12072_TheEcologicalFarm

Vegetable Crop Diseases and Interventions

Most of the diseases described in this chapter are common throughout North America. Many, such as late blight, are found worldwide. As with insect pests, I categorize interventions for plant disease into minimal impact, moderate impact, and heaviest impact, based on their potential to disrupt ecological systems. I include information and research about suppressive soil microorganisms because these microorganisms are a natural part of a functioning and healthy soil organic matter system. These beneficial microorganisms suppress disease in several ways: They improve plant health so plants can better resist disease, and they directly compete with and attack disease microorganisms. I also focus on suppressive soil microorganisms because many researchers and farmers are experimenting with commercial formulations of these microorganisms, which are now or will become available for farmers and gardeners.

Note that for some diseases, I list only minimaland moderate-impact interventions. This is because the heaviest-impact interventions available to organic growers, such as copper fungicides, are not really worth the ecological impact risk. For other diseases, there are no moderate-impact options available. The “Physiological Disorders and Nutrient Deficiencies” section beginning on page 260

covers deficiencies and other growth problems that may sometimes be mistaken for disease.

Alternaria Diseases: Early Blight, Leaf Blight, Purple Blotch

Alternaria is a common fungal genus with many species; individual species attack different plant families. For example, Alternaria cucumerina attacks melon and cucumber, A. solani attacks tomatoes and potatoes, and A. porri prefers onion and garlic. Alternaria causes leaf spotting and wilting. Leaf spots progress from older leaves to newer ones. Circular spots start as small, sunken, water-soaked areas and progress to purple to brown spots surrounded by yellow rings that create a diagnostic bull’s-eye pattern seen in figure 11.1. Entire leaves may die and drop from the plant. Spots are also found around stem ends on fruits and on the plant stem. This fungus is spread by splashing water and by walking through plants when wet. Alternaria survives in plant debris and may be spread by insects, or wind, and by rain/irrigation. Spores germinate in several hours during wet, highhumidity conditions. Infection is encouraged if water remains on plant tissue for longer than a couple of hours. Optimal temperatures for infection are 60 to 80°F (16–27°C) (depending on the

Angular Leaf Spot

Alternaria Diseases: Early Blight, Leaf Blight, Purple Blotch

reportedly improve tomato growth and suppress early blight disease.1

• Fruit rot on field-grown chili peppers: Application of Trichoderma harzianum IMI 392432 to pepper seeds before planting significantly suppressed disease caused by A. tenuis 2

• Tests at Cornell University report suppression of Alternaria on basil using an organic product containing the beneficial bacterium Streptomyces lydicus.

• Tests at Cornell University report suppression of Alternaria on brassica crops using an organic product containing the beneficial bacteria Bacillus amyloliquefaciens strain D747.

HEAVIEST-IMPACT INTERVENTION

Alternaria species) plus high humidity. The disease is encouraged by low soil fertility.

HOSTS

Depending on the fungus species, cucurbits (cucumber, squash, melon, and pumpkin), solanaceous crops (eggplant, pepper, potato, and tomato), onion, and other vegetables such as peas and cabbage.

MINIMAL-IMPACT INTERVENTION

Keep water off foliage; irrigate with soaker hoses or drip irrigation. Follow 3- to 4-year rotations between susceptible crops in the same plant family. Remove infected leaves. Maintain balanced nutrient levels and high soil organic matter levels to enhance native soil biological control microorganisms.

SUPPRESSIVE SOIL MICROORGANISMS

Some specific soil microorganisms have shown suppression of Alternaria species.

• Early blight on tomato: Bacillus amyloliquefaciens strain F727 has shown some suppression of A. solani. Plant-growth-promoting rhizobacteria

Sulfur (or as a very last resort, copper) can be sprayed when temperatures are between 55 and 85°F (13–29°C) and weather is wet, to protect leaves from infection.

Angular Leaf Spot

Angular leaf spot (Pseudomonas syringae) is a bacterial disease causing dark, angular spots between leaf veins. One good diagnostic is that tear-shaped droplets ooze from infected tissue. As the leaf dries, tissue tears and shrinks. Fruit may have circular spots or rotted areas. The bacteria overwinters in plant debris and can persist for more than 2 years on dry leaves.

HOSTS

Cucurbits (cucumber, squash); a similar strain infects beans.

MINIMAL-IMPACT INTERVENTION

Follow 2- to 3-year rotations between cucurbit crops. Avoid wetting foliage with irrigation water. Use resistant cultivars when possible. Plant on raised beds. Excessive nitrogen fertilizer increases disease severity. Angular leaf spot is also seed-borne; treat seed for 20 minutes with 120°F (49°C) water.

Figure 11.1. Early blight symptoms on tomato leaves.

Angular Leaf Spot

Maintain balanced nutrient levels and high soil organic matter levels to enhance native soil biological control microorganisms.

SUPPRESSIVE

SOIL

MICROORGANISMS

Beneficial soil bacteria have been reported to induce systemic resistance against angular leaf spot disease of cucumber.3 (See the section “Epiphytic Bacteria and Fungi” on page 141 for a discussion of induced systemic resistance.)

HEAVIEST-IMPACT INTERVENTION

As a very last resort, if a crop is already showing symptoms and wet weather is expected, copper can be sprayed to protect leaves.

Anthracnose

Small, depressed, and circular fruit spots are the most common symptom of anthracnose, which is caused by Colletotrichum species fungi. Spots become much larger, with centers ranging from tan or orange to brown or black. Anthracnose prefers temperatures of around 80°F (27°C) and wet conditions. The disease is seed-borne and can also overwinter in the soil. Rain splash spreads anthracnose spores from infected soil, plant debris, and/or fruits onto susceptible plants.

HOSTS

Vegetables, members of the cucurbitaceae family, and peppers.

MINIMAL-IMPACT INTERVENTION

Rotate crops, allowing a 2- to 3-year gap period between susceptible crops (including strawberry). Avoid wetting foliage with splashing irrigation water. Use resistant cultivars when possible. Plant on raised beds. Use straw or living mulch or plastic mulch to avoid splashing of soil onto plants. Avoid excessive nitrogen fertilization.

HEAVIEST-IMPACT INTERVENTION

During wet, warm weather, copper can be sprayed to protect leaves as a very last resort if anthracnose is a common problem for your crops.

