Radish Alternaria
Three species of Alternaria cause serious damage to crucifers: Alternaria brassicicola, A. brassicae, and A. raphani. A. raphani is most often found on radish, but can infect other cruciferous 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.
Alternaria species are simple parasites that survive saprophytically outside the host with diseased crop debris as 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.
- Buy seed certified as disease-free or treat seed with hot water.
- Practice long rotations with non-cruciferous crops.
- Incorporate diseased plant debris into the soil.
- Eliminate cull piles.
- Control cruciferous weeds.
- Minimize the length of leaf wetness periods by reducing plant density, orienting rows with prevailing winds, and irrigating in the morning when leaves can dry quickly.
For Current information on disease recommendations ins specific crops including information on chemical control & pesticide management, please visit the New England Vegetable Management Guide website.
Diseases
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