Fungal Disease of Cruciferous Crops
Table of Contents
Disease management is important for producing acceptable yield and quality of cruciferous crops such as cabbage, cauliflower, canola, rutabaga, grown for the fresh market, the processor, and for storage. Diseases caused by viruses, bacteria, and fungi, as well as physiological disorders, are all found in cruciferous crops in Ontario. This Factsheet discusses the biology, recognition, and management of diseases of crucifers caused by fungi.
Damping-off disease affects crucifer seedlings grown in flats and seedbeds, or in the field. Damping-off is normally caused by soil fungi such as Pythium and Rhizoctonia. Seed and young seedlings are attacked and may rot before they emerge or topple over a few days afterwards. When older seedlings are attacked by Rhizoctonia, the lower stem becomes constricted and dark-brown near the soil surface, a symptom called wire-stem (Figure 1). Such plants may die when stressed, break over in strong winds, or produce a stunted, unmarketable crop.
Black-leg, caused by Phoma lingam (Leptosphaeria macutans), is of major concern in crucifer production. Sources of the fungus include infested seeds, cruciferous weeds, and residues of cruciferous crops remaining in or on the soil. The fungus often kills seedlings or produces sunken, black cankers at the stem base which stunt the growth of surviving plants (Figure 2). Yellow to brown, circular spots with grey centres appear on the leaves. The presence of tiny, black bodies (pycnidia) on these cankers or leaf spots is characteristic of black-leg (Figure 3). The black bodies contain millions of spores of the black-leg fungus which ooze out and spread during wet weather.
Control measures for black-leg are similar to those for bacterial black rot and include treatment of seeds with hot-water or fungicide, and avoiding work in an infected field when foliage is wet. However, rotation with non-cruciferous crops and complete control of susceptible cruciferous weed hosts for four years are recommended if black-leg has been severe. Because disease progresses rapidly in wet conditions, crucifers should be planted in well-drained fields which dry out quickly.
Club-root, caused by Plasmodiophora brassicae, is a destructive soil-borne disease which affects nearly all cultivated, as well as many wild and weed members of the cabbage family. The fungus enters root hairs and wounded roots, and multiplies rapidly, causing abnormal enlargement of the underground stem, taproot, or secondary roots (Figure 4 and Figure 5). These roots often decay before the crop has matured, releasing many resting spores which can survive for a decade in the absence of a susceptible host plant.
Because there are several races or strains of the club-root fungus, cruciferous crops bred for resistance often give inconsistent results when grown in different locations. Due to its long persistence (10 years or more) in soil, the fungus is not readily avoided by crop rotation. Growing crucifers in well-drained, warm soil, eradication of related weed hosts, fungicidal soil drenches, and lime application to maintain pH above 7.2 are, when combined, the most successful methods of control. The addition of limestone to bring soil pH over 7.2 is the best means of control. Agricultural limestone takes a minimum of one year to effectively change the pH. For faster activity a minimum of 1700 kg/ha hydrated lime can be added (regardless of soil pH) at least six weeks prior to field transplanting. Hydrated lime changes soil pH only temporarily. Agricultural limestone is effective for several years.
It is important, however, to use clubroot-free locations for outdoor seed beds. Do not add hydrated lime in seedbeds, as it may mask the presence of the fungus, and allow it to move with the transplants to the field, where subsequent infection may occur if soil pH is favorable.
When transplanting, discard all plants in a lot if clubroot is found on any seedling. Others may be infected and not yet show symptoms. If transplanted, they will infest that field with the fungus.
Symptoms of Fusarium yellows or wilt, a soil-borne fungus, caused by Fusarium oxysporum f.sp. conglutinans, resemble those of black rot. Affected plants are stunted, lopsided, yellowed, lose most of their lower leaves, and have a brown to black discoloration in the veins (Figure 6 and Figure 7). However, yellows may be distinguished from black rot because leaf dieback progresses from the petiole or midrib outwards, affected leaves are usually curved laterally, leaf margins may have reddish-purple discoloration, and pockets of dark discoloration are not associated with the vascular system when crucifers are infected with the yellows fungus. Fusarium yellows is a "hot weather" disease, and thus is rarely seen in early cole crops.