Aster Yellows

Aster yellows is a disease caused by an organism called a phytoplasma. It is spread only by leafhopper feeding. This disease causes twisted distorted new growth (including leaflike petals) and a yellowing/reddening of leaf tissue; it also causes hairy roots in carrots. Leafhoppers that spread the disease overwinter in warmer regions. After feeding on infected plants, they migrate north and can then transmit the disease through continued feeding on uninfected plants. Peak infection periods are in late summer and early fall.

HOSTS

Carrot, celery, and various flowers, including asters and zinnia.

MINIMAL-IMPACT INTERVENTION

Plant resistant cultivars when possible. At the Frontier Herb Company Experimental Farm in Iowa, planting several crop species together in growing beds greatly reduced the incidence of aster yellows.

HEAVIEST-IMPACT INTERVENTION

Control leafhoppers that spread this disease by applying insecticidal soap plus neem.

Bacterial Spot or Bacterial Blight

Leaves, stems, and fruit infected with bacterial spot (Xanthomonas campestris) develop brown, circular spots or specks, but without concentric zones (as for fungal leaf spot diseases). On beans there is a diagnostic yellow ring around the brown spots. The

Bacterial Wilt of Cucurbits

Bacterial Spot or Bacterial Blight

bacterium overwinters on plant debris and is seed-borne. It is favored by wet conditions, high humidity, and temperatures of 75 to 86°F (24–30°C).

HOSTS

Tomato and pepper. Another race of the bacteria also affects beans.

MINIMAL-IMPACT INTERVENTION

Keep water off foliage, stake tomatoes and peppers to keep them off the ground, and follow a 3-year rotation. Treat seed with hot (122°F [50°C]) water or bleach.

SUPPRESSIVE SOIL MICROORGANISMS

Tests at Cornell University report suppression of bacterial spot on tomato using an organic product containing the beneficial bacterium Bacillus amyloliquefaciens strain D747.

HEAVIEST-IMPACT INTERVENTION

Copper can be sprayed to protect leaves as a very last resort, during wet weather when temperatures exceed 75°F (24°C). Copper is only moderately effective on bacterial spot, however, and may have phototoxic effects on tomato plants.

Bacterial Wilt of Cucurbits

Infection by Erwinia tracheiphila bacteria causes wilting and death of most cucurbits. White ooze is visible at any cut in the plant tissue. Bacterial wilt is spread by and depends upon the presence of cucumber beetles and overwinters in their guts. In fact, the bacterium survives the winter only in the digestive tract of cucumber beetles. In spring, when beetles begin to feed on new cucumbers and melons, the overwintered bacteria are spread to new cucurbit leaves from the beetles’ fecal droppings.

The bacteria can then infect plants through wounds caused by beetle feeding. Bacteria are unable to infect plants through normal plant openings (such as stomata) and the disease is not carried on seed. The disease can be more severe with excessive levels of nitrogen, but it also seems to be promoted by very low levels of nitrogen and potassium in unbalanced nutrient situations.

HOSTS

Cucumber, squash, muskmelon, pumpkin, and gourd. Watermelon is immune.

MINIMAL-IMPACT INTERVENTION

Use tightly secured row covers over seedlings and transplants to prevent cucumber beetles from gaining access.

HEAVIEST-IMPACT INTERVENTION

Control populations of cucumber beetles (see the “Cucumber Beetles” entry on page 226). When the first cucumber beetles are observed, spray with neem and pyrethrum.

Black Canker

Black canker fungus (Tersonilia perplexans) causes brown, black, or purple black cankers to form mainly on the surface of root crowns and shoulders. The disease organism overwinters in infected parsnip and carrot roots or as spores in the soil. Disease development is enhanced by cool, wet weather.

HOSTS

Carrot and parsnip.

MINIMAL-IMPACT INTERVENTION

Use of resistant varieties is the best suppression practice. Rotate parsnips and carrots with non-host crops.

Black Rot

Black Rot

Black rot (Xanthomonas campestris) turns young plants yellow, then brown, and eventually kills them. Yellowing begins first on older leaf edges and progresses inward in a wedge-shaped pattern. Leaf veins turn black. Black rings and yellow ooze are present in cut stems. The most common source of the infection is infested seeds or infected transplants. The bacteria can remain infectious for up to 2 years. Infection occurs during wet weather. Optimal temperatures for disease development are 80 to 86°F (27–30°C) when dew, rain, or sprinkler irrigation are present. Transmission occurs via insects, water, and gardening or farming tools.

HOSTS

A wide range of brassica crops and some brassica weeds (weeds can be a source of inoculum).

MINIMAL-IMPACT INTERVENTION

Keep water off leaves or water in the morning to allow maximum rapid drying. Follow 2- to 3-year rotations and destroy infected plants. Grow resistant varieties. Treat seed with hot (120°F [49°C]) water for 15 to 20 minutes before planting. Note that mustards are susceptible to heat-induced seed damage and require shorter duration hot water treatment. Low nutrient content in soils and plants aggravates black rot disease. In one study, disease severity diminished significantly when organic sources of foliar nitrogen and potassium (from manure-based compost) were added to kale plants, and lignin cotent increased. The compost appears to have activated defense mechanisms in kale plant tissues.4

SUPPRESSIVE SOIL MICROORGANISMS

An antagonistic, endophytic strain of Bacillus subtilis suppressed three strains of black rot bacteria in cabbage, cauliflower, rape, and broccoli. However, when encouraging or applying these

microorganisms, details matter; this beneficial bacteria was less effective when crops were grown on heavy soils during heavy rainfall periods.5 Tests at Cornell University report suppression of black rot on brassica crops using an organic product containing the beneficial bacteria Bacillus amyloliquefaciens strain D747.

HEAVIEST-IMPACT INTERVENTION

Spray copper only if you have had severe problems with black rot in the past and weather is wet. Copper is ineffective once the disease is well established and works only to limit spread to uninfected plants.

Botrytis

Senescent leaves, fruits, and petals are susceptible to botrytis (Botrytis cinerea), which is also commonly called gray mold. Under cool, moist conditions a soft, brown decay develops, covered by a dense gray to light brown mass of spores. Germination of spores and infection requires moisture for 8 to 12 hours, relative humidity of 85 percent or greater, and temperatures 55 to 75°F (13–24°C). Growth of this fungus is inhibited at temperatures above 89°F (32°C). Healthy, actively growing green plant parts are seldom infected directly by Botrytis fungi.

