Cabbage, Broccoli & Other Cole Crop Diseases

Black rot (Xanthomonas campestris pathovar campestris) symptoms are yellow, V-shaped lesions that appear along the tips of the leaves with the point of the V directed toward a vein.
Mary Katherine Bamberg, ©2018, Clemson Extension

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.

Pythium Damping-Off

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. Use new potting soil and new or thoroughly cleaned and disinfested containers and trays. Wash used containers with soapy water to remove all traces of old soil mix, and then briefly submerse containers in a 10% bleach solution. Allow to dry before planting in the containers.

If the disease has been severe enough in the past to warrant control, the biocontrol fungicide, Bacillus subtilis QST 713, can be used before or at the time of seeding. It can be used to treat seedlings in trays or young transplants in the garden. See Table 2 for examples of products containing this biofungicide. Apply all products according to directions on the label.

Downy Mildew

Downy mildew symptoms on a collard leaf.
Justin Ballew, ©2020, Clemson Extension

This disease is caused by the fungus Hyloperonospora brassicae 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 the disease becomes severe enough to warrant chemical control, chlorothalonil or mancozeb will give good control, and copper fungicides 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 90 °F. Wait 7 days after spraying before harvesting if using chlorothalonil or mancozeb. See Table 2 for examples of products containing these fungicides. Apply all chemicals according to directions on the label.

Alternaria Leaf Spot

Alternaria leaf spot on a collard leaf.
Justin Ballew, ©2020, Clemson Extension

This disease is caused by the fungus Alternaria brassicicola, 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). Blackened areas may develop on cauliflower curds and on broccoli heads. 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 for three years will also reduce disease. If the disease is severe enough to warrant chemical control, chlorothalonil will give good control, and copper fungicides will give fair to poor control. Spray every seven to 10 days after transplants are set. Wait 7 days after spraying before harvest if using chlorothalonil. See Table 2 for examples of products containing these fungicides. Apply all chemicals according to directions on the label.

Black Rot

Black rot is a bacterial disease that affects all vegetables in the crucifer family, including collards.
Mary Katherine Bamberg, ©2018, Clemson Extension

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 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 seeds and transplants. If the source of the seeds is unknown or infested seed lots 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.

Black Leg

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. Use a four-year crop rotation.

Wirestem

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. Collards, kale, cabbage, broccoli, and cauliflower are the most susceptible brassicas to wirestem.

Fusarium Yellows

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.

Viruses

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.

Table 1. Some Disease-Resistant Varieties for South Carolina.

Table 2. Examples of Fungicides Labeled for Use in Home Gardens for Cole Crop Disease Control.

Pesticides are updated annually. Last updates were done on 5/21 by Joey Williamson.

Originally published 09/99