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