f\m Wm&Hi Extension Bulletin E-1668, January 1983, 55$ Disorders of Cole Crops

COOPERATIVE EXTENSION SERVICE • MICHIGAN STATE UNIVERSITY

By Christine T. Stephens, Department of Botany and Plant Pathology Bernard H. Zandstra, Department of Horticulture

1. Molybdenum deficiency—cauliflower 2. Magnesium deficiency—Chinese cabbage

1. Molybdenum deficiency occurs occa­ lbs actual boron per acre in broadcast fer sionally on cole crops. It is most common tilizer to avoid boron deficiency. 3. Boron deficiency—broccoli on soils with a pH below 6.0. Early stages look like nitrogen deficiency with yellow mottling between the veins of all leaves. As 4. Internal tipburn is a result of an inter­ the plants grow, the edges of young leaves nal calcium imbalance in cabbage. Leaves turn inward, turn yellow and then brown; inside the heads turn brown and dry up. soft rot often infects the affected leaf There may be a single dry leaf, or several at margins. If molybdenum deficiency is different locations in the head. Affected known to exist, treat seed with a sodium heads appear to be sound externally. molybdate slurry before planting. If the Although most Michigan soils contain problem appears in the field, apply 3 high calcium levels, tipburn still occurs, ounces of sodium molybdate per acre usually during hot, dry periods. Maintain­ weekly in foliar sprays until symptoms ing adequate soil moisture levels helps disappear from young leaves. reduce the incidence of tipburn, but does not eliminate it. Some newer varieties are reported to be somewhat resistant to 4. Internal tipburn—cabbage 2. Magnesium deficiency appears as inter- tipburn. veinal chlorosis in older leaves. Cauli­ flower and Chinese cabbage are especially susceptible. Magnesium deficiency occurs 5. Buttoning is premature heading in most often on soils with low pH and high cauliflower and broccoli. Small heads (1 to potassium levels. It can usually be avoided 4 inches in diameter) develop and foliar by applications of dolomitic limestone. If growth ceases. The heads may continue to the problem occurs during a cropping enlarge but are of poor quality. In season, apply 10 to 20 lbs magnesium cauliflower, there are insufficient leaves to sulfate (Epsom salts) in 100 gallons of blanch the curds. Buttoning can occur water per acre as a foliar spray. anytime from seeding until the plants are almost mature, but most often occurs 3. Boron deficiency causes hollow stem in within a month of transplanting in the cauliflower, broccoli, and cabbage. Light field. This problem is a result of stress on brown spots on the curd are evidence of young plants induced by low or high boron deficiency in cauliflower and broc­ temperatures, moisture stress, transplant coli. The brown spots can expand and shock, or nutrient deficiencies. It can be cover the entire head. As heads mature, avoided by growing varieties well adapted soft rot bacteria often infect the hollow to Michigan, and taking good care of the stem making the heads unsalable. Apply 3 plants in the greenhouse and field. 5. Buttoning—cauliflower 6a. Black rot—cabbage 9a. Blackleg—cabbage

7. Leafy curd—cauliflower

6. Black Rot (Xanthomonas campestris) symptoms first appear along the leaf margins 10-14 days after invasion by the 8. Thrip damage—cabbage bacteria. The bacteria enter the plant through water pores or wounds produced by insects or weather damage. The bacteria then move rapidly through the Michigan. Some varieties are better suited water conducting (vascular) system of the to warm season production and are less plant, eventually plugging it. Veins turn susceptible to leafy head. black as water flow is restricted and plant tissue around the point of invasion turns 8. Thrip damage is a major concern in yellow in a V-shaped lesion. If the weather mid-and late-season cabbage. Damage con­ is cool (below 70°F daytime high) external sists of small brown scars on inside leaves, 9b. Blackleg—cabbage symptoms may disappear although the caused by rasping and sucking of the small organism remains in the vascular system. (1/16 inch), tan insects. Damage is most Plants infected in the seedbed may show severe during hot, dry summers. Thrips the fibrous root system. The disease causes no symptoms except for premature enter cabbage early in head development wilting, stunting and death of many cotyledon or leaf drop. When temperatures and remain in the heads until harvest. affected plants. rise, lesions appear on leaf margins; the Once thrips are inside the heads, it is im­ Disease control is best achieved by use of plant wilts, loses leaves and dies. Heavily possible to control them. Damage is usual­ disease free seed and transplants, and a infected plants will stop growing. The ly visible only by peeling off a number of 4-year crop rotation. heads discolor internally and plant tissues outer leaves. Regular insecticide applica­ break down. tions, commencing at heading, should con­ 10. Downy Mildew (Peronospora parasit­ Control is best achieved by use of hot- trol thrips. ica), The initial symptom of this fungal water-treated seed which has tested disease is the appearance of small, ir­ negatively for the bacterium, 3-year crop 9. Blackleg (Phoma Jingam) symptoms can regularly shaped grayish-purple spots on rotations to non-cruciferous (mustard) appear early in the growing season on stems and the undersides of leaves. Under crops, use of resistant varieties, elimina­ seedlings not yet transplanted in the field. cool, moist conditions, the spots enlarge tion of cruciferous weed hosts from field Rather inconspicuous, small, circular, and become covered with fluffy, grayish- margins, and spraying with approved dark lesions appear on leaves of plants white mycelium. The upper leaf surface preventive spray materials. affected by the fungus. The spots gradually yellows and dries out. Heavily infected enlarge, becoming well defined with a gray leaves eventually drop off. The organism 7. Leafy head of broccoli and leafy curd center filled with numerous black, pimple­ may move systemically in the plant caus­ of cauliflower are a result of head forma­ like, spore-bearing structures. The lesions ing internal discoloration of broccoli, cab­ tion during hot weather. Small leaves on stems are oval in shape and are often bage and cauliflower heads. develop and protrude from the heads. surrounded by a purplish margin. Spots Avoid low-lying fields with heavy soils, Because of their sensitivity to hot weather, spread over the whole plant including the do not overwater, rotate to non-cruciferous broccoli is usually grown as a spring or fall root system. The dark cankers which form crops and apply appropriate fungicides to crop and cauliflower only as a fall crop in on affected roots may eventually destroy control downy mildew.

