Disease Management

Cabbage Alternaria

ALTERNARIA DISEASES OF BRASSICAS

The Plant Diagnostic Clinic

Revised December 2005

Three species of Alternaria cause serious damage to brassicas: Alternaria brassicicola, A. brassicae, and A. raphani.  Alternaria brassicicola and A. brassicae infect broccoli, Brussels sprouts, cabbage, cauliflower, Chinese cabbage, kohlrabi, kale, rutabaga, and turnip. A. raphani is most often found on radish, but can infect other brassica crops. The most common symptom of Alternaria diseases is yellow, dark brown to black circular leaf spots with target like, concentric rings. Lesion centers may fall out, giving the leaf spots a shot-hole appearance. Individual spots coalesce into large necrotic areas and leaf drop can occur. Lesions can occur on petioles, stems, flowers, flower pedicels, and seed pods. Pod infection causes distortion, premature shattering, and shriveled, diseased seed that germinates poorly.

Alternaria species are simple parasites that survive saprophytically outside the host, diseased crop debris is the primary site of survival from year to year. Resting spores (chlamydospores, microsclerotia) have been reported. The disease is favored by warm temperatures (60-78° F) and at least 12 hours of relative humidity of 90 % or more. The fungi sporulate profusely and are spread throughout fields by wind, splashing water, equipment, and workers. The main means of introduction into new areas is on infested seed.

Management:

• Buy seed certified as disease-free or treat seed with hot water.
• Practice long rotations with non-brassica crops.
• Incorporate diseased plant debris into the soil.
• Eliminate cull piles.
• Control brassica weeds.
• Avoid overhead irrigation during head development.
• Keep seedbeds disease-free to prevent the spread of disease and locate seedbeds so as to avoid wind-borne inoculum.
• Control of Alternaria leaf spot on cabbage heads in the field is necessary for long-term storage.

Chemical recommendations:

azoxystrobin (Amistar): 2-5 oz/A (0 dh, REI 4h). Apply prior to disease development and continue at 7-14 day intervals. Do not make more than one application of Amistar before alternating with a fungicide with a different mode of action.

chlorothalonil (Bravo Ultrex 82WDG): 1.4 lb/A (7 dh, REI 12h). Apply at the first sign of disease and repeat at 7-10 day intervals.

cyprodinil plus fludioxonil (Switch 62.5WDG): 11-14 oz/A (7dh, REI 12h).  Apply at the first sign of disease and repeat at 7-10 day intervals. Only turnip varieties harvested for their leaves may be treated.

maneb (Maneb, Manex): Rates vary depending on the formulation. (7 dh, REI 24h).

Prepared by M. Bess Dicklow, UMass Extension, 107 Fernald Hall, University of Massachusetts, Amherst, MA 01003-9320. Tel. 413-577-1827 Fax. 413-545-2115. mbdicklo@umext.umass.edu