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

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