host parasite interactions can be traced back to different causes. The number, size or chlorophyll content of the chloroplasts may decrease.Their shape, inner membrane structure may alter to some degrees, some of their metabolites may accumulate. Often the starch grains enlarge, or accumulate for example in the case of the cauliflower mosaic caulimovirus (CaMV) infected cabbage plants (Conti et al., 1972).

Effect of virus infection on CO2-fixation and on some biosynthetic processes 

In several virus-host interactions the disturbances of CO2-fixation and of related metabolic ways, or the alteration in ratio of certain products was reported. At the early stage of systemic TMV infection (virus replication) more CO2molecules assimilated by the photosynthetic processes (Doke and Hirai, 1970a). As the infection had been progressed this tendency inverted: CO2-uptake was decreasing in the infected.

 The same changes as well as the increase of res-piration were observed in tobacco leaves infected with tobacco etch potyvirus (TEV).The enzyme activity of phosphoenol-pyruvate-carbox-ylase (PEP-carboxylase) increased in the leaves of chinese cabbage infected with TYMV.

In contrast, in tobacco leaves infected with TSWV activity loss of PEP-carboxylase was measured and it was explained as the early senescence induced by the virus infection.

Changes in the chlorophyll protein complexes and chloroplast proteins

Virus infected monocotyledons (e.g. barley infected with BSMV or WSMV) contained reduced amount of chlorophyll protein complexes compared to the healthy controls.

Effect of virus infection as a biotic stress factor on photosynthesis

Stress has been described by Osmond et al. (1987) as all parameters that suppress the maximum, potential genomial value of growth and reproduction of a plant. Plants adopt to stress conditions in two ways, either by tolerance or avoidance. In the case of tolerance the plant’s reaction to a moderate stress is the upregulation, while against a severe stress the down regulation of metabolic processes.

At the cellular level the damage in the membranes and the fluorescence of chlorophyll built in the thylakoid membranes indicated the post stress conditions. In the early phase of virus infection both the non-photochemical quenching of fluorescence and the predominantly reduced state of QAthe primary electronacceptor of PSII, indicated the development of symptoms and the rapid chloroplast destruction due to photoinhibitory conditions (Balachandran et al., 1997).

 

 The build up of carbohydrates and the brakeup of the equilibrium between synthesis and brakedown probably blocks the gen regulation, which effects the level of chlorophyll-protein complexes and photosynthetic metabolic enzymes.

 

Interaction between nutrient and plant diseases

Interactions between plants, nutrients, and disease pathogens are very complex and not completely understood. Nutrition, although frequently unrecognized, has always been a primary component of disease control. Plants suffering a nutrient stress will be less vigorous and more susceptible to a variety of diseases. The severity of most diseases can be reduced and the chemical, biological or genetic control of many plant pathogens can be enhanced by proper nutrition.
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Elements Relation with diseases
Nitrogen Excessive N enhanced tissue susceptibility to blast disease N suppresses fungal disease and feeding intensity of sucking insects.
  
Phosphorus
  
P lowers the susceptibility to fungal attacks because it is a component of phospholipids in cellular membranes.
Potassium K application reduces insect attacks and fungal and nematode infections, because is has a role in cellular functions and thick- cuticle development.
Calcium Ca as a component of calcium pectate in the middle lamella, its levels were related to lower susceptibility of leaves to fungus.
Sulphur S used as fungicide to control and suppress fungal diseases 
Accumulation of S in xylem vessel invading fungal pathogen.
Zinc Zn prevents leakage of sugar onto the plant surfaces, which can enhance the invasion of fungus and bacteria.
Copper Cu fertilization decreases the severity of the Pyricularia oryzae on rice due to its role as a component of polyphenol oxidase Copper has the ability to denature the spores and conidia of fungus.
Silicon Diseases such as blast, brown spot and sheath blight can be ex- tremely threatening to rice cultivation if Si is deficient in soil. Silicon has physiological roles in disease resistance.

Essential nutrient elements besides their growth enhancing and yield maximizing roles may also have secondary effect on survival and virulence of pathogens or the tolerance of the host plant to the disease. Deficiency of nutrients has often been regarded as predisposing factors in plant susceptibility to many plant diseases. Application of nutrient elements has shown reduction in disease incidence in rice plant.

Element Diseases Causal organism
K Leaf spot Helminthosporium spp.
Brown spot Cochliobolus miyabeanus
Sheath blight Rhizoctonia solani
Stem rot Sclerotium oryzae
Brown spot Ophiobolus miyabeanus
Blast Pyricularia grisea
Stem rot Sclerotium oryzae
Bacterial leaf blight Xanthomonas oryzae pv. oryzae
Cu 
 
Mn
Blast Pyricularia grisea
Blast Pyricularia grisea
  
 
Zn
 
 
Si
Leaf spot Alternaria spp.
Leaf spot Cochliobolus miyabeanus
Stem/sheath blight Rhizoctonia solani
Blast Pyricularia grisea
Brown spot Cochliobolus miyabeanus

Pesticides and Vegetable Disease Control

Fungicides can be a great help in preventing diseases when properly applied to the plant foliage. Since fungicides are preventive, they should be applied before the disease occurs, or as soon as the first symptoms of disease appear. Some vegetable diseases require specific fungi- cides for their control. An outline of diseases of specific vegetables, disease description, suggested cultural and chemical control and comments follows this section.
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Fungicides are available primarily as wet- table powders, dry flowables and dusts, but a few are sold as emulsifiable concentrates, flowables and liquids. Wettable powders (WP) and dry flowable (DF) are formulated in such a manner as to be readily suspendable in water. Dusts (D) should not be mixed in water, but applied directly to the plant. Emulsifiable con- centrate (EC) fungicides contain an emulsifying agent that makes them readily suspendable in
water. Flowable (F) fungicides are finely ground wettable powders that are suspended in a liquid.
Some soil fungicides are available as gran- ules and are applied in the furrow at planting. Dust and spray fungicides may also be used as in-furrow treatments for seedling disease control.
Table II gives trade names and rates of broad spectrum fungicides for controlling foliar dis- eases of vegetables. Applying foliar fungicides to vegetables is best done by spraying, because it provides coverage of all plant surfaces. Foliar fungicides are available in dust formulations, but dusts are usually not as effective as sprays.