HOSTS

Many vegetable, fruit, and flower crops.

MINIMAL-IMPACT INTERVENTION

Plant on raised beds, maintain low humidity, and keep water off leaves or maximize drying conditions when you irrigate. Organic biofungicides containing the beneficial fungus Ulocladium oudemansii (U3 strain) have shown some suppression of Botrytis.

HEAVIEST-IMPACT INTERVENTION

If botrytis has been a past problem, spray sulfur or potassium bicarbonate when humid, cloudy weather persists.

Cercospora Leaf Spot

Cercospora Leaf Spot

Leaf spots typical of cercospora leaf spot are pale, round circles with dark margins (see figure 11.2). Symptoms appear on older leaves first and can cause defoliation. Cercospora fungi overwinter on plant residue. Spores are carried on the wind relatively long distances. Infection requires free water on leaf surfaces and warm nights combined with high humidity. Optimal daytime temperatures for disease development are 77 to 95°F (25–35°C) with night temperatures above 61°F (16°C) and a relative humidity of 90 to 95 percent. Inoculum occurs on residue from a previously infected crop, but the fungus can be carried on seed.

HOSTS

Beet, Swiss chard, spinach, carrot, celery, cucurbits (cucumber, squash), tomato, and lettuce (different

species on each vegetable). Also infects several weed species, such as lambsquarters, pigweed, mallow, and bindweed.

MINIMAL-IMPACT INTERVENTION

Grow resistant varieties. Soak seed in 122°F (50°C) water for 25 minutes before sowing. Rotate crops, allowing a 3-year break between susceptible vegetables. Keep water off leaves. Increase air movement by staking crops if possible.

SUPPRESSIVE SOIL MICROORGANISMS

An antagonistic strain of Bacillus subtilis has been reported to suppress cercospora leaf spot.6 Repeated foliar applications of a liquid culture of two Trichoderma fungi isolates also suppressed cercospora in one study.7 Tests at Cornell University report suppression of this disease using an organic product containing the beneficial bacteria Streptomyces lydicus.

HEAVIEST-IMPACT INTERVENTION

Spray sulfur at first sign of the disease if it has been a serious problem in your crops in the past.

Clubroot

Clubroot (Plasmodiophora brassicae) is not seedborne, but the main means of spread is contaminated transplants. The disease causes wilting as a result of deformed and constricted roots. It is favored by wet, heavy soils and rain events that cause temporarily flooded soils.

HOSTS

All brassica crops.

MINIMAL-IMPACT INTERVENTION

Choose transplants carefully or grow your own. Adjust soil pH to 6.8 with lime. Some research indicates that in acidic soils, annual application of

Figure 11.2. Cercospora leaf spot on Swiss chard.

Curly Top (Virus)

1,500 pounds per acre (1,680 kg/ha) of hydrated lime (calcium hydroxide) before planting helps to diminish clubroot. A rotation of at least 7 years out of susceptible brassicas is also necessary.

Curly Top (Virus)

Infection by the curly top virus causes dwarfing, leaf vein swelling, curling, puckering, distortion, and yellowing; death occurs if very young plants are infected. The causal viruses are curtoviruses (family Geminiviridae), which are spread by leafhopper insects. Tomato, melon, and cucurbit fruits in general appear to ripen prematurely, but have an odd taste. Younger plants seem to be more susceptible to damage and develop more symptoms from the virus compared with adult plants. High light intensity, prolonged summer heat, and high evaporation lead to severe infection. Relative humidity above 50 percent reduces curly top; humidity below 35 percent increases it. High

humidity may also delay visits of leafhoppers. The virus is not seed-borne; it overwinters in crop debris and weed hosts, such as lambsquarters, Russian thistle, and four-wing saltbush.

HOSTS

Chard, beet, spinach, watermelon, tomato, cucurbits (cucumber, squash, melon).

MINIMAL-IMPACT INTERVENTION

Grow resistant varieties.

Remove diseased plants immediately. There are reports that crops grown in high tunnels or under plastic tunnels may resist this virus better than those grown unprotected. Increasing humidity may help avoid this virus.

HEAVIEST-IMPACT INTERVENTION

Some recommend spraying for leafhoppers, but this has not been effective for control of curly top and is probably not worth the ecological impact.

Figure 11.3. The deformed kohlrabi roots on the right plant are caused by clubroot disease. The kohlrabi roots on the left are normal, without disease.

Damping Off

Damping Off

Damping off is caused primarily by Pythium fungi, but also Fusarium and Rhizoctonia species. Only young seedlings are affected. Once plants have a well-developed root system and mature leaves, they naturally resist the fungus or mold that causes damping off. Infected seedling stems look watersoaked and thin, and roots are stunted or have dark, sunken spots. Young leaves wilt and turn greengray to brown. The microorganisms that cause damping off thrive in cool wet conditions and are encouraged by slow plant growth due to low light, overwatering, and high salt from overfertilizing. See also the “Rhizoctonia Diseases” entry on page 255.

HOSTS

Many vegetables and flowers.

MINIMAL-IMPACT INTERVENTION

Plant seeds under temperature conditions that favor rapid germination, or pre-sprout seeds before planting outside. For indoor plant production, use a heating pad under trays to warm the soil to 65 to 75°F (18–24°C). Wait until the outside soil has reached optimal temperatures for specific crop germination. Instead of a sterile growing media, use a compost-based growing media with good drainage and water to keep it moist but not so wet that you can squeeze water out of it. Provide good light sources to your seedlings at least 12 hours a day. In some cases, ambient light from windows does not provide enough light. Wait until late morning to water plants to maximize drying conditions.

SUPPRESSIVE SOIL MICROORGANISMS

Tests at Cornell University report suppression of damping off using an organic seed treatment containing the beneficial bacteria Streptomyces lydicus.

Downy Mildew

Symptoms of downy mildew begin as early as seedling stage, and it can be mistaken for other leaf diseases. Downy mildew has darkly colored, angular spores that cause blackish to gray spots on leaves compared with the white, diffuse spores of powdery mildew disease that cause white patches or a white film on foliage.

