Prepared by Nancy Doubrava, HGIC Horticulture Specialist, and James H. Blake, Extension Plant Pathologist, Clemson University. Revised by Joey Williamson, HGIC Horticulture Extension Agent.(New 09/99. Revised 05/04.)
Cole crops, — such as broccoli, Brussels sprouts, cabbage, and cauliflower — are easily grown in South Carolina. Tips for growing healthy cole crops in the garden are available in HGIC 1301, Broccoli, HGIC 1326, Cauliflower and HGIC 1303, Cabbage & Chinese Cabbage. Several disease problems commonly affect these plants, especially when resistant varieties are not planted. Some of the more common problems are covered here, including root and stem rots of young plants, black rot, downy mildew and viruses.
This disease commonly affects seeds and young transplants and is caused by the soil-borne fungus Pythium species. Infected seeds decay in the soil. Seedlings and young transplants will "damp-off" or rot at the soil line, before they eventually collapse and die.
Prevention & Treatment: Cultural controls include planting on raised beds and providing good drainage. Start seeds in commercial potting soil, not in garden soil. If disease has been severe enough in the past to warrant chemical control, metalaxyl can be used before or at the time of seeding. Use for outdoor seeding only, not for transplants. Apply all chemicals according to directions on the label.
This disease is caused by the fungus Peronospora parasitica and can attack both seedlings and mature vegetable plants. Infected plants develop a gray mold on the lower leaf surface. The upper leaf surface of infected plants first turns yellow and then may turn brown or necrotic. Leaves wither and die. Symptoms differ from powdery mildew in that the downy mildew fungus grows only on the lower surface of the leaf. Development of the disease is favored by moist conditions.
Prevention & Treatment: Use varieties with resistance or tolerance to this disease (Table 1). Rotate with crops other than cole crops or greens. Remove plant debris immediately after harvest. Use wide plant spacing to promote drying of leaves. Avoid wetting the leaves when watering. If disease becomes severe enough to warrant chemical control select one of the following fungicides: chlorothalonil will give good control; maneb or fixed copper will give fair control. Spray every seven to 10 days after transplants are set. Make sure that the lower leaf surface is covered with fungicide. Do not apply copper when temperatures are above 85 °F. Wait 7 days after spraying before harvest if using chlorothalonil or maneb. Apply all chemicals according to directions on the label.
This disease is caused by the fungus, Alternaria species, and occurs during warm, moist conditions. On seedlings, the symptoms are small dark spots on the stem that can cause damping-off or stunting of the plant. On older plants, the bottom leaves are infected first with brown circular spots on the leaves. Spots have characteristic concentric rings (target spots). Infected leaves soon turn yellow and drop. Bright sunshine, frequent dews or showers, and temperatures between 60 and 90 °F favor disease development.
Prevention & Treatment: Remove and destroy all crop debris immediately after harvest, since this disease overwinters on plant residue. It is easily spread by tools, wind, splashing water or insects. Seed treatment and rotation with crops other than cole crops or greens will also reduce disease. Select one of the following fungicides, if disease is severe enough to warrant chemical control: chlorothalonil will give good control; maneb or fixed copper will give fair control. Spray every seven to 10 days after transplants are set. Wait 7 days after spraying before harvest if using chlorothalonil or maneb. Apply all chemicals according to directions on the label.
Black rot is caused by a bacterium, Xanthomonas campestris pathovar campestris, and can affect all vegetables in the crucifer family. Above-ground parts of the plant are primarily affected, and symptoms may vary depending on the type of plant, age of the plant and the environmental conditions. In general, yellow, V-shaped lesions appear along the tips of the leaves with the point of the V directed toward a vein. When lesions enlarge, wilted tissue expands toward the base of the leaves. Veins turn black or brown. Infection may spread into the stems. Cutting into the stems often reveals a black-brown discoloration with a yellowish slime present. Symptoms on cauliflower may appear as numerous black or brown specks, black veins and discolored curds.