Outline for Control of  Vegetable Diseases

 
Disease
 
Symptoms
Chemical & Formulation1 Approximate
Rate/Gallon2
 
PHI3
 
Limitations
 
ASPARAGUS
Root Rot
(Fusarium)
Deterioration of root system and poor growth of plants. Mancozeb 75 DF 1 TBSP   Pre-plant root dip.
Rust Numerous small, reddish- yellow spots on spears and ferns. Mancozeb 75 DF 2 TBSP 180 Apply to ferns after harvest of spears. Make applications at 7-10 day intervals, beginning at first appearance of disease.
 
BEAN (SNAP)
Anthracnose Brown spots on leaves, sunken with pinkish ooze. Basic copper sulfate 53WP
 
Chlorothalonil 54 F
mfg directions
 
 
3 tsp
0
 
 
7
Use western-grown seed. If plants become infected, do not work while plants are wet. Spray at weekly intervals. Do not plant beans in
field for at least two years following occurrence of disease. Plow infested bean trash deeply
into soil. Do not graze treated areas or feed treated plants to livestock.
Bacterial Blights Water-soaked spots on leaves and pods. Red margin and sometimes a yellow halo around spot. Fixed Copper mfg directions 0 Same applies as for anthracnose.
Gray Mold (Botrytis) Gray moldy growth on pods and stems. Chlorothalonil 54 F 3 tsp 7 Begin at 25 to 50% bloom. Repeat at peak bloom. Do not graze treated areas or feed treated plants to livestock.
Mosaic Virus (bean yellow mosaic, bean common mosaic, and peanut stunt viruses) Yellowed leaves, with or without crinkling or speckling. Leaves cupped, runners killed.       Use resistant varieties. Half runners are most susceptible, particularly
“  Pink.’   Make successive plantings, as mosaic is more severe at certain times of the year.
Powdery Mildew White, powdery mold on surface of leaves. Sulfur Dust
 
Wettable Sulfur
mfg directions mfg directions 0
 
0
Use resistant varieties. Spray or dust with sulfur when disease first appears and repeat treatment 10 days later.
Root Rot & Seedling
Disease (Rhizoctonia)
Rots of seeds death of seedlings (damping off), and root rots. Terraclor 75 WP mfg directions   Apply Terraclor in-furrow at planting. Rotate crops, avoid double cropping beans and turn under plant debris well in advance of planting. (See Extension factsheet SP277-O)
Rust Reddish brown pustules develop on leaves. Chlorothalonil 54 F 3 tsp 7 Spray plants when rust appears and repeat at 7-10 day intervals. Do not graze treated areas or feed treated plant parts to livestock.
White Mold
(Sclerotinia)
White moldy growth on pods and stems. Terraclor 75 WP mfg directions   Spray at first appearance of disease.
 
Disease
 
Symptoms
Chemical & Formulation1 Approximate
Rate/Gallon2
 
PHI3
 
Limitations
 
BEAN (lima) AND BUTTER PEA
Stem Anthracnose Reddish brown spots on leaves and pods. Small black specks sometimes can be seen. Basic copper sulfate 53 W mfg directions 0 Plant disease-free seed and rotate lima beans with other crops. Start applications of fungicide at first bloom and continue at 7-day intervals.
Root-knot Swollen, galled root system.       See section above on nematode control.
Root Rot         See Bean, Snap.
 
BEET
Downy Mildew Leaf
Spots
Various leaf spots. Fixed Copper mfg directions 0 Spray at first appearance and repeat at 7-10 day intervals.
Rust Orange to rust colored spots that will rub off on hands. Sulfur mfg directions 0  
BROCCOLI (SEE CABBAGE)
BRUSSELS SPROUTS (SEE CABBAGE)
CABBAGE
Alternaria Leaf Spot Target spots on older leaves. Small, black spots may also occur. Chlorothalonil 54 F Maneb 80 WP 1.5 tsp
 
3-4 tsp
0
 
7
Start spraying when this disease first appears. Continue spray at 7-
10 day intervals until disease is under control. Remove excess residues by washing.
Downy Mildew Yellow areas on upper side of leaves; downy growth on lower surface.
Bacterial Soft Rot Soft, watery rot occurs, favored by hot, wet conditions. Begin in center of broccoli head.       Control of black rot will also help control bacterial soft rot. Avoid damage to the crop which will provide infection sites for soft rot bacteria.
Black Leg Lower stem turns brown and rots causing a canker.       Use certified disease-free seeds or transplants.
Black Rot Yellow V-shaped spots on edge of leaves. Veins near spots turn black.       Use certified disease-free seeds or transplants. Follow good sanitation practices. Don’t grow cabbage in locations where black rot occurred the previous year. See Extension factsheet SP277-P.
Club Root Galls or clubs on roots. Plants have light green color and are stunted. Hydrated lime
 
Terraclor 75 WP (transplant solution)
mfg directions
 
6 TBSP/gal. water
  Broadcast and work lime into soil by tilling within 3 days before planting. Apply 3/4 pint of Terraclor solution per plant as a transplant solution.
 
Disease
 
Symptoms
Chemical & Formulation1 Approximate
Rate/Gallon2
 
PHI3
 
Limitations
CABBAGE CONT’D
Damping-Off Young plants are rotted off at soil line. Terraclor 75 WP (seedbed drench) 1 TBSP/gal. water/50 sq. ft. seedbed   Sterilize seedbed soil. Apply
Terraclor drench after seeding.
Wire Stem Brown discoloration of stem. Entire stem becomes hard and constricted. Terraclor 75 WP 10-15 lbs./10,900 ft. of row   Sprayed as 8-inch band centered on row.
Yellows (Fusarium) Leaves turn yellow. Cross section of stem shows brown discoloration in veins.       Grow resistant varieties in areas where soil is infested. Secure disease-free transplants.
CANTALOUPE
Alternaria Leaf Spot Tan target spots on leaves, followed by blighting. Chlorothalonil 54 F Mancozeb 75 DF Maneb 80 WP 1.5-3 tsp
 
2-3 TBSP
 
4-6 tsp
0
 
5
 
5
Use disease-free seed. Fungicide applications can be delayed until first sign of disease if adequate survey of garden is maintained.
Anthracnose Sunken spots on fruit and tan leaf spots.
Downy Mildew Yellow, irregular spots on leaves.
Gummy Stem Blight Brown, round leaf spots. Cracks on stems with gummy ooze.
Bacterial Wilt Individual runners suddenly wilt and die.       Apply insecticide to control cucumber beetles, which spread the bacterium. See Extension factsheet SP277-C.
Powdery Mildew White, powdery mold on surface of leaves. Sulfur mfg directions 0 Apply at first sign of disease and repeat at 7-14 day intervals. Do not apply sulfur if temperatures exceed
90 degrees F.
CAULIFLOWER (SEE CABBAGE)
CARROT
Alternaria Blight Numerous dark brown spots which may coalesce. Chlorothalonil 54 F 1.5-2 tsp 0 Spray at first appearance and repeat at 7-10 day intervals. Alternaria blight can spread rapidly.
Cercospora Leaf Spot Small, dark brown to black spots on leaves.
Southern Blight White mycelial growth on lower stem; death of plant.       Use crop rotation.
 