Different species of fungi cause downy mildew on different crops. Downy mildew on spinach is caused by Peronospora effuse, which does not colonize other crops in the same family (beets and Swiss chard) or weedy relatives, such as lambsquarters, and nettleleaf goosefoot. On lettuce, downy mildew is caused by Bremia lactucae. On cucurbits, it is caused by Pseudoperonospora cubensis. The story is further complicated by downy mildew’s ability to evolve different races. For example, in cucurbits, there are two main races, one that infects cucumbers and melons, and another that infects pumpkins and squash. Hence it is possible to have a healthy pumpkin field alongside a diseased cucumber field. Downy mildew is ecologically adaptable! New races continue to occur as the disease changes.

The optimal temperature range for this disease is 55 to 70°F (13–21°C). However, spores of the downy mildew pathogen have been observed on plants over a very wide temperature range, from less than 40°F up to 118°F (4–48°C). The disease needs moisture on the leaf surface in order to germinate and start a new infection. It is favored when temperatures are low and there are long periods of leaf wetness caused by overnight dew. Since these conditions are common in late fall and in cool-season greenhouses, that is often where we find downy mildew. It is a seed-borne disease.

HOSTS

Cucurbit crops, spinach, basil.

MINIMAL-IMPACT INTERVENTION

An integrated approach is vital to suppressing this disease.

• The use of resistant cultivars is the most effective way to suppress spinach downy mildew. There are many races, however, and most varieties are not resistant to all races.

• Nitrogen fertilizer seems to encourage downy mildew by boosting lush vegetative growth, which encourages higher downy mildew infection. Nitrogen reportedly increased the disease in both grape and pearl millet.8

• Soaking seeds in hot water (122°F [50°C]) for 25 minutes is useful for minimizing the downy mildew inoculum in spinach seeds.9

• Keep water off leaves. Drip irrigation and wide row spacing help to dry leaves and encourage good air movement around plants. You can also trellis vining plants to improve air circulation and avoid wet foliage.

• Follow a 3-year crop rotation. The only place I ever see downy mildew is in my high tunnel where I insist on growing spinach every late fall and winter. I need to forgo spinach for a winter, or build another high tunnel.

SUPPRESSIVE SOIL MICROORGANISMS

Tests at Cornell University report suppression of downy mildew on basil using an organic product containing the beneficial bacteria Streptomyces lydicus and on other vegetable crops in products containing Bacillus amyloliquefaciens.

HEAVIEST-IMPACT INTERVENTION

Potassium-bicarbonate-based fungicides have been used on this disease. Potassium bicarbonate has been reported to kill downy mildew on contact by pulling water from spores and their growing strands.

Fusarium Wilt or Yellows

Fusarium oxysporum, and crop-specific subspecies, are fungi that affect both seedlings and mature plants. Infection causes top growth to wilt, yellow, and die. Lesions form at the plant base or slightly below the soil line. Reddish brown streaks are present internally in the root, stems, and leaf petioles. The earliest symptom is the yellowing of old leaves, often on only one side of the plant. The symptoms can be mistaken for verticillium wilt (see the “Verticillium Wilt” entry on page 259).

Fusarium is most prevalent on acidic sandy soils. It can survive several years in soil and is favored by warmer weather (80 to 85°F [27–30°C]), high levels of phosphorus, certain micronutrients, and ammonium forms of nitrogen.

HOSTS

Broccoli and other brassicas, celery, tomato, eggplant, pepper, potato, cucumber, and cantaloupe; occasionally in pumpkin, squash, and watermelon and weeds such as pigweed, mallow, and crabgrass.

Figure 11.4. Downy mildew on spinach.

Fusarium Wilt or Yellows

MINIMAL-IMPACT INTERVENTION

Use of resistant varieties is the best suppression practice. Follow a 5- to 7-year rotation. In general, raise the pH to 6.5 to 7.0 if the soil is acid. In studies on cucumber, higher levels of nitratenitrogen seemed to protect cucumber plants against fusarium wilt disease by suppressing colonization of cucumber Fusarium oxysporum ssp. cucumerinum. Remove and destroy infected plants.

SUPPRESSIVE SOIL MICROORGANISMS

Many microbial species, including Bacillus spp., Pseudomonas spp., Trichoderma spp., Streptomyces spp., and Acinetobacter spp. have been shown to effectively suppress plant-disease-causing Fusarium species.10 Commercially available biocontrol products containing Gliocladium virens and Trichoderma harzianum can reduce disease when granules are incorporated into potting medium at 0.2 percent (weight/volume). Co-inoculation of an antagonistic bacteria strain, Bacillus amyloliquefaciens, and the fungus Pleurotus ostreatus suppressed cucumber fusarium wilt in a greenhouse pot study.11 Organic products containing Bacillus amyloliquefaciens strain F727 and Streptomyces lydicus can be applied as a foliar spray or root drench. A biofungicide with extract of giant knotweed (Reynoutria sachalinensis) can also be applied as a foliar spray or root drench.

Late Blight

Late blight (Phytophthora infestans) first manifests as water-soaked spots on older leaves; spots enlarge into brown blotches. Leaf undersides may be covered with a gray to white moldy growth. Infected leaves, petioles, and stems shrivel and die. Tomato fruit develops dark, greasy-looking spots that enlarge until the fruit rots. Potato tubers show irregular, slightly depressed areas of brown to purplish skin.

The fungus overwinters on crop debris. It thrives at 91 to 100 percent humidity and optimal

temperatures of 65 to 72°F (18–22°C) with cool days plus warm nights. Inoculum carried by wind and water infects young plants. When weather is favorable, infection moves so fast that plants appear to have been damaged by frost. Hot, dry days with temperatures above 86°F (30°C) decrease infection.

HOSTS

Potato, tomato, occasionally eggplant.

MINIMAL-IMPACT INTERVENTION

Use of resistant varieties is the best suppression practice. For potatoes, keep tubers covered by hilling with soil or mulch throughout the growing season. A biofungicide with extract of giant knotweed (Reynoutria sachalinensis) can also be applied as a foliar spray or root drench. In tests at Cornell University, organic products containing Bacillus amyloliquefaciens strain F727 applied as a foliar spray were somewhat effective against late blight. Researchers in Germany reported that compost extracts provide some control against late blight on tomato leaves.

Figure 11.5. Water-soaked spots and brown blotches on older tomato leaves at summer’s end caused by late blight.

Phytophthora Root Rot

Late Blight

HEAVIEST-IMPACT INTERVENTION

Spray copper only if you have had severe problems with the disease in the past and weather is wet and humid with temperatures between 60 to 75°F (16–24°C).