Prevention & Treatment: There are no chemical controls available, so disease prevention is very important. The bacteria survive the winter on plant debris and on weeds, such as wild mustard and Shepherd’s purse. It also can survive in and on seeds from infected plants. It can remain alive on plant residue buried in the soil for up to two years. The disease is easily spread by splashing water, wind, insects and garden tools. High temperatures and humidity favor development of the disease.
Use certified disease-free seed and transplants. If source of the seeds is unknown, or infested seedlots must be used, treat seed with hot water to eradicate pathogenic bacteria. Cabbage, broccoli, and Brussels sprouts can be treated at 122 °F for 25 minutes, while seeds of cauliflower, kale, turnip, and rutabaga are treated for 15 minutes. However, this treatment may reduce the viability of seed. Choose varieties tolerant to black rot (Table 1). Do not plant cole crops where black rot has occurred in the past two to three years. Select well-drained sites with good air circulation. Good sanitation practices in the garden are very important to minimize disease development and spread. Remove nearby weeds and any volunteer plants from previous seasons. Destroy all plant debris from the garden after harvest that may have been diseased. Not handling plants when they are wet will reduce the spread of disease-causing organisms.
The fungus, Phoma lingam, causes black leg. The symptoms of black leg are ash gray spots speckled with tiny black dots on the leaves and stem. Stems become girdled, and the plants wilt and die. Moist conditions favor development of this disease.
Prevention & Treatment: Same as described for black rot.
The fungus, Rhizoctonia solani, causes wirestem. Stems of plants become constricted and brittle at the soil line. The plant becomes stunted and may rot at the soil line. This disease is more severe on fall cole crops when the soil is warm.
Prevention & Treatment: Use certified disease-free transplants, preferably greenhouse grown
Yellows or wilt is a much less important disease today due to the development of varieties resistant to the disease. The disease is caused by the fungus, Fusarium oxysporum forma specialis conglutinans, and most members of the cabbage family are susceptible. The fungus usually enters the plant through young rootlets or wounds in older roots at transplanting time and then moves up the stem and throughout the plant. Symptoms include leaf yellowing, defoliation of older plants, stunting and death of seedlings. Stems are often twisted to one side. On susceptible plants, symptoms may not appear until the soil warms up, close to the time of crop maturity. It is easily confused with black rot, except discoloration inside of the stem appears more yellow-brown instead of black. Yellows are more likely to cause a curve in the midrib, resulting in a plant that is stunted to one side. Disease development is most severe between 80 to 85 °F.
Prevention & Treatment: Once the disease is present in the garden, the only successful control is to use yellows-resistant varieties (Table 1). Conventional methods of crop rotation, seed treatment, sanitation and fungicide applications do not control yellows. In gardens where the disease is not present, extreme care should be taken to exclude infected seedlings.
There are several common viruses that can affect cole crops including Turnip mosaic virus (TuMV) and Cauliflower mosaic virus (CaMV). Infected plants may be stunted and have distorted leaves. Black spots that look like pepper develop on the heads of plants.
Prevention & Treatment: There are no chemicals available to control viruses. Aphids spread the virus, so controlling the insects that spread the virus can help to minimize the disease. This control method is difficult because infection occurs immediately after an insect feeds, and insects migrate freely between plants. A good control strategy is to maintain healthy and vigorous plants, to plant recommended varieties and to monitor your garden for any unusual symptoms as they occur. Keep the area clear of crucifer weeds, such as wild mustard, that can harbor the virus. Always wash your hands after handling diseased plants.
|Broccoli||'Green Valiant'||Resistant to head rot. Performs well in spring and fall|
|'Everest'||Resistant to head rot and downy mildew.|
|'Green Comet'||Resistant to downy mildew & heat tolerant|
|'Premium Crop'||Resistant to downy mildew & heat tolerant|
|Resistant to Fusarium yellows|
'Solid Blue 760'
'Solid Blue 780'
|Resistant to Fusarium yellows & tolerant to black rot|
|Brussels Sprouts||'Jade Cross'||Resistant to Fusarium yellows|
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