Disease
 
Symptoms
Chemical & Formulation1 Approximate
Rate/Gallon2
 
PHI3
 
Limitations
COLLARD, KALE, MUSTARD & TURNIPS
Alternaria Leaf Spot Brown target spots on leaves. Fixed Copper
 
Maneb 80 WP
mfg directions
 
3-4 tsp
0
 
10
Maintain thin plant stand. Avoid low-lying or poorly-drained soils. Avoid hot part of growing season. Maneb for kale only. Begin sprays when disease is first noticed and repeat at 7-10 day intervals. Use a spreader-sticker for better fungicide coverage.
Anthracnose Small, tan spots with dark brown margins on leaves.
Cercospora Leaf Spot Tan leafspots with yellow haloes.
Downy Mildew Yellow spots on upper surface of leaf with white, downy growth on underside.
CORN (SWEET)
Blights (Helmintho- sporium) Spots on leaves and drying or blighting of leaves. Chlorothalonil 54 F Mancozeb 75 DF Maneb 80 WP 0.75-2 tsp
 
1.5 TBSP
 
3 tsp
14
 
7
 
7
Begin applications when conditions favor disease development and repeat at 4-7 day intervals. Do not apply chlorothalonil to sweet corn to be processed. Do not graze treated areas or feed treated plants to livestock. Use Maneb on crop
for seed production only.
Maize Dwarf Mosaic Alternating light and dark green streaks in leaves. Plants are stunted.       Plant early and preferably in fields not heavily infested with Johnsongrass. Silver Queen, a white variety, has fair tolerance to this disease.
Rust Bronze, elongate spots. Chlorothalonil 54 F Mancozeb 75 DF
Maneb 80 WP
0.75-2 tsp
 
1.5 TBSP
 
3 tsp
14
 
7
 
7
See “Blights.”
Stewart’s Wilt Brown streaks in leaves parallel to veins.       Control corn flea beetle. See insect section of this publication. Also called bacterial blight.
CUCUMBER
Alternaria Leaf Spot Brown, target spots on leaves. Chlorothalonil 54 F Mancozeb 75 DF Maneb 80 WP 1.5-3 tsp
 
2-3 TBSP
 
2-4 tsp
0
 
5
 
5
Use disease-free seed. Fungicide applications can be delayed until first sign of disease if an adequate survey of garden is maintained.
Anthracnose Brown, irregular spots on leaves, sunken spots on fruit.
Angular Leaf Spot Brown, angular spots on leaves. Fixed Copper mfg directions 0 Spray at first appearance and repeat at 7-14 day intervals. Use disease- free seed. (Copper can injure young plants).
 
Disease
 
Symptoms
Chemical & Formulation1 Approximate
Rate/Gallon2
 
PHI3
 
Limitations
CUCUMBER  CONT’D
Downy Mildew Yellow, irregular spots on leaves. Chlorothalonil 54 F Mancozeb 75 DF Maneb 80 WP 1.5-3 tsp
 
2-3 TBSP
 
2-4 tsp
0
 
5
 
5
Use disease-free seed. Fungicide applications can be delayed until first sign of disease if an adequate survey of garden is maintained.
Gummy Stem Blight Brown, circular leaf spots. Cracks on stems with gummy ooze.
Bacterial Wilt Starts on individual
runners and eventually the entire plant wilts and dies.
      Apply insecticide to control cucumber beetles, which spread the disease. See Extension factsheet SP277-C and the insect control section of this publication.
Powdery Mildew White, powdery mold on surface of leaves. Sulfur mfg directions 0 Apply at first sign of disease and repeat at 7-14 day intervals. Do not apply sulfur if temperatures exceed
90 F.
Scab Sunken spots on fruit. Chlorothalonil 54 F 2-3 tsp 0 Resistant varieties widely available. Apply at first sign of disease and repeat at 7-10 day intervals.
EGGPLANT
Leaf Blights, Fruit Rots Spots develop on leaves and fruits. Fixed copper
 
Maneb 80 WP
mfg directions
 
3-4 tsp
0
 
0
Begin spraying before disease appears.
KALE (SEE COLLARD)
MUSTARD (SEE COLLARD)
OKRA
Pod Blight Young pods fail to develop and deteriorate.       Many times this blight is associated with poor pollination. The fungus Choanephora attacks the flowers
and young pods. Providing good air drainage will help prevent infection by Choanephora.
Root-knot Nematode Swollen, galled root system.       See section above on nematode control.
Verticillium Wilt Yellowing of leaves. Inside of stem will have brown discoloration.       Rotate okra with crops which are not susceptible to Verticillium wilt.
 
