Diseases of cole crops
Figure 1. Note the dark ring in the conductive tissue of the stem. This symptom is commonly associated with bacterial black rot disease of cole crops.
This is a very serious disease on cole crops, especially cabbage, cauliflower, broccoli, turnip, and brussels sprouts. Other hosts include kohlrabi, collard, rutabaga, and radish. The disease is caused by the bacterium Xanthomonas campestris pv. campestris.
Symptoms. Plants may be affected at any stage of growth. The bacterium usually enters through the water pores around the leaf margins. Once the bacterium has entered the leaf, the tissue turns yellow. Infection progresses toward the center of the leaf, forming a V-shaped pattern. The veins within the yellow-colored tissue turn black. The bacterium moves down the leaf and is distributed throughout the entire plant. The infected leaves turn yellow and eventually drop off; some plants may be almost completely defoliated. The affected stem, when cut crosswise, reveals a black ring where infection has followed the water-conducting tissue (Fig. 1). The bacterium is carried on and in seed. When infected seed is planted, the bacteria pass from seed parts into the leaves of the small seedling and symptoms develop as previously described.
Disease cycle. The pathogen overwinters in and on the seed and in the plant debris left in the field. The bacterium may become established in a field by planting infected seed, planting infected transplants, or planting in fields where the disease was a problem the previous year. The bacteria are spread by splashing or running water and insects.
Control. Control practices include (1) planting western-grown, disease-free seed, (2) using a hot water seed treatment, (3) applying a fungicide treatment, (4) maintaining good sanitation of planting beds, (5) inspecting plants and handling plants carefully, (6) using crop rotation, and (7) planting resistant cultivars when available.
Plant only western-grown seed. Seed grown in the western United States is usually free of the pathogen, while seed grown in Europe and the eastern United States usually has a low percentage of infected seeds.
Hot water seed treatment. The grower should request hot water-treated seed.
Fungicide treatment. Copper products may help slow the progress of the disease. Refer to Midwest Vegetable Production Guide for Commercial Growers, BU-7094-S.
Plant bed sanitation. Take care to avoid planting beds that have been used for cole crop production. Wait at least three years. Drainage water from old compost heaps and old cabbage fields can contaminate the soil. If disease-free soil is not available, soil fumigation may be an option.
Plant inspection and handling of plants. Inspect the plants thoroughly and look for disease symptoms described earlier. Handle plants carefully to avoid injury.
Crop rotation. A three-year crop rotation with unrelated crops is required since the bacterium can overwinter in the soil for two years.
Figure 2. Note enlarged, distorted roots associated with club root disease.
Seed rot and damping off
The pre-emergence rotting of seeds or the post-emergence damping-off (collapse) of seedlings are diseases caused by soil-borne fungal organisms. As noted under black rot control, using a good sanitation program, crop rotation, and treated seed when available are important precautions against all seed and seedling diseases. Fungicide treatments may also be used to protect seedlings. A fungicide such as PCNB is effective against wirestem, Rhizoctonia sp. root rot. Incorporate this fungicide into the top 3- 6 inches of soil. A fungicide such as mefenoxam is effective against Pythium sp. damping-off. Always follow label directions.
This disease, caused by the fungus Phoma lingam, is a major concern in areas involved with cole crop production because the pathogen can infect a variety of cole crops.
Symptoms. Plants may become infected in the seedbed or in the field at anytime during the growing season. Usually the first symptom is a circular depressed canker that develops at the base of the stem, enlarges, and eventually surrounds the entire stem. Yellow spots with gray centers appear on the foliage. The infected tissue on both the stem and leaves is marked with small dots. These black dots are fungal structures and indicate the presence of the pathogen. Severely infected plants usually topple over as the pathogen destroys the supportive stem tissue.
Disease cycle. The fungus overwinters in the soil on old, infected plant debris for at least three seasons. When infected seed is planted, the fungus infects the seedling and produces spores which, in turn, are disseminated to other plants. Rain and surface drainage water spread the pathogen. The disease can spread very rapidly even though only a few plants may be infected initially.
Control. The methods needed to control black leg (fungus) are the same as those described for black rot (bacteria) control. These control measures include (1) planting western-grown, disease-free seed, (2) using a hot water seed treatment, (3) applying a fungicide treatment, (4) maintaining good sanitation of planting beds, (5) inspecting plants and handling plants carefully, (6) using crop rotation and (7) planting resistant cultivars.
This important cole crop disease is caused by the fungus Plasmodiophora brassicae.
Symptoms. Infected roots enlarge, become distorted, and resemble clubs, hence the name (Fig. 2). Often disease development on the roots of affected plants can be extensive before above-ground portions show any symptoms. Leaves on infected plants turn yellow, wilt, and die.
Disease cycle. The fungus gains entrance into the plant by attaching to the root hairs. As the fungus begins to develop in the roots, it produces spores. These spores are released and can be disseminated by water and infested soil. Acid soils and cool wet weather favor pathogen development. The disease is not seed-borne.
Control. Location of the seedbed is very important in club root control. To avoid local and wide-spread distribution of the pathogen, plant in disease-free soil. Hydrated lime incorporated into the soil to raise the pH to 7.2 reduces club root; however, on muck soils the application of lime is of little value because of the high soil buffering capacity. Club root can be reduced by using PCNB fungicides per label recommendations. These control programs should be coupled with a long crop rotation.
This disease is caused by the fungus Alternaria brassicae. It is widespread and affects all cole crops.
Symptoms. On seedlings, symptoms appear as small dark spots on the stems which eventually cause the seedling to topple over. Symptoms on the foliage also appear as small dark spots which enlarge rapidly. Within these infected areas, large masses of dark spores are produced. The pathogen may attack the heads of cauliflower and broccoli rendering them unmarketable. The disease is also a problem in storage of cole crops.
Disease cycle. The pathogen can overwinter on old, infected plant debris and in seed. The fungus produces masses of spores which are easily disseminated by wind, rain, or equipment. Warm, moist weather conditions favor disease development.
Control. Since the pathogen is carried in the seed, hot water seed treatment is an effective control measure. Application of fungicides such as maneb, chlorothalonil, or copper products will help control this disease (see Midwest Vegetable Production Guide for Commercial Growers, BU-7094-S). Begin fungicide application at the first sign of disease and repeat every seven to ten days, according to label directions.
This disease, which affects all cole crops, is caused by the fungus Peronospora parasitica. Plants may become infected during any stage of growth, but usually the disease is more of a problem on early-seeded plant beds or on late-maturing crops.
Symptoms. Infection usually takes place on the underside of lower leaves where the fungus produces a white fluffy growth. As the disease develops, yellow spots develop on the upper leaf surface which later turn tan in color.
When the pathogen attacks cabbage heads, symptoms appear as small, dark sunken spots. Similar symptoms can be observed on curds of cauliflower and broccoli.
Disease cycle. The fungus overwinters on old crop debris where it produces masses of spores which can be disseminated by wind and rain. Cool, moist weather conditions favor disease development.
Control. Fungicides such as maneb, chlorothalonil, aluminum-tris, mefenoxam, or copper products (see Midwest Vegetable Production Guide for Commercial Growers.) provide good control of downy mildew. Begin application at the first sign of disease and repeat according to label directions. Plant resistant varieties when possible.
Read the pesticide label and follow the instructions as a final authority on pesticide use.