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Diseases of Leafy Crucifer Vegetables (Collards, Kale, Mustard, Turnips)

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Diseases of Leafy Crucifer Vegetables (Collards, Kale, Mustard, Turnips) Oklahoma Cooperative Extension Service EPP-7666 Diseases of Leafy Crucifer Vegetables (collards, kale, mustard, turnips) John Damicone Extension Plant Pathologist Oklahoma Cooperative Extension Fact Sheets are also available on our website at: Vegetable crops in the crucifer family, grown for their ed- http://osufact.okstate.edu ible leaves include collards, kale, mustard, turnips, and turnip x mustard hybrids. These cool-season crops are well adapted cabbage, cauliflower, broccoli, radish, etc.) are avoided for spring and fall production in Oklahoma. While most of the for at least two years. Rotations with corn, grain sorghum, production is for processing, both processing and fresh markets or another summer grass crops particularly are beneficial demand high-quality produce free of blemishes. Diseases are for reducing levels of root-knot nematodes. important factors limiting the production of leafy greens. Dis- Site selection and preparation - Selecting well-drained soils eases mainly cause damage by reducing crop quality. Severe and forming raised beds helps avoid damping-off, root disease development can reduce quality to the point where rot, and wilt diseases promoted by water-logged soils. the crop is unmarketable. Preparing a good seed bed promotes rapid seedbed Agents (pathogens) that cause the most common dis- germination and seedling growth. Acid soils should be eases of leafy greens are molds (fungi) and bacteria, but avoided or corrected with lime. Maintaining records on diseases caused by viruses and nematodes also can be the disease history of fields helps avoid disease problems a problem. This Fact Sheet is intended to aid growers in and the timely use of control measures. identifying these diseases and to provide general guidelines Seed health - Seed contamination, particularly for bacterial for managing them. Accurate identification of a disease is diseases such as black rot, is an important means of critical to an effective management program. For example, introducing diseases into non-infested fields. Purchasing use of a fungicide to control a leaf spot disease will not be high-quality seed from reputable sources helps ensure effective if the pathogen is a bacteria. Some diseases can that seed is pathogen-free. Many seed companies pro- routinely be identified in the field with a little experience. For duce seed in dry climates where disease pressure is low diseases that are difficult to identify in the field, consider sub- and implement quality control programs to minimize the mitting samples through local Extension offices to the OSU chances for seed contamination. Some seed companies Plant Disease Diagnostic Laboratory. Consult Fact Sheet directly test crucifer seed for the presence of the black EPP-7612, Plant Disease and Insect Diagnostic Services, rot bacterium. Seed lots passing the test are certified for recommended procedures in taking and submitting good black-rot free. Questionable seed lots should soaked in samples. Because registrations of pesticides for disease man- hot water for 20 minutes at exactly 122°F. agement change from year to year, consult the latest edition Sanitation - Foliar diseases can be spread by equipment of the Extension Agent’s Handbook for Insect, Plant Disease, and workers moving in and between fields. Soil cling- and Weed Control (Extension Circular E-832). ing to equipment and boots is a means of introducing pathogens such as nematodes into clean fields. Equip- Strategies for Disease Management ment and boots should be cleaned when moving from Diseases are best managed by minimizing pathogen infested fields to clean fields. Clean fields also can be populations and/or slowing the increase of disease during the worked before entering infested fields. Fields should not growing season. Integrated pest management (IPM) refers be worked when leaves are wet to limit disease spread. to the use of several different management strategies within Irrigation - In sprinkler irrigation, excessive irrigation, or a crop management system and the judicious use of pesti- frequent irrigations with small amounts of water favors cides where needed. Better, more economical pest control is spread and development of many diseases and should achieved when IPM is practiced, compared to reliance on a be avoided. Water should be applied during periods when single management strategy such as pesticide application. foliage dries rapidly. Disease management strategies that are effective components Weed control - Weeds in and around fields may harbor viruses of IPM programs are listed below. Many of these strategies and bacteria. For example, common weeds in the cruci- help prevent diseases from