Mosaic Virus: Tobacco Mosaic Virus, Cucumber Mosaic Virus

Specific virus diseases are difficult to distinguish, but generally they cause stunted, slow-growing plants, twisted, crinkled, cupped, or deformed leaves, and yellow/green mottling, puckering, and distortion of leaves. Viruses can also cause colored circles or mottling and streaks on fruits. There are many species of mosaic virus, and they are very persistent. The virus is spread either through insects (aphids, thrips, and leafhopper feeding) or mechanically (infected tools, hands and clothing). The virus reproduces within plant cells and disrupts the cell’s normal function. Viral diseases are systemic and symptoms usually progress and worsen through the growing season.

HOSTS

Cucurbits (cucumber, squash), solanaceous crops (eggplant, pepper, potato, tomato), celery, corn.

MINIMAL-IMPACT INTERVENTION

Use of resistant varieties is the best suppression practice. Control aphids and cucumber beetles, which help spread the virus. Pre-soak seeds in a 0.5 percent bleach solution. Remove and destroy infected plants.

Phytophthora Root Rot

Several species of soilborne pathogens in the genus Phytophthora cause root rot diseases. Stunted, yellowing, wilting leaves are the first sign of this disease. Leaves may also turn dull green or in some cases red or purplish.

Phytophthera fungi overwinter as spores in soil or diseased plant material. Species that cause root and crown rots enter host plants near the root collar via wounds or the succulent parts of small roots. Fungal spores move in water and are attracted to

Figure 11.6. Wilted and yellowing pepper leaves and fruit due to infection by Phytophthora. Photo by Doug O’Brien.

Phytophthora Root Rot

the root exudates from stressed plants. These spores can survive for years in moist soil without host plant roots. However, if the soil is completely dried out, these spores are less likely to survive for more than a few months. Phytophthera can be spread in splashing rain or irrigation water. Flooded and saturated soils favor the spread. Different Phytophthora species are favored by different temperatures and conditions. Root rot of tomato is favored by warm conditions.

HOSTS

Many vegetable crops, including asparagus, tomato, pepper, eggplant, beans, and brassica crops.

MINIMAL-IMPACT INTERVENTION

The most important factor in reducing Phytophthora disease is proper water management. Plant in raised beds to provide for good drainage. Group your crop plants according to their specific irrigation needs. Separate those needing frequent, light irrigations, such as potatoes, from those needing less frequent, but deep irrigations, such as tomatoes. Avoid prolonged saturation of the soil or standing water. Keep the soil pH above 6.0. Remove and destroy infected plants, including the roots. Follow a 2-year rotation that includes a resistant crop such as corn.

Potato Scab

The bacterium that causes potato scab, Streptomyces scabies, is inhibited at soil pH higher than 7.4, and disease severity is reduced in soils with pH levels of 5.2 and below. There are usually no aboveground symptoms, but potato tubers have roughened, russeted areas and scab-like protuberances with corky tissue.

HOSTS

Potato mainly, but also other root crops, including beet, radish, carrot, and parsnip.

MINIMAL-IMPACT INTERVENTION

Use resistant varieties. Maintaining soil moisture near field capacity during the 2 to 6 weeks following tuber initiation will inhibit infection by potato scab. Bacteria that flourish at high soil moisture outcompete potato scab on the tuber surface. Mulching with straw (as long as the soil has good drainage) may help to maintain higher moisture levels and discourage scab. Applying manure to potato fields has been shown to cause an increase in scab infection. This is probably because Streptomyces bacteria are involved in the decomposition of soil organic residues and hence stimulated by its presence. Rotation with small grains, corn, or alfalfa help to decrease potato scab, but red clover stimulates it and should be avoided where potato scab has been a problem. Also do not rotate with alternate hosts of potato scab, such as radish, beet, and carrot. Limit the use of soil amendments, such as lime and manure that raise soil pH. Light-textured soils that dry out easily and those with high levels of organic matter are favorable to scab infection.

Powdery Mildew

Powdery mildew is a common occurrence on plants in the squash family at the end of the season. It is a good reason to rotate crops, but not to panic. There are many species of fungi that cause powdery mildew; each attacks specific plant families. White, powdery spots form on both upper and lower surfaces of leaves and on shoots, flowers, and fruit. All powdery mildew species can germinate and grow without the presence of water. In fact, spores of some powdery mildew fungi are inhibited when plant surfaces remain wet for extended periods. Temperatures of 60 to 80°F (16–27°C) and shady conditions favor powdery mildew development. The disease is inhibited at temperatures above 90°F (32°C) and in extended direct sunlight.

Powdery Mildew

HOSTS

Beans, beet, carrot, cucumber, eggplant, lettuce, melon, parsnip, peas, pepper, pumpkin, radish, squash, tomato, and turnip.

MINIMAL-IMPACT INTERVENTION

Plant in full sun and avoid shade for susceptible crops. Avoid excess fertilizer. Overhead sprinkling can reduce powdery mildew because spores are washed off the plant (but it could encourage other diseases!).

SUPPRESSIVE SOIL MICROORGANISMS

Bacillus subtilis may suppress the microorganisms that cause powdery mildew.

MODERATE-IMPACT INTERVENTION

Potassium-bicarbonate-based fungicides have been used on this disease. Potassium bicarbonate has been reported to kill powdery mildew on contact by pulling water from spores and their growing strands. Many organic materials have been tested for powdery mildew over the years, but in comparative studies on zucchini at Purdue University, only the potassium bicarbonate treatment had significantly less powdery mildew than the untreated control. JMS Stylet-Oil, insecticidal soap and sulfur were effective in tests at Cornell University.

HEAVIEST-IMPACT INTERVENTION

If powdery mildew is a consistent, serious problem in your fields or garden, consider use of sulfur or copper sprays. Sulfur was very effective in tests at Cornell University. Copper sprays were most effective in tests at Purdue University.

Rhizoctonia Diseases

The Rhizoctonia fungus can be subdivided into strains based on the plant family preferred and optimal temperature to cause disease. For example, strains of Rhizoctonia that attack potato do not attack

brassica crops. The strain of Rhizoctonia that causes bottom rot and head rot of cabbage grows at temperatures ranging between 48 and 91°F (9–33°C), while lettuce Rhizoctonia is favored by warm temperatures between 77 and 81°F (25–27°C). Generally, Rhizoctonia causes damping-off-like symptoms on seedlings, root rots, and aboveground stem cankers and leaf and fruit rots. Rhizoctonia persists in soil and in plant debris. It is persistent over very long periods. Cool, moist soils favor disease development; dry or waterlogged soil discourages it.