Disease
 
Symptoms
Chemical & Formulation1 Approximate
Rate/Gallon2
 
PHI3
 
Limitations
ONION
Downy Mildew Pale green, oval, sunken spots on leaves. Purplish mold may be in spots. Chlorothalonil 54 F Mancozeb 75 DF Maneb 80 WP 1.5-3 tsp
 
3 TBSP
 
2-6 tsp
14
 
7
 
7
Apply when disease first appears and repeat at 7-10 day intervals. Do not apply Mancozeb or Maneb on exposed bulbs. Do not apply Chlorothalonil within seven days of harvest on dry-bulb onions or
within 14 days on green onions, leeks or shallots. Do not apply Chlorothalonil to sweet Spanish onions.
Leaf Blast (Botrytis) White to tan streaks on leaves. Leaves eventually die.
Purple Blotch
(Alternaria)
Purple target spots on leaves.
PEA (SOUTHERN)
Cercospora
Leaf Spot
Light to dark brown spots on leaves. Chlorothalonil 54 F 1.5-2 tsp 42 Spray at early bloom and repeat at
7-10 day intervals. For use on beans harvested dry with pods removed (blackeye pea only). Only Bravo 720 or Bravo DG is labeled. Do not graze treated areas or feed treated plants to livestock.
Rust Bronze to rust-colored spots that will rub off on hand.
Mosaic Virus Mosaic of leaves (alternate light and dark green
areas). Distortion of leaves and pods.
      Use virus-free seed. Plant resistant varieties.
Powdery Mildew Dull white, felt-like growth on leaves. Sulfur mfg directions 0 Spray at first appearance at 7-10 day intervals.
Root Rot Brick-red lesions on lower stem and roots. Roots later die.       See Bean, Snap.
PEPPER
Bacterial Spot Black, angular spots appear on leaves. Dark raised spots also occur on fruits. Plants shed infected leaves. Fixed Copper
Plus
Maneb 80 WP
mfg directions mfg directions 0 Use disease-free seed or buy disease-free transplants. Spraying with fixed copper will help prevent spread.
Blossom-end Rot Tan sunken areas on blossom end and side of fruit.       Lime soil to provide adequate calcium. Avoid planting on droughty soils. Do not use excessive nitrate fertilizer. Irrigate and follow culture practices that tend to provide the most uniform soil moisture.
Cercospora
Leaf Spot
Circular spots with gray centers develop on leaves. Fixed copper
 
Maneb 80 WP
mfg directions
 
3-6 tsp
0
 
7
Apply fungicide when disease appears and continue as needed on a 7-10 day schedule.
Anthracnose Sunken spots on ripening fruit.
 
Disease
 
Symptoms
Chemical & Formulation1 Approximate
Rate/Gallon2
 
PHI3
 
Limitations
PEPPER CONT’D
Phytophthora Blight Rapid wilt and death of plants in wet areas of field.       Plant on raised bed.
Root-knot Swollen, galled root system.       See previous section on nematode control.
Southern Blight Plants wilt and die. White mold can often be seen on base of stem. Terraclor 75 WP (soil treatment) mfg directions   Turn soil deep to bury plant debris that might harbor disease organism. Use Terraclor in setting water at planting. Rotate with sod crops.
Sun Scald Dry, white bleached spot on side of pod exposed to sun.       Fruits on plants kept in a healthy, vigorous condition are not likely to be injured by sun scald. Control of bacterial spot will prevent loss of leaves and keep the fruit protected from the sun.
POTATO, IRISH
Early Blight Small brown, target spots appear on leaves in hot, wet weather. Chlorothalonil 54 F Mancozeb 75 DF Maneb 80 WP 1-1.5 tsp
 
1-2 TBSP
 
3-4 tsp
0
 
14
 
14
Start applications when plants are
4-6 inches high and continue at 7-
10 day intervals.
Late Blight Water-soaked irregular spots on leaves in cool,
wet weather. Plants appear scalded.
Black Leg Stem turns black. Plant wilts and dies.       Plant certified seed.
Rhizoctonia Canker
(Black Scurf)
Black cankers girdle stem, causing poor growth. Terraclor 75 WP mfg directions    
Scab Rough, scabby spots on tubers.       Use disease-free seed. Where soil is infested with scab organism, use resistant varieties and rotate crops. See SP277-G.
Seed Piece Decay Rotting of seed piece. Mancozeb
 
Maneb
Use dust formulations according to mfg directions   Treat seed pieces with fungicide dust before planting. Plant seed pieces immediately after treating. Do not use treated seed pieces for food, feed or oil purposes.
Ring Rot Brown discoloration of vascular tissue in tuber.       Plant certified disease-free seed and practice crop rotation.
Virus Diseases Curling, streaking, or mosaic in leaves. Plants may be stunted.       Plant certified disease-free seed and practice crop rotation.
 
Disease
 
Symptoms
Chemical & Formulation1 Approximate
Rate/Gallon2
 
PHI3
 
Limitations
PUMPKIN
Downy Mildew Yellow, irregular spots on leaves. Chlorothalonil 54 F Maneb 80 WP 1.5-3 tsp
 
3-4 tsp
0
 
5
Begin spraying at first sign of disease. See label for spray intervals. Use higher rates for scab control.
Gummy Stem Blight Black, circular spots on fruit.
Microdochium Blight White dashes on stem surface.
Scab Sunken or raised spots on fruit.
Mosaic Virus Green patterns on fruit.       Reflective mulches, aphid control, and weed control may be helpful. Control is difficult.
Powdery Mildew White, powdery mold on leaf surface. Sulfur mfg directions 0 Begin spraying at first sign of disease.
SPINACH
Downy Mildew (Blue
Mold)
Yellow spots on upper leaf surface. Gray downy
fungus on underside of leaf.
Basic copper sulfate 53 WP mfg directions 0 Start fungicide applications at first sign of disease and continue at 7-10 day intervals as necessary. Where white rust has been a problem in
the past, spraying should start when the first true leaves develop. “Fall Green,” a fall variety, is moderately resistant to white rust and several races of blue mold.
White Rust Yellow spots on upper leaf surface. White powdery mass on underside of leaf.
SQUASH
Downy Mildew Yellow, irregular spots on leaves. Chlorothalonil 54 F Mancozeb 75 DF
Maneb 80 WP
1.5-3 tsp
 
2-3 TBSP
 
3-4 tsp
0
 
5
 
5
Begin spraying at first sign of disease. See label for spray intervals.
Powdery Mildew White, powdery mold on leaf surfaces. sulfur mfg directions 0 Apply at first sign of disease and repeat at 7-14 day intervals. Do not apply sulfur if temperatures exceed
90 F.
Scab Sunken or raised spots on fruit. Chlorothalonil 54 F 2-3 tsp 0 Begin spraying at first sign of disease. See label for spray intervals.
Blossom Blight Blossoms rot and stick to young fruit. The end of the fruit turns black.       Provide good air circulation for rapid drying.
Mosaic Virus Greening of fruit. Leaves mottled, veins stunted.       Reflective mulches may be of some value. The yellow-stemmed
varieties Multipik, Superpik and Supersett are tolerant of watermelon mosaic virus.
 
