Introduction and significance
Disease caused by Botrytis usually has a minor impact on crucifers because infected, older leaves are removed during harvest. More severe problems can occur if crops are affected by other problems, such as chemical burn, nutrient deficiency, or weather damage, and are subsequently colonized by gray mold. Gray mold is an important problem in stored produce, particularly on white cabbage.
Symptoms and diagnostic features
Symptoms are brown blotches on leaves or stems that extend in size from 1–2 cm in diameter to very large sections of diseased tissue. Gray mold results in a soft, mushy rot in which tissues disintegrate if handled. Gray fungal growth is usually present in tufts near the center of foliar lesions or generally over cauliflower heads (170) or other organs. On stored cabbage (171) the outer wrapper leaves may bear extensive sporulation. Gray mold symptoms are almost always associated with senescence or physical damage to host tissues.
Causal agent
Gray mold is caused by the ascomycete fungus
Botryotinia fuckeliana. However, this stage is rarely
observed on crucifers and the disease is associated with the anamorph Botrytis cinerea. Conidiophores are long, can measure up to 1–2 mm, become gray-brown with maturity, and branch irregularly near the apex. Conidia are clustered at the branch tips and are single- celled, pale brown, ellipsoid to obovoid, and measure 6–18 x 4–11 μm. If formed, sclerotia are black, oval or irregular in shape, and measure 4–15 mm. On host tissue the fungus produces characteristically profuse mycelial growth that is initially fluffy, white or pale gray, later becoming darker gray- brown when it sporulates.
Disease cycle
Botrytis cinerea survives in and around fields as a sapro-
phyte on crop debris, as a pathogen on numerous crop and weed plants, and as sclerotia in the soil. Conidia develop from these sources and become windborne. When conidia land on senescent or damaged crucifer tissue, they will germinate within a few hours if free
moisture is available and rapidly colonize this food base. Once established in the senescent tissue, the pathogen will progress into adjacent healthy stems and leaves, resulting in disease symptoms and the produc- tion of additional conidia. Cool temperatures, free moisture, and high humidity favor disease develop- ment. Factors that predispose crucifer crops to gray mold are physical damage due to passing farm equipment, burn from fertilizers and pesticides, pest infestations (especially from boring insects such as root maggots), drought stress, frost damage, overmaturity of the crop, and tipburn due to nutritional imbalances.
Control
Prevent or minimize injury to the crop. Provide suffi- cient fertilizers for the crop, and harvest the crop before the commodity becomes overmature. For cabbage, handle and harvest the crop under dry conditions and before the occurrence of frost. Postharvest fungicide dips are used on cabbage in some countries.
BRASSICACEAE
F
UNGAL
D
ISEASES
170 Gray mold on cauliflower head.
170
171 Gray mold on stored white cabbage.
168
References
Davies, R. M. and Heale, J. B. 1985. Botrytis cinerea in stored cabbage: the use of germ tube growth on leaf discs as an indi- cation of potential head rot. Plant Pathology 34:408–414. Yoder, O. C. and Whalen, M. L. 1975. Factors affecting post-
harvest infection of stored cabbage tissue by Botrytis cinerea.
Canadian Journal of Botany 53:691–699.
Erysiphe cruciferarum
POWDERY MILDEW
Introduction and significance
Crucifer crops are frequently affected by powdery mildew, which occurs in all production areas. There are large differences in susceptibility between different species and cultivars. Severe epidemics commonly occur in swede and turnip that result in reduced yield and quality. More commonly, the foliage and stems of brassicas may exhibit disease, though these infections have limited effects on quality and yield.
Symptoms and diagnostic features
Powdery mildew is initiated from airborne conidia and appears as small, radiating, diffuse colonies of superfi- cial white mycelium that can measure from 5–15 mm in diameter. These ectophytic colonies are readily removed by rubbing the plant surface. Severe, advanced infec- tions produce a dense white powdery covering of leaves (172, 173), stems, and seed crop pods. Severe attacks
cause chlorosis, early defoliation, and necrosis of the tips of young leaves of cauliflower and cabbage. Powdery mildew colonies may be gray and restricted in size on resistant cultivars as the host reaction produces black speckling beneath the colony. Conspicuous gray or purple symptoms occur on the stems of Brussels sprout, while on sprouts there may be white colonies or fine black speckling in radiating lines.
Causal agent
Powdery mildew is caused by the ascomycete fungus
Erysiphe cruciferarum. The asexual conidia measure
30–52 x 11–17 μm and are borne singly or in chains of only two spores. There appears to be physiological spe- cialization within the pathogen; for example, isolates from turnip will not infect Brussels sprout, and vice versa. The sexual stage has only been reported occa- sionally and consists of spherical cleistothecia that are brown or black. Like all powdery mildews, E. crucifer-
arum is an obligate pathogen.
Disease cycle
The pathogen survives on overwintering crops and vol- unteers. The main period of powdery mildew activity in northern Europe is usually from July until October, but there is great seasonal variation for the disease. Conidia readily spread via winds and can travel significant distances. Powdery mildew conidia do not require water for germination, which can take place at low relative humidity. Rain or free water can actually inhibit DISEASES OFVEGETABLECROPS
F
UNGAL
D
ISEASES
172 Severe powdery mildew of swede.
172
173 Close-up of powdery mildew on underside of
Brussels sprout leaf.
169 powdery development on the upper leaf surface. Germ-
tube growth occurs mainly at 98–100% relative humidity. In some areas of California, powdery mildew is most severe on greenhouse-grown vegetable crucifers. The exact role of cleistothecia in the disease cycle is not known, but this spore stage could provide ascospores as primary inoculum.
Control
Apply fungicides such as sulfur, triazole, and morpho- line products. Less susceptible crops such as cauliflower and cabbage may not require fungicide treatment. For swede and turnip, delay planting dates as this can reduce disease severity. Plant resistant cultivars, partic- ularly for Brussels sprout, cabbage, and swede. Two forms of resistance have been recognized: suppression of fungal development beyond the appressorial stage, and delayed spore production.
References
Dixon, G. R. 1974. Field studies of powdery mildew (Erysiphe
cruciferarum) on Brussels sprouts. Plant Pathology
23:105–109.
Koike, S. T. and Saenz, G. S. 1997. First report of powdery mildew, caused by Erysiphe cruciferarum, on broccoli raab in California. Plant Disease 81:1093.
Rudyard, S. A. and Wheeler, B. E. J. 1985. The development of
Erysiphe cruciferarum on field-grown Brussels sprouts and
associated changes in soluble amino-acids in foliage leaves.
Plant Pathology 34:616–625
Vakalounakis, D. J. 1993. First record of Erysiphe cruciferarum on Lunaria biennis in Greece. Plant Pathology 43:424–425.
Fusarium oxysporum f. sp. conglutinans, F. oxysporum f. sp. raphani