Symptoms and Signs
The two Alternaria species, A. brassicae and A. brassicicola, cause similar symptoms. Small, dark specks first develop on leaves and later enlarge into circular, tan spots (0.25-0.5 inch in diameter). The spots caused by A. brassicicola tend to be darker than those caused by A. brassicae. If conditions are favorable, dark green spores of the pathogen will grow on the spots. Such growth causes the spots to have concentric rings in them. Old leaf spots become papery in texture and may tear. When the dry tissue falls out, a shot hole effect results.
Comments on the Disease
Alternaria leaf spot is usually not an economic concern on cole crops. It occasionally is a problem on cabbage during cool, rainy months. The pathogen can also infect Brussels sprouts, broccoli, and cauliflower. Leafy crucifers that are harvested for their leaves (red mustard, Chinese cabbage, tat tsoi, and mizuna mustard) are also susceptible to Alternaria brassicae and can be seriously damaged by this pathogen. Disease is favored by moist conditions. Spores are spread by winds and splashing water. The fungus does not survive in soil but is carried over in crucifer seed, on weeds or volunteer hosts, or on undecomposed crop residue.
Management
Use clean seed and practice crop rotation. Fungicides applied as foliar sprays will control this disease.
| Common name | Amount per acre | REI‡ | PHI‡ | |
|---|---|---|---|---|
| (Example trade name) | (hours) | (days) | ||
| Not all registered pesticides are listed. The following are ranked with the pesticides having the greatest IPM value listed first—the most effective and least likely to cause resistance are at the top of the table. When choosing a pesticide, consider information relating to the pesticide's properties and application timing, honey bees, and environmental impact. Always read the label of the product being used. | ||||
| A. | IPRODIONE | |||
| (Rovral) | 2 pt | 24 | 0 | |
| MODE-OF-ACTION GROUP NAME (NUMBER1): Dicarboximide (2) | ||||
| COMMENTS: For broccoli only. Apply immediately after thinning (2- to 4-leaf stage) as a directed spray to the base of the plant and adjacent soil surface. | ||||
| B. | CHLOROTHALONIL | |||
| (various products) | Label rates | 12 | 7 | |
| MODE-OF-ACTION GROUP NAME (NUMBER1): Multi-site contact (M5) | ||||
| COMMENTS: Use at 7- to 10-day intervals. | ||||
| ‡ | Restricted entry interval (REI) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing. Preharvest interval (PHI) is the number of days from treatment to harvest. In some cases the REI exceeds the PHI. The longer of the two intervals is the minimum time that must elapse before harvest. |
| 1 | Group numbers are assigned by the Fungicide Resistance Action Committee (FRAC) according to different modes of action. Fungicides with a different group number are suitable to alternate in a resistance management program. For fungicides with mode-of-action group numbers 1, 4, 9, 11, or 17, make no more than one application before rotating to a fungicide with a different mode-of-action group number; for fungicides with other group numbers, make no more than two consecutive applications before rotating to a fungicide with a different mode-of-action group number. |