Disease
 
Symptoms
Chemical  & Formulation1 Approximate
Rate/Gallon2
 
PHI3
 
Limitations
SWEET  POTATO
Black Rot Black spot on roots. Dry, black decay extends in flesh of root.       Use certified  seed. Do not replant for 3 years in locations  where this disease has been present.
Scurf Brownish  black discoloration  on surface of potato.
Cork Small, black, corky spots in potatoes.  Spots do not extend to surface.       Use certified,  disease-free  seed.
Root-knot  Nematodes Longitudinal  cracking  of potato.       See previous section on nematode control.
Soil Rot (Pox) Circular  sunken areas on fleshy roots. Feeder roots are blackened.       Rotate sweet potatoes  with other crops. Maintain  pH below 5.2 in infested fields. Grow a resistant variety.
Stem Rot (Fusarium
Wilt)
Plants are yellow and stunted. The interior of the vine shows a brown discoloration.       Select disease-free  seed stock by examining  vines of each hill. If cross section of vine shows a brown discoloration,  do not use roots for seed stock.
Southern  Blight (Plant
Bed)
Plants die in spots in bed. White growth on lower stem.       Use new soil in beds each year.
TOMATO
Bacterial  Spot Small, raised spots on fruits. Water-soaked spots on foliage. Fixed Copper plus
Mancozeb  75 DF OR plus
Maneb 80 WP
mfg directions
 
1.5-2 TBSP
 
3-4 tsp
0
 
5
 
5
Use hot water seed treatment  or use disease-free  seed.
Blossom-End  Rot Black spot on blossom end of small green fruit.       Maintain  adequate  calcium  level in soil and uniform soil moisture. Plant least susceptible  varieties. Avoid droughty  soils.
Early Blight Brown target spots followed  by blighting  of foliage. Chlorothalonil 54 F Mancozeb  75 DF Maneb 80 WP 1.5-2 tsp
 
1.5-3 TBSP
 
3-6 tsp
0
 
5
 
5
Start preventative  spraying as soon as plants are established  in the garden. Spray at 7-10 day intervals. During periods favorable  for
disease development,  shorten the spray interval.  Maneb 80 WP can be used in the greenhouse  and in the garden.
Anthracnose Circular,  sunken spots on ripe fruit.
Septoria  Leaf Spot Small, gay circular leaf spots with dark borders.
Buckeye  Fruit Rot Circular,  zonate bands within large spot on fruit, worse on lower clusters.       Mulch and stake plants to keep soil off fruit clusters.
 
Disease
 
Symptoms
Chemical & Formulation1 Approximate
Rate/Gallon2
 
PHI3
 
Limitations
TOMATO CONT’D
Fusarium Wilt Yellowing and wilting of foliage. Inside of stem has brown discoloration.       Use a resistant variety and rotate tomato fields. Obtain disease-free plants and plant on disease-free soil.
Gray Mold (Botrytis) Leaves turn brown from tip back; gray mold can be
seen on foliage during humid weather.
Chlorothalonil 54 F 3 tsp 0 Begin fungicide spray applications at first sign of disease.
Late Blight Irregular, watersoaked spots on leaves. Occurs in mold, wet weather. Chlorothalonil 54 F Mancozeb 75 DF
Maneb 80 WP
1.5-3 tsp
 
1.5-3 TBSP
 
2-6 tsp
0
 
5
 
5
Follow spray schedule for early blight. If weather conditions are favorable for late blight, shorten spray interval. Obtain disease-free plants and plant in non-infested soil.
Leaf Mold Yellow spots on upper surface of leaves. Olive to gray mold on underside of leaves. Primarily in greenhouses. Chlorothalonil 54 F 1.5-2 tsp 0 Fungicides will control leaf mold in the garden, but are not adequate in greenhouse production. The best control in greenhouse tomatoes is the
use of resistant varieties and regulation of humidity.
Pythium Stem Rot Dark, water rot of lower stem of young plants.        
Southern Blight Plants wilt and die. White mold can often be seen on base of stem. Terraclor 75 WP (soil treatment) mfg directions   Turn soil deep to bury plant debris which might harbor disease organisms. Use Terraclor in setting water. Rotate with turf.
Verticillium Wilt Yellowing of leaves; inside of stems will have brown discoloration.       Use resistant varieties. Practice long rotations of tomatoes with other crops.
TURNIP (SEE COLLARDS)
WATERMELON
Alternaria Leaf Spot Brown, target spots on leaves. Chlorothalonil 54 F Mancozeb 75 DF Maneb 80 WP 1.5-3 tsp
 
2-3 TBSP
 
2-4 tsp
0
 
5
 
5
Begin at first sign of disease. See label for spray intervals. Plant anthracnose- resistant varieties. Mancozeb and Maneb are not labeled for scab control.
Anthracnose Brown irregular spots on leaves and sunken spots on fruit.
Cercospora Leaf Spot Tiny, dark brown spots.
Downy Mildew Yellow, irregular spots on leaves.
Gummy Stem Blight Irregularly circular leaf spots. Cracks on stems with gummy ooze.
Scab Small, raised warts on fruit.        
Fusarium Wilt Yellow and wilting of leaves. Brown discoloration in interior of vine.       Grow resistant varieties. Long rotations should be practiced even where
resistant varieties are used.

1   Common names are  used  for  chemical recommendations. Trade names are  listed in  Table II.
2   Consult label for  precise rate of application. The  rates given in  teaspoon or tablespoon per  gallon in  the  publication are  only  approximate
and  the  label of the  specific  product to  be  used  should be  consulted before  mixing and  applying any  fungicide.
3   PHI  (pre-harvest interval) is  the  minimum number of days  between last application and  harvest Never attempt to use  dust formulations of fungicides  in  spray solutions. Dusts will  not  suspend in the  spray solution. Wettable powder  formula- tions may  appear similar to dusts, but  they are formulated to be suspended in  spray solutions.