HOSTS

Many vegetable crops, including lettuce and brassica crops as well as beet, beans, carrot, celery, cucumber, eggplant, onion, peas, pepper, rhubarb, spinach, tomato, and sweet potato.

MINIMAL-IMPACT INTERVENTION

Practice a 3-year or longer rotation to non-susceptible crops such as sweet corn or onions. Plant crops like lettuce on raised beds to promote air movement and drainage and to minimize foliage contact with

Figure 11.7. White powdery mildew patches on both surfaces of cucumber leaves.

Root Knot Nematodes

the soil. Keep irrigation water off foliage and, for lettuce heads, avoid irrigation near harvest, when crops are most susceptible to Rhizoctonia.

SUPPRESSIVE SOIL MICROORGANISMS

Treat seed with organic products containing Bacillus subtilis or Pseudomonas fluorescens. Organic products containing Bacillus amyloliquefaciens strain D747 can be applied to soil on 14- to 28-day intervals right up until harvest.

Root Knot Nematodes

Nematodes are microscopic roundworms. They can move only short distances on their own but may be spread in transported soil or plant debris. They persist over a fairly long time in gelatinous, sacklike structures when conditions are unfavorable. Females, eggs, and juveniles survive in plant roots. Eggs and juveniles are released into the soil when plants decompose. Nematodes are active when soil is moist and warm. Plants affected by root knot nematodes (Meloidogyne spp.) show symptoms of stunting, wilting, and yellowing. Closer inspection of roots reveals swollen, distorted areas, called galls or knots.

HOSTS

Many kinds of vegetables and flowers, woody shrubs, and weeds.

MINIMAL-IMPACT INTERVENTION

An integrated approach is best for root knot nematodes.

• Practice rotations that include a summer fallow and/or winter cover crops of grains, such as wheat. Winter grains planted when soil temperatures are below 65°F (18°C) help to decrease nematode populations.

• In hot weather heat the soil with solarization techniques (covering the soil with clear plastic

for 3 to 5 weeks), then uncover the soil and leave it to dry.

• Apply parasitic nematodes. (See the “Parasitic Nematodes” section on page 125.)

• Several bacterial genera, namely Pasteuria, Pseudomonas, Burkholderia, Arthrobacter, Serratia, Achromobacter, and Rhizobium, are known to suppress nematodes. Application of Bacillus cereus strain BCM2 in tomato decreased nematode populations by 60 percent and reduced nematode damage.12 A study testing a commercial biocontrol product containing Bacillus subtilis, Bacillus licheniformis, and Trichoderma longibrachiatum also inhibited nematode reproduction on tomato.13

• Trichoderma longibrachiatum decreased nematodes by 88 percent during in vitro experiments and reduced nematodes in cucumber in a greenhouse study.14 Trichoderma strains have also been proven effective both as plant growth promoters and to suppress nematodes in pepper.15

• Cover crops of French marigolds can reduce the number of root knot nematodes. Marigolds release a chemical that is highly toxic to root knot nematodes and prevents egg hatching. Also, root knot nematodes do not seem to be able to develop properly in marigold roots. Other cover crops increase the diversity of microorganisms in the soil and encourage the growth of certain bacteria and fungi that feed on root knot nematodes and parasitize their eggs.

• Several varieties of nematode-resistant tomatoes are available, and some resistant varieties of peppers, peas, and beans are on the market.

Rust Diseases

Though different rust diseases infect different crops, most can be identified by the minute, circular to elongate, golden or reddish brown pustules that form on the upper and/or lower leaf surfaces (as seen in figure 11.8). White rust (Albugo occidentalis) on spinach shows up as small yellow spots on upper leaf surfaces

and white pustules on lower leaf surfaces. Asparagus rust (Puccinia asparagi) and corn rust (P. sorghi) disease are favored by temperatures near 80°F (27°C) with high humidity and frequent dews. Optimal conditions for infection by P. porri, which causes rust in garlic, onion, and leek, occur around 59°F (15°C) with 100 percent relative humidity for at least 4 hours. Spinach white rust is favored by warm (72°F [22°C]), sunny days followed by cool nights with dew.

HOSTS

Asparagus, corn, onion, garlic, chives, spring onion, leek, spinach.

MINIMAL-IMPACT INTERVENTION

Plant resistant varieties. Rotate away from susceptible crops for 2 to 3 years. Plant in well-drained soils. Till under infected plant residues or remove and destroy infected plants in smaller areas. Organic products containing the beneficial bacterium

Bacillus amyloliquefaciens strain D747 have been used on rust disease with variable results.

Sclerotinia Disease: White Rot, Lettuce Drop

Initial symptoms of sclerotinia disease or white mold (Sclerotinia sclerotiorum) are small, circular, light green spots that appear water-soaked. Affected plant parts dry, turn brown, and become covered with a white, cottony fungal growth, hence the common name. Infected fruits and foliage rapidly disintegrate with a watery rot, giving rise to the common name of lettuce drop.

The disease is favored by moist conditions and temperatures of 68 to 77°F (20–25°C). Sclerotinia overwinters in the soil as resting structures that can persist there for 5 to 8 years. The microorganism infects susceptible crops in the spring. Then the cycle continues as the resting structures, called sclerotia, fall from infected crops onto the soil surface, and become incorporated by soil cultivation and tillage. The sclerotia can germinate after 20 to 100 days, depending on soil moisture and temperature. In the soil, sclerotia can be degraded by

Figure 11.8. Rust disease on garlic.

Figure 11.9. Sclerotinia disease is just starting to infect the bottom leaves of this lettuce plant that are touching the soil.

Sclerotinia Disease: White Rot, Lettuce Drop

bacterial decomposition and fungal parasitism and also eaten by mites, collembolans, and earthworms.16

HOSTS

A wide range of plants, including lettuce (lettuce drop), beans (white mold), and tomato (white mold). Other crops are susceptible to one of several Sclerotinia species, including Brussels sprouts, cauliflower, cabbage, carrot, collards, eggplant, pepper, potato, squash, and melon.