2 12. Yellows—cabbage

10a. Downy mildew—cabbage head 11a. Wirestem—cauliflower

13. Soft rot—Chinese cabbage

yellowish-brown. Plants either die prematurely or form imperfect heads. lib. Rhizoctonia head rot—cabbage Varietal resistance is available in both cabbage and radish and is a successful means of yellows control. port and the plant remains erect. At this stage, the disease is called wirestem. Stems are discolored and wiry above the soil line. 13. Soft Rot (Erwinia carotovora) bacteria The plant grows very slowly and usually cause a watery, soft, foul-smelling rot of does not develop to maturity. Bottom drop Chinese cabbage and other cruciferous occurs as a carryover from wirestem. hosts. Bacterial infection often occurs after 10b. Downy mildew—cauliflower leaf Lower leaves wilt, rot and darken, but re­ chemical, mechanical, insect or other in­ main on the plant. A head rot may develop, jury. In Chinese cabbage, the rot follows causing a darkening and decaying of the internal tip burn. The bacteria liquify the mainstem at the base of the heads and spot­ pectic substances in the cell wall, resulting ting and wilting of the leaves in the center in rapid softening of the invaded tissues of the head. and their collapse into a slimy mass. To avoid infection, grow Chinese cab­ Damping off and wirestem can be con­ bage on light, well-drained soils and main­ trolled in the greenhouse and field tain adequate soil moisture to avoid tip seedbeds by using sanitized soil and con­ burn. Select varieties suitable to the tainers and treating seed or soil with a Michigan climate and avoid injury to the fungicide. Avoid overwatering. Avoid plants. Rotate fields with nonhost crops field sites that are low lying with heavy such as corn, wheat or soybeans. 10c. Downy mildew—cauliflower head soils and do not use diseased transplants.

14. Sclerotinia Head Rot (ScJerotinia 11. Wirestem (Rhizoctonia solanij causes a 12. Yellows (Fusarium oxysporum /. con- scJerotiorum). Cabbage and other cruci­ number of closely related diseases of cole glutinansj primarily affects cabbage and ferous hosts can be attacked late in the crops, including damping off, wirestem, radish, although other cruciferous crops growing season by this soil-borne fungus. bottom drop and head rot. If the fungus are susceptible. Initally, the plants turn off- The fungus invades the main stem of the attacks very young seedlings, the disease is color (light green or yellow), with lower plant near the soil line, causing a soft called damping off. The fungus penetrates leaves affected first. Yellowing generally decay of the outer layer of the stem. An in­ seedlings near the soil line causing water- occurs on one side, although the entire fected plant may succumb rapidly or soaked constrictions of the stem. This plant may be uniformly affected. Curling become stunted and decline slowly. Lower results in wilting and toppling over of the and distortion of affected leaves is com­ leaf drop accompanies the decline in plant plants. If plants survive this initial attack, mon. In either case, leaves continue to vigor. A cottony, white mycelium is visible the center of the stem decays and collapses turn yellow. The water conducting on infected plant parts. Black fruiting while the stalk continues to provide sup- (vascular) systems of infected plants turn structures Vi to Vz inch in diameter, called

3 14. Sclerotinia head rot—cabbage 15b. Wilting caused by club root- 16b. Alternaria head rot—cauliflower Chinese cabbage sclerotia, are often found embedded in the using clean transplants, avoiding move­ On cauliflower, tiny brown sunken le­ mycelium growing on the outer leaves of ment of machinery from infested areas in­ sions appear on the curds. On broccoli, cabbage heads. to clean fields, dipping transplants in these lesions are yellow. The spots enlarge Avoid low lying fields with heavy soils approved fungicides and maintaining a rapidly and are eventually covered with and rotate crops to reduce disease high soil pH (7.3 or above). Crop rotation is the black spores of the fungus. incidence. not very effective because the resting To avoid the Alternaria diseases, use spores can survive in the soil for many high quality, disease-free seed; irrigate ear­ 15. Club Root (Plasmodiophora brassicaej years. ly in the day so that leaves dry rapidly; tie may be quite advanced before foliar symp­ cauliflower heads loosely and as late as toms appear because only roots become in­ 16. Alternaria Diseases (Alternaria spp.J, possible to reduce free moisture on the fected. Infected plants wilt in the middle of Alternaria fungi cause both leaf spotting head, and apply fungicides when hot, sunny days and leaves turn pale green and head rotting of crucifers. The initial necessary. to yellow. Eventually, infected plants wilt disease symptom is the appearance of Other MSU Extension Bulletins with in­ permanently and die or survive in a small dark spots on older leaves. The spots formation on cole crops: stunted condition. are generally circular, ranging from xh E-312 Control of insects, diseases and Clubroot zoospores enter roots through inch to IV2 inch in diameter. A brown- or nematodes on commercial vegetables, 75

Fertilizer recommendations for root system and blocks the water conduct­ face. The margins of tightly wrapped inner vegetables and field crops in Michigan, ing system. In addition, the clubbed roots leaves of cabbage may become colonized 40

Some of the photos in this publication were contributed by Paul Williams, Dept. of Plant Pathology, U of Wisconsin, and Sandra Perry, Plant Diagnostic Clinic, Dept. of Botany and Plant Pathology, MSU

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