Table II. Common Names and Trade Names of  Chemicals for Controlling Diseases of  Vegetables

 
Common Names
 
Trade Names1
 
benomyl
 
Note:  Home  garden benomyl products are  no longer available.
chlorothalonil Bravo 720  (54%) Bravo 75 WP Bravo 90 WDG
Bravo Flowable Fungicide
Broad Spectrum Liquid Fungicide
Fungi-Gard
Lawn, Ornamental and  Vegetable Fungicide
Vegetable Disease Control
fixed  coppers Basicop Blue Shield Kocide
Tri-Basic Copper  Sulfate
mancozeb2 Dithane DF Manzate 200  DF Penncozeb
maneb2 Maneb 80 WP Maneb Spray
Maneb Liquid Fungicide
PCNB Terraclor 10 G Terraclor 75 WP

1   Follow  label instructions when mixing and  applying fungicides. One  gallon of fungicide spray will usually cover  400-450  square feet  of garden area (130-150  linear feet  in  three foot wide  rows).  The application rate varies with plant size.  Some  product labels stipulate higher quantities of weaker sprays than other similar products. Always follow  the  label. Labels may  limit the  number of applica- tions or the  total allowable amount of fungicide/unit area.
 
2  The  Environmental  Protection Agency is requiring that all  mancozeb, maneb and  metiram product registrations and  labels bearing homeowner uses  be amended to bear the  following protective clothing and  hygiene language:  “Home  gardeners applying this product must wear long-sleeved shirt, long pants and  chemical-resistant gloves.  The  gloves  must be washed thoroughly with soap  and  water before removing. Clothes must be changed immediately after using these EBDC  products and  must be laundered separately from  other laundry items before  reuse.”
 
This  listing does  not  imply any  preference or discrimination to other products of similar suitable composition, but  is provided solely  as  a reference. All  formulations of a particular  fungicide may  not be labeled for all  crops;  check  label before  purchasing or using any  fungicide.
 
Foliar sprays will  aid  in  controlling leaf spots,  rusts, mildews, anthracnose and  fruit rots. Foliar sprays are  not effective against  vascular wilts or root  rots.  Foliar sprays are  protectants, because they form  a protective layer of fungicide over  the  surface of the  fruit and  foliage. Disease agents (bacteria and  fungi) that land on these fungicide coated surfaces are  killed or prevented from  infecting the  plant.
Most  fungicides are  not  effective in  inhibit- ing  disease organisms once  they have infected a plant. It is imperative that foliar fungicides be applied prior to infection of the  plant. A spray schedule should be followed  that maintains a protective fungicide layer on the  foliage and fruit during favorable infection periods. By carefully monitoring their vegetables, some gardeners can  usually delay the  first fungicide application until the  first sign  of disease. Then a 7-14  day  spray schedule should be followed. During rainy or humid weather, spray applica- tion  intervals should be shortened.
Proper pesticide mixing and spraying plays a very  important part in  achieving disease control. Most  hom  gardeners will  find  a 1-2 gallon compressed-air sprayer adequate for applying foliar sprays. A nozzle  with a cone pattern will  provide the  most  effective coverage of plant foliage. Keep  the  pressure up  to insure small spray particle size  and  good coverage.
Sprayers should be cleaned and  rinsed after each  use.  Hose-end sprayers are  not  very  effec- tive  in  applying fungicides to vegetables. Never use  the  same sprayer for fungicides and  insecti- cides  that has  been  used  for herbicides. Residues of certain types of herbicides are  very  difficult to remove from  sprayers. These residues may  cause crop  injury if a herbicide-contaminated sprayer is used  in  applying fungicides or insecticides.
Homeowners should protect themselves when mixing or applying pesticides. Always read the label for use directions and precaution statements. As of spring 1992,  the  Environmental Protec- tion  Agency is requiring that all  mancozeb, maneb and  metiram product registrations and  labels bearing homeowner uses  be amended to bear the  following protective clothing and hygiene language:
“Home gardeners applying this product must wear long-sleeved shirt, long pants and chemical-resistant gloves. The gloves must be washed thoroughly with soap and water before removing. Clothes must be changed immediately after using these EBDC products and must be laundered separately from other laundry items before reuse.”

Early blight of potato and its economic impact

[ads_dropcap]T[/ads_dropcap]he primary damage of early blight is due to premature defoliation of the plant. Photosynthesis rates increase and respiration rates decrease in apparently healthy tissues. Physiological changes are difficult to measure and evaluation of crop loss is based on the level of disease. When potato tubers become infected, the quantity and quality of marketable products is decreased and the number of secondary pathogens increases. Control of early blight has been shown to increase yield and increase profit margin to farmers.

[ads-quote-center cite=’Hafiz Muhammad Rizwan Mazhar’]Department of Plant Pathology, University of Agriculture, Faisalabad-Pakistan (E-mail: rizwanmazhar084@gmail.com)[/ads-quote-center]
[restabs alignment=”osc-tabs-left” responsive=”false” tabcolor=”#81d742″]
[restab title=”Pathogen” active=”active”]Alternaria solani [/restab]
[restab title=”Host”]Potato (Solanum tuberosum)[/restab][/restabs]
 

Symptoms

Symptoms of early blight occur on foliage and tubers of potatoes.  Initial symptoms on leaves appear as small 1-2 mm black or brown lesions. The lesions will enlarge and are often surrounded by a yellow halo. Lesions greater than 10 mm in diameter often have dark pigmented concentric rings called “bull’s-eye” type lesion is highly characteristic of early blight.  Lesions occurring on stems are often sunken and lens-shaped with a light center, and have the typical concentric rings. Beneath the surface of the lesion the tuber tissue is leathery or corky with a brown discoloration.  Early blight lesions on tubers tend to be dry and are less prone to attack by secondary organisms than lesions of other tuber rots.  After prolonged storage severely diseased tubers may become withered.

 

Pathogen biology

            The causal pathogen of early blight is the fungus Alternaria solani.  There is no known sexual stage and hence it is classified as a Deuteromycete.  The genus Alternaria is a large and important group of pathogenic fungi, which cause a significant number of important diseases.  The fungus is readily cultured on artificial media such as V8 juice where it produces a deeply pigmented gray/black hairy colony.  The mycelium is haploid and septate, becoming darkly pigmented with age.  Sporulation in culture can be stimulated by exposure to fluorescent light.  The asexual conidia are borne singly or in a chain of two on distinct conidiophores.  The beaked conidia normally possess 9–11 transverse septae. Morphological and pathogenic variability among isolates of A. solani has given rise to claims of the existence of races, although this remains unproven.