MINIMAL-IMPACT INTERVENTION

Standard crop rotation practice is not effective because of the wide host range of this disease, but a very long rotation of up to 8 years with non-hosts, such as grasses and grains, may be effective. Avoid excessive irrigation and irrigate in mid- to late morning to allow plants to dry as quickly as possible. Mulch or living mulch can decrease this disease by preventing soil from splashing up on plants. Sclerotinia’s ability to persist in the soil for extended periods makes deep plowing ineffective. However, some brassica cover crops are reported to suppress this disease. Indian mustard (Brassica juncea) reduced potato stem rot caused by Sclerotinia sclerotiorum by more than 50 percent in one study.17 Field mustard (B. campestris) and rapeseed (B. napus) also reduced Sclerotinia in the same study. Creating a soil organic matter system with an active microbial community and undisturbed habitat for earthworms is the best preventive against Sclerotinia.

SUPPRESSIVE SOIL MICROORGANISMS

Tests at Cornell University report suppression of Sclerotinia on lettuce using an organic product containing the beneficial bacteria Coniothyrium minitans applied once as soil spray. These beneficial bacteria can also be incorporated into the soil in the fall to help protect crops the next season. Tests at Cornell report suppression of white mold

on bean crops using an organic product containing the beneficial bacteria Bacillus amyloliquefaciens strain D747.

Septoria Leaf Spot

Septoria leaf spot (Septoria spp.) appears initially on lower leaves after the first tomato fruit forms. The small circular leaf spots have dark brown borders and tan to gray centers. This disease is favored by wet weather and temperatures of 72 to 79°F (22–26°C). Irrigation water, rain, high humidity, and dew on leaves lead to rapid disease development. Septoria may overwinter on solanaceous weeds such as groundcherry. Septoria leaf spot can be confused with early blight (see the “Alternaria Diseases” entry on page 243). Early blight causes fewer, larger circular spots with concentric rings surrounded by a yellow halo.

HOSTS

Tomato, eggplant, potato.

MINIMAL-IMPACT INTERVENTION

Use resistant cultivars. Iron Lady tomato has some tolerance to Septoria. Keep water off leaves. Avoid irrigating in late afternoon or evening. Stake plants and space plants well to reduce the amount of time leaves remain wet.

SUPPRESSIVE SOIL MICROORGANISMS

Tests at Cornell University report moderate suppression of Septoria on tomato using organic products containing the beneficial bacteria Streptomyces lydicus and Bacillus subtilis GB03. Another organic product containing extract of giant knotweed (Reynoutria sachalinensis) was also moderately effective.

HEAVIEST-IMPACT INTERVENTION

Copper sprays have been recommended for septoria leaf spot.

Smut

The corn smut fungus (Ustilago maydis) causes swelling in aboveground plant tissues (ears of corn). Plant cells become spongy gray, then black as the spores mature. Galls can be up to 4 inches (10 cm) in diameter. Smut overwinters in the soil in corn residue and is seen after damage to the plant, such as hail damage.

HOSTS

Corn. Pasture grasses are susceptible to a similar smut fungus.

MINIMAL-IMPACT INTERVENTION

There is no control for this disease after infection. Remove and destroy all infected plants. Do not compost these plants, unless your composting practice creates high enough temperatures to kill this disease. Soil applications of raw manure favor infection. Plants grown with high nitrogen levels or with high rates of manure are more susceptible to the disease. Use resistant corn varieties.

Verticillium Wilt

Verticillium wilt (Verticillium spp.) symptoms vary depending upon the host crop, but foliar symptoms typically include wilting, curling, yellowing, marginal or interveinal browning, and death. Overall, these symptoms may resemble water stress and can occur on only one side of a plant. Inside stems, you may see discolored streaks or bands that range in color from light tan to grayish olive to brownish black. Yellowing and defoliation usually progress upward. Verticillium survives in plant debris and in the roots and trunks of killed trees (for as long as several years). As a resting structure (microsclerotia), the fungus can persist in the soil for 10 years or longer. Water-stressed or wounded plants are most susceptible. Wet, warm (65 to 72°F [18–22°C]) weather encourages this fungus.

HOSTS

Solanaceous crops (eggplant, pepper, potato, tomato) are especially susceptible. Several other vegetable crops can be hosts, including cantaloupe, pumpkin, watermelon, mint, spinach, and strawberry. Some cover crops can be hosts, including common vetch. Weed species such as dandelion, groundsel, lambsquarters, nightshade, pigweed, sagebrush, and shepherd’s purse are also hosts to Verticillium species.

MINIMAL-IMPACT INTERVENTION

There is no cure for verticillium wilt. Use resistant cultivars. Resistant varieties are available for alfalfa, mint, potato, strawberry, sunflower, tomato, and other crops. Soak seeds in hot water or a 0.5 percent bleach solution before planting. Do not plant out transplants until soil temperatures are 65 to 70°F (18–21°C). Rotate crops on a 4- to 5-year basis with non-susceptible plants such as sweet corn, spinach, beans, peas, grasses, asparagus, carrot, and sweet potato. Rotation and incorporation of cover crops or other organic amendments prior to planting has been shown to reduce verticillium wilt in some crops. Several cover crops or crops, including broccoli, Sudan grass, and various mustards, may suppress verticillium wilt. High-nitrogen fertilizers can increase wilt severity. Remove and destroy infected plants. Soil solarization for 4 to 6 weeks in midsummer can reduce but does not completely eliminate Verticillium inoculum.

White Rot

White rot (Stromatinia cepivora) is favored by cool, moist soil conditions. The soil temperature range for infection is 50 to 75°F (10–24°C), with an optimum of 60 to 65°F (16–18°C). As soil temperatures rise above 78°F (26°C), the disease is naturally suppressed. Unfortunately, the temperature and soil moisture conditions that are good for onion and garlic growth

also favor white rot development. Leaves of infected plants turn yellow, wilt, and die back. Older leaves and bulbs collapse with a watery decay.

HOSTS

Onion, garlic, leek.

MINIMAL-IMPACT INTERVENTION

Long rotations may help, but since white rot has long-lived resting structures (sclerotia), once it is present in a field, it is very difficult to grow Allium species there successfully. Some work is being done with sclerotial germination stimulants, which are stimulants to get the disease’s resting structures to germinate rapidly rather than over a long period of time. In one study, onion oil, garlic oil, and Allium crop waste were the most effective treatments. Within 6 months of treatment, more than 70 percent of the sclerotia germinated and then died in the treated plots. But onions and garlic planted in those plots the following year still became infected with white rot.

