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

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.

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.

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.

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