 

Disease Cycle and Epidemiology

Alternaria solani overwinters primarily on infected crop debris. The dark pigmentation of the mycelium increases resistance to lysis which extends the survival time in the soil to several years. Warm, humid (24-29°C/ 75-84°F) environmental conditions are conducive to infection. In the presence of free moisture and at an optimum of 28-30°C (82-86°F), conidia will germinate in approximately 40 minutes. Germ tubes penetrate the leaf epidermis directly or enter through stomata. Infection of potato tubers usually occurs through wounds in the tuber skin inflicted during harvest. Wet conditions at harvest provide a favorable environment for spore germination as well as causing swollen lenticels on the tubers which are easily invaded.

Early blight is principally a disease of aging plant tissue. Lesions generally appear quickly under warm, moist conditions on older foliage and are usually visible within 5-7 days after infection.   

A long wet period is required for sporulation but it can also occur under conditions of alternating wet and dry periods.  Secondary spread of the disease results from conidia being dispersed mainly be wind and occasionally by splashing rain or overhead irrigation.  Early blight is considered polycyclic with repeating cycles of new infection.  This is the period when the disease has the potential to spread rapidly and build up to damaging levels in the crop.

Disease Management

Cultural practices

In many cases employing sound cultural practices that maintain potato plants in good health will keep early blight losses below economic levels.  Because the pathogen over winters on infected crop debris, in-field sanitation procedures that reduce initial inoculum in subsequent crops are beneficial.  Consideration should be given to removing potentially infected material.   Optimal tuber maturity is the most important factor for control of tuber infection.   Tuber infection can be reduced by careful handling during harvest to minimize wounding as well as avoiding harvesting during wet conditions.  Tubers should be stored at 50 to 55 ºF, at high relative humidity and with plenty of aeration to promote wound healing which will reduce the amount and severity of tuber infections that develop in storage.

Chemical control

Fungicides with protectant and curative properties are registered for use against early blight on potato. The cheaper protectant fungicides such as mancozeb and chlorothalonil are the foundation of most early blight management programs. These fungicides must be reapplied every 7-10 days to provide protection of new growth as well as to counter the effects of weathering which progressively removes the chemical from the leaf surface.  Fungicides which are registered for Alternaria control in potato include azoxystrobin, pyraclostrobin, trifloxystrobin, fenamidone and famoxidone.  They are weakly curative and use rates are considerably lower than the traditional protectant products, although cost per acre is typically higher.

Significance

In modern agriculture the potato are relatively recently adopted crops. Wide scale cultivation gained prominence largely in the 19th century some 300 years after their introduction into Europe from the Andean region of South America. The potato is a dietary staple in nearly all temperate countries with annual production worldwide in 2017 of around 381 million metric tones (FAOSTAT, 2017). Early blight is distributed worldwide and essentially occurs wherever potatoes are grown. Crop losses due to early blight in unsprayed fields vary enormously from 5 –78%.   Best estimates suggest annual expenditure globally on fungicides for control of Alternaria spp. is around $45 million in potatoes.

Social Impact and Economic Importance of Blight

The possible economic and social impact of this disease is best demonstrated by the significant role it played in the Irish Famine in the middle of the 19th century.  Irish farmers depended on the potato for their primary source of food.   As a result of the famine, millions of Irish died or move to another country.

Late blight is a serious economic threat in the vast majority of potato production systems.  In locations where disease pressure is high, a susceptible potato variety may require fungicide applications every 3–5 days.  In spite of the obvious destructive potential of late blight, it is extremely difficult to measure losses due to this plant disease because other factors simultaneously affect yield.

 

Bacterial Leaf Spot

Bacterial Leaf Spot

Cause.

Xanthomonas campestris pv. alfalfae (syn. X. alfalfae and Phytomonas alfalfae) survives in infected residue that has been incorporated into soil or is lying on the soil surface, in hay, and in debris associated with seed. During warm, wet weather, bacteria are splashed or blown onto leaves and enter them through small wounds that have been made by any means. During dry weather, bacteria may enter leaves through wounds made by windblown soil particles. The bacteria multiply inside the leaf and frequently ooze to the leaf surface, where they may be splashed by rain or rubbed by leaf to leaf contact onto adjacent healthy leaves.

Distribution.

Wherever alfalfa is grown under warm, wet conditions.

Symptoms.

Diseased seedlings are often killed or stunted at high temperatures. Initially, small, water soaked spots occur in chlorotic areas on leaflets. The spots enlarge to irregular shaped

lesions (23 mm in diameter) with chlorotic margins that are pronounced on the underside of leaflets. Eventually, the lesions become light yellow to tan, often with a lighter center, and have a translucent, papery texture. Lesions often glisten because of the dried bacterial exudate on their surface. Diseased leaves defoliate prematurely. Stem lesions are water soaked initially, then turn brown or black.

Management

1. Grow resistant cultivars. Plants within a cultivar differ in susceptibility

2. Sow in the spring.

Alfalfa Sprout Rot

Alfalfa Sprout Rot

Cause.

Erwinia chrysanthemi survives for up to 2 weeks on inoculated dried seeds. Air, the water supply, or greenhouse workers may introduce bacteria into a sprouting house. Spread within a tray is by seed to seed contact. Rot is most severe at high moisture and temperatures of 28°C and above. Little disease occurs below 21°C.

Symptoms.

Radicles are a translucent yellow as they emerge from the seed. In 24 to 48 hours, seeds stop growing and turn into a yellowish, odiferous mass that contains numerous bacteria. Disease initially occurs in a few trays, but it may spread throughout all trays in a few days.

Management

1. Control temperature in the sprouting house.

2. Practice sanitation. Bacteria likely survive in water remaining in tanks used for soaking seeds.

3. Soak seeds for 2 hours in 0.5% sodium hypochlorite or calcium hypochlorite. However, hypochlorite is not currently registered for alfalfa seed treatment in the United States.

Establishing the Cause of Disease

The correct diagnosis of a plant disease and its cause is not always an easy task. In the first instance symptoms may be ill defined which make their association with any organism problematic (Derrick and Timmer, 2000) and, secondly, plants grow in environments which are notably non-sterile. In particular, besides supporting a microflora on their aerial parts, the phylloplane, they are rooted in soil which may contain in excess of 1 million organisms per gram. The plant pathologist is therefore faced with trying to determine which, if any, of the organisms associated with the diseased plant is responsible for the symptoms. This is normally achieved by the application of the postulates of Robert Koch, a German bacteriologist of the 19th century, which for plant pathogens may be stated as follows:

  1. The suspected causal organism must be constantly associated with symptoms of the disease.
  2. The suspected causal organism must be isolated and grown in pure culture.
  3. When healthy test plants are inoculated with pure cultures of the suspected causal organism they must reproduce at least some of the symptoms of the disease.
  4. The suspected causal organism must be re isolated from the plant and shown to be identical with the organism originally isolated.