Physiological Disorders and Nutrient Deficiencies

Sometimes symptoms such as leaf and fruit spots and discoloring are caused not by disease microorganisms, but instead by a plant’s physiological

response to environmental stress (like too much heat or cold) and to nutrient deficiencies. Refer to “Deciding When a Nutrient Intervention Is Needed” on page 82 for descriptions of crop nutrient deficiency signs in detail and in a larger ecosystem context. Here I explain how to fix the most common nutrient deficiencies and physiological disorders.

BLOSSOM-END ROT

Scientists used to think that calcium deficiency directly caused the physiological disorder known as blossom-end rot. Calcium can play a part in causing blossom-end rot, but the situation is a bit more complicated than that. A review of the recent literature concluded that calcium deficiency is not the cause but rather a result of blossom-end rot in tomato and pepper fruit.18 Blossom-end rot is actually a physiological disorder aggravated by several interacting conditions of environmental stress, including drought, high light intensity, heat, ammonium-nitrogen nutrition, excessive nitrogen fertilization, uneven soil moisture, cold stress in the spring, and poor root development caused by an inactive soil organic matter system. Often blossomend rot occurs even when soil calcium levels are high. If that is the case in your garden or farm soil, adding calcium won’t prevent symptoms. The way to manage blossom-end rot is to create an effective soil organic matter system and avoid tomato and pepper crop environmental stress. Blossom-end rot has virtually disappeared on my farm the past 7 years since I began to grow my own fertilizers that are high in carbon. If soil calcium levels are actually deficient, however, calcium addition can help to alleviate blossom-end rot. For example, research indicates that spray applications of calcium chloride solution on a weekly basis reduced blossom-end rot symptoms by 50 percent.19 Note that calcium chloride is not considered an acceptable fertilizer amendment for use by certified organic farms, with the caveat that “calcium chloride, from brine process is natural and prohibited for use except as a

Figure 11.10. Blossom-end rot on tomato.

Physiological Disorders and Nutrient Deficiencies

foliar spray to treat a physiological disorder associated with calcium uptake.”20 Unless your fields or garden beds suffer from severe soil calcium deficiency, it does not make much sense to spray calcium chloride on tomato or pepper foliage (see “Calcium Deficiency” below).

BORON DEFICIENCY

Some vegetables, such as brassicas, are heavy boron feeders. You can correct a deficiency with the careful addition of borax to the soil in liquid form (1 tablespoon per 100 square feet [9.3 m2] in 1 gallon [4 L] water) or 1 tablespoon Solubor in 1 gallon water. Or add boron more slowly by adding kelp (10 pounds per 100 square feet [49 kg/100 m2]) or wood ashes (note that wood ashes also increase soil pH and soil potassium, sometimes excessively). Foliar seaweed sprays as soon as transplants are set, and repeated several times until head or fruit formation, may help to prevent deficiency problems. While boron is essential for root growth and fruit development, it can become toxic if overapplied. Always test the soil and apply only the recommended amount.

CALCIUM DEFICIENCY

Add gypsum if your pH is above 6.2, or calcitic lime if your soil pH is below 6.2 and a soil test shows that soil calcium is low.

IRON DEFICIENCY

In high-pH soils, iron forms chemical compounds that are unavailable for uptake by most plants. High pH-induced iron deficiency is common in high-pH western US soils. You can lower soil pH by adding sulfur. For an emergency intervention, apply a root drench or spray leaves with iron chelate. Iron that has been treated with lignin derived from woody plants is usually allowed by organic certification, but check with your certifier first. Iron that has been chelated with synthetic materials, such as EDTA, is not allowed. Apply 1 tablespoon of

chelated iron in 1 gallon (4 L) of water as a soil drench or as a spray on new foliage, in the evening when humidity levels are high.

NITROGEN DEFICIENCY

Nitrogen is the most commonly found nutrient deficiency in vegetable crops. Be careful not to overdo nitrogen, though, because too much nitrogen causes rapid leaf tissue growth with little or no root growth as well as low flower and fruit production. Apply liquid nitrogen sources as a root drench, such as 1/4 pound (113 g) of alfalfa meal plus

Figure 11.11. Nitrogen deficiency causing yellow, pale leaves on newly transplanted heirloom tomatoes.

Physiological Disorders and Nutrient Deficiencies

1/4 pound of soybean meal mixed in 20 gallons (76 L) of water. Let sit at 65 to 70°F (18–21°C) for 12 to 36 hours before application. Covering transplants with row cover also helps to warm the soil and allows the plants better uptake of slowly available organic residue sources of nitrogen (see “Helen’s Magic Mix Liquid Fertilizer Intervention” on page 87).

PHOSPHORUS DEFICIENCY

Cool spring soils and high or low soil pH can make phosphorus unavailable to plant roots. Sometimes this does not affect yield if it occurs in older plants. But leaf purpling of young plants can decrease yields. To improve phosphorus availability, warm soil with plastic mulch before planting and use row cover after planting. Phosphorus can be added in liquid form as a root drench, such as 2 cups (0.5 L) of rock phosphate mixed in 10 gallons (38 L) of water. Let the mixture sit at 65 to 70°F (18–21°C) for 12 to 36 hours before application.

(See “Helen’s Magic Mix Liquid Fertilizer Intervention” on page 87.)

SUNSCALD

Sunscald is a physiological disorder caused by sudden exposure of fruits to intense direct sunlight. Rapid growth also encourages sunscald. On hot, sunny days, fruit exposed to the sun becomes extremely hot, in contrast with fruit protected by a dense canopy of foliage. Optimal crop fertilizer and irrigation help develop a thick foliage canopy that protects fruit from direct sunlight. On a small scale, shade cloths that provide 35 percent shade reduce sunscald significantly, especially when plants are in the midst of a quick growth spurt and it suddenly turns very sunny and hot. In a study at the University of Georgia, bell pepper fruit levels of nitrogen, phosphorus, and potassium increased with increasing shade level. And, phytophthora blight in plants and fruit sunscald decreased with shade. However, fruit soluble solids and fruit weight decreased with increasing shade level, too. Shading bell peppers reduces light intensity, both air and soil temperatures, and heat stress in the plants. Balanced nutrition and avoidance of excessive nitrogen also seem to reduce sunscald.

Figure 11.12. Minor phosphorus deficiency shows up as purpling in older leaves of mature fall-planted broccoli.

Figure 11.13. Sunscald on pepper fruits.