Figure 6. Plant and leaf of cabbage affected by Fusarium yellows. Note stunted, lopsided plant, lower leaves missing, brown vascular and yellow leaf discoloration, and curvature of petiole and midrib. (0.2x actual size)
Due to its persistence in soil without host plants, this fungus is very difficult to control by crop rotation and other methods. Monogenic dominant resistance has been incorporated into many varieties of cabbage and some radish and Brussels sprouts, but none is currently available in cauliflower and broccoli. This resistance is noted in many cultivars listed in OMAFRA Publication 363, Vegetable Production Recommendations.
This disease, also known as white mold or white rot, is caused by the fungus Scierotinia sclerotiorum. This fungus attacks not only crucifers but also a wide variety of other crop plants in field and storage. Plants are infected from seedling to maturity by wind-blown spores, or directly by fungal strands arising from hard, black fungal bodies called sclerotia. Water-soaked spots appear anywhere on the plant, usually on leaves nearest the ground, or on the head. Affected tissue often turns grey, giving rise in wet weather to fluffy white mold which eventually is dotted with black sclerotia in the field or storage (Figure 8 and Figure 9). Pale grey-bleached spots, stem-rot, stunting and premature death may occur in fields of canola, beginning during flowering, especially in dense plantings.
Downy mildew, caused by Peronospora parasitica, may be a serious foliar disease of all cruciferous crops. Susceptible hosts include canola, cabbage, broccoli, Brussels sprouts, kale, cauliflower, rutabaga, radish, horseradish, Chinese cabbage and mustards, ornamentals such as stock, wallflower, and aubretia, and many cruciferous weeds. However, there are several pathogenic varieties (physiologic races) of the fungus which attack different groups of, but not all the aforementioned, cruciferous hosts.
Overwintering mainly in crop debris, on cruciferous weeds, and occasionally on crop seed, downy mildew fungus becomes a problem in early spring, late summer, and fall during damp, cool weather. Beginning with lower leaves, small yellow-brown spots appear (Figure 10) which eventually expand and develop greyishblack lace-like markings (Figure 11). In moist weather, bluish-white downy mold is apparent on the underside of these leaf spots (Figure 12). Abundant sporulation and rapid disease development occur at greater than 98% relative humidity, when leaves are wet, and at 8-16°C. Downy mildew becomes severe in several days under these conditions especially when plants remain wet until mid-morning. In cauliflower and broccoli, symptoms may occur as pale brown or greyish discoloration on the curd (flower), or grey to black spots and streaks on the stems below the curd. Cabbage heads in storage also may be penetrated by greyish-black discoloration, as well as becoming susceptible to secondary rot pathogens.
In contrast to downy mildew, Alternaria leaf spots, caused by Alternaria brassicae and other related species, usually occur during warm, moist weather. Yellow-brown spots with target-like concentric rings appear on leaves (Figure 13), as well as dark brown sunken spots on heads of Brussels sprouts (Figure 14), broccoli, and cauliflower (Figure 15). These spots contain many spores which are spread by wind, rain, or on equipment and people. Spores require at least 9 hrs. of moisture to germinate and infect the plant. Older, senescing plant parts are more susceptible to infection.
A generalized disease control program for cruciferous crops is as follows:
A) For producing transplants or direct seeding:
B) In addition, transplant or direct seed into fields which have:
C) When buying transplants, insist on the following precautions in the contract:
By using the above management tools appropriately, disease problems in cruciferous crops can be minimized. For further information, consult the book, Diseases and Pests of Vegetable Crops in Canada, ISBN 0-9691627-3-1, the OMAFRA publication, Integrated Pest Management for Crucifers in Ontario, Order No. 701, or an OMAFRA Vegetable Specialist.
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