Clearly, these criteria can only be met with organisms that can be cultured, ruling out all obligate pathogens which include a number of important fungi, many phytoplasmas and all viruses and viroids. Establishing these organisms as causal agents of disease usually involves purification of the suspected agent rather than culture and the demonstration that these purified preparations reproduce at least some of the disease symptoms.

Reference: Introduction to Plant Pathology. Richard N. Strange. 2003. John Wiley and Sons Ltd.

Cultural Methods of Vegetable Disease Control

Most vegetables are susceptible to one or more diseases. You can, therefore, anticipate disease problems sooner or later in your vegetable garden. By following good cultural practices and taking preventive measures, your chances of garden failure due to disease problems can be reduced.

Cultural Methods of Vegetable Disease ControlGarden site selection is important to pro-duce high yields of healthy vegetables. Trying to grow vegetables on a poor site is one of the main causes of garden failure. Although few people will have ideal garden sites, they should select the best site available.

Garden sites should not be within the drip line of large trees. Avoid planting near black walnut trees, since they produce a root sub-stance that is toxic to certain vegetables, especially tomatoes. The garden site should be slightly sloped to provide good water and air drainage through the soil.

Excess soil moisture can damage vegetable roots, as well as promote root diseases caused by certain fungi. Air movement through the garden is also important to help dry the foliage, thus reducing the chances of fungal and bacterial infections. Garden sites with good air drainage are less likely to be damaged by late frosts.

Most garden vegetables require full sunlight for maximum production. Sunlight also hastens drying of foliage. Soil tillage should be done early enough, prior to planting, to allow decomposition of raw organic matter such as manure or green plant material. This usually requires about six weeks under warm temperatures and longer at low temperatures. Organic material that has not decomposed can be a source of disease organisms and can also promote development of certain diseases such as root and stem rots. Applying nitrogen fertilizer before plowing or tilling green plant material into the soil will hasten its de-composition.Cultural Methods of Vegetable Disease Control2

Crop rotation will help prevent the buildup of disease-causing organisms in the soil. Some disease causing organisms affect one vegetable or group of vegetables, but may not affect an-other. Several vegetables of the same family, such as squash, cucumbers and cantaloupes, may be affected by the same disease. Therefore, it is not a good practice to grow plants of the same family in rotation. Table 1 gives crop groupings for rotation to control soil-borne diseases. At least a three-year rotation is suggested for vegetable crops.

Sanitation is very important in controlling vegetable diseases. Many disease-causing organ-isms survive the winter in plant debris, cull fruit or plant stubble left in the garden. Any practice that will eliminate these overwintering sites for fungi, bacteria, viruses and nematodes will reduce the occurrence of disease problems the following year. Removal or plowing-under of crop stubble and trash helps destroy overwintering populations of disease organisms. Some disease-causing organisms are able to survive the off season on contaminated equipment or containers. Equipment that has been used in disease-infested vegetable gardens or containers used in handling diseased vegetables should be disinfested before being used again.

Disease-free seed and transplants are a must in vegetable production. Seed should not be saved from diseased plants. Always buy seed from a reputable dealer, since you normally cannot tell from their external appearance if seed are contaminated with disease-causing organisms.

Certain geographical areas, such as the arid western states, can produce disease-free seed because of climatic conditions. Seed from these areas should be stipulated in your seed orders. Gardeners starting their crop from transplants should, likewise, insist on disease-free plants.

Seed treatments vary, depending on the crop as well as the disease to be controlled. Some disease-causing organisms are carried on the surface of seed and can be controlled by a simple fungicide treatment. Fungicides are not effective against those organisms carried beneath the seed coat.

Fungicides applied to seed also give young seedlings some protection from soil-borne disease organisms as they germinate and emerge. Such treatments, however, do not control organisms that attack the plant after the seedling stage.

A seed treatment is usually applied by the company from which the seed is purchased. Home-grown seed can be treated at home with relative ease. Thiram or Captan fungicides can be used as seed treatments on most vegetable crops. Use these protectant fungicides according to instructions on the label. For small quantities of seed, such as packets, apply sufficient fungicide to coat the seed surface. Simply place a small quantity (comparable to the size of a match head) in the packet, reclose and shake to coat the seed with the fungicide.

Planting dates can be an effective tool in reducing diseases of vegetables. Okra, for in-stance, requires warm soil for good germination and growth. If planted when the soil is still cold, the seeds will rot, or if they do germinate, they will probably develop damping-off or stem rot. Some crops, such as corn and beans, should be planted as early as the weather permits to escape severe virus infections. Aphids that transmit viruses are usually at lower population levels early in the season.

Mulches can be used to conserve moisture, keep fruit clean and prevent diseases. Mulches reduce fruit rot on crops, such as strawberries, tomatoes, squash, cucumbers and melons by preventing direct contact with the soil. Mulching will reduce splashing of soil onto lower fruit and foliage by rain.

Staking or trellising tomatoes, pole or half runner beans and cucumbers will prevent soil contact with the foliage and fruit. Air circulation will be better if these plants are trellised, thus promoting better drying of foliage and reducing diseases. Pesticides can be more effectively applied to trellised plants.

Watering can influence the development and severity of many foliage diseases. Wet foliage is favorable for the development of most diseases. To reduce infections, apply irrigation water to the soil rather than the foliage. If water must be applied to the foliage, then it should be done in late morning or mid-afternoon to allow the foliage to dry before evening.

Maintaining uniform soil moisture can re-duce problems such as blossom end rot of pe-pers and tomatoes. Excessive soil moisture can result in increased root and stem rot diseases. It is best to work in the garden when the foliage is dry to reduce disease spread. Bacterial diseases of tomatoes, beans and other crops are readily spread on hands and clothing of workers when the foliage is wet.

Use of resistant varieties is one of the most economical ways of controlling vegetable diseases. Resistant varieties should be used in areas where diseases are present or where the soil is known to be infested with disease-causing organisms. Resistant varieties should be used even when rotation is practiced.