becoming established. fer family such as swinecress and Virginia pepperweed Crop rotation - Fungi, bacteria, and nematodes that cause are known to harbor black rot bacteria. Practicing good root and foliar diseases persist in soil or crop debris in weed control minimizes chances of disease carryover the soil. These pathogens build to damaging levels with and spread. repeated cropping of crucifer crops. Crop rotation is ef- Scouting - Crops should regularly be inspected (at least fective where crops in the crucifer family (also includes once a week) for symptoms and signs of diseases and Division of Agricultural Sciences and Natural Resources • Oklahoma State University other pests. Scouting permits the early detection of Root-knot (Meloidogyne spp.) pest problems so timely management practices can be Root-knot is caused by several species of nematodes, implemented. which are microscopic roundworms that live in soil and feed on Chemical control - Fungicide sprays may be needed for plant roots. Crucifers are susceptible to all root-knot species. In effective management of diseases caused by fungi. Cop- Oklahoma, both northern and southern root-knot nematodes per compounds are used to reduce diseases caused by are present and both cause damage to vegetable crops in bacteria as well as fungi. Fungicides and copper sprays isolated fields. Where nematode populations are high, yield help prevent healthy plants from becoming infected, loss results from poor plant growth. Nematodes are most but cannot cure plants already diseased. Therefore, severe on sandy, well-drained soils. Fortunately, nematodes applications should be made before symptoms appear are most active during the summer when these crops are not where disease is anticipated or shortly after the disease grown. is first detected. Because fungicides degrade over time, Symptoms - Affected plants are pale green to yellow, repeated applications on a seven to 14-day schedule stunted, and may occur in patches along rows or in circular typically are required to maintain protection. Sprays areas. Roots of affected plants show the characteristic galls should be applied before anticipated rains rather than or swellings caused by nematode feeding. The southern after, since protection is afforded during wet periods that root-knot nematode causes large galls that may be up to a favor infection. Sprays should be applied in a sufficient half-inch thick, while galls caused by the northern root-knot volume to achieve thorough plant coverage. The number nematode are finer in appearance. of fungicides registered for use on leafy greens is limited; Control - Crop rotation with grass crops for at least two therefore it is critical to use IPM strategies along with years effectively reduces nematode populations below dam- chemical control. aging levels. Rotations with other broadleaf vegetable crops Seed and soil-applied fungicides also are used to should be avoided, since most are susceptible to root-knot. Soil manage soilborne diseases. Seed treatments are rec- should be tested for nematodes prior to planting in fields with ommended to help prevent damping-off of seedlings. a history of nematode problems to ensure that the populations Soil-applied fungicide is pre-plant incorporated, applied are below damaging levels. See OSU Extension Fact Sheet to the seed furrow, or banded over the row and watered EPP-7610 for more information on sampling procedures for in. Soil-applied fungicide provides additional damping-off nematodes. control, and may help reduce root-rot diseases. Residue management - Since many pathogens survive in and on crop debris, crop residue should be incorporated Bacterial Leaf Diseases by disking or plowing soon after harvest to promote rapid Bacterial diseases have been important foliar diseases decomposition. of leafy greens in Oklahoma. Bacterial diseases are serious, since they spread rapidly and cause direct damage to the Soilborne Diseases marketed portion of the crop. In addition, chemical controls are not very effective. While the causes and symptoms of each disease are different, management programs are similar. Damping-off (Fusarium, Rhizoctonia, and Pythium spp.) Black rot (Xanthomonas campestris Damping-off is a term used to describe the different phases pv. campestris) of seed and seedling disease. Several fungi that commonly Black rot is a widespread and destructive disease of inhabit agricultural soils may contribute to damping-off prob- crucifer crops in Oklahoma. Entire fields can be destroyed lems within a single field. Damage results from reduced plant when conditions are favorable for disease development. The stand, which in severe cases results in reduced yield or the disease is caused by a bacterium that survives on seed, weedy added expense of replanting. Damping-off is favored by poor crucifers, and crop debris in infested fields. Extended
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