Brassica dark leaf spot
|Brassica dark leaf spot|
|Brassica dark leaf spots (Alternaria brassicicola) on Broccoli|
| Black leaf spot
Alternaria brassicae f. microspora
Brassica dark leaf spot (Alternaria brassicicola) is a plant pathogen causing leaf spots on most Brassica species including broccoli, Brussels sprouts, cabbage, cauliflower, Chinese cabbage, kohl rabi, kale, swede, and turnip.
The pathogen can affect host species at all stages of growth, including seeds. On seedlings symptoms include dark stem lesions immediately after germination, that can result in damping off, or stunted seedlings.
When older plants become infected, Alternaria symptoms often occur on the older leaves, since they are closer to the soil and are more readily infected as a consequence of rain splash or wind blown rain. Spots caused by Alternaria brassicicola tend to be darker and have a more irregular margin than those cause by Alternaria brassicae. Brussels sprout yields can be seriously reduced due to a low tolerance for spots and blemishes.
Fruit-bearing branches and seed pods show dark or blackened spots that result in yield loss due to premature pod ripening and shedding of the seeds. Infection can also occur on the fruit, before or after harvest. A common symptom of broccoli and cauliflower infection is a browning that occurs on the head.
In addition to destruction of a seed crop, the pathogens can live within the seed, spread the disease to other fields, and cause a loss of seedlings
Late infection, or infection of older leaves, does not characteristically reduce yields, and can be controlled through intensive removal of infected leaves.
- Hot-water treatment
Seed treatment with hot water is one method of controlling spores on the seed coat. However, this treatment sometimes depresses germination.
- Crop rotations
Rotation with noncruciferous crops and eradication of cruciferous weed hosts can help control these pathogens. Since spores can survive on leaf tissue for 8 to 12 weeks and stem tissue for up to 23 weeks, fields that are replanted soon after harvest often coincide with a large amount of inoculum which is likely to effect the crop's emergence and early growth stages.
- Biological control
Preliminary studies with the Actinomycete fungus, Streptomyces arabicus, indicated an antifungal effect on A. brassicae and A. brassicicola in both laboratory and field studies. In Finland, surface treatment with powdered Streptomyces griseoviridis (at 15 mg/g seed) has been shown to provide control against A. brassicicola. Further work is needed before this nonchemical approach may be practical.
The following fungicides can be used to control A. brassicicola:
- Thiram at 0.55 lb/100 lb seed
- Iprodione at 0.25 lb a.i./100 lb seed
- Fenpropimorph at 0.25 lb a.i./100 lb seed
- Aoxystrobin (Amistar) 2-5 oz/A. 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. Apply at the first sign of disease and repeat at 7-10 day intervals.
- Cyprodinil plus fludioxonil 11-14 oz/A. 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.
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