Black leaf spot (Brassica)

Alternaria raphani is one of the causes of Black leaf spot on most Brassica species. It is most often found on radish, but can infect other brassica crops.

Symptoms
A. raphani produces black stripes or dark brown, sharp-edged lesions on the hypocotyl of the seedling. It grows in the vascular system and rapidly infects the entire seedling.

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.

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.

The disease may also be a means by which bacterial soft rot enters the stem, which may lead to plant death.

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

Treatment
Late infection, or infection of older leaves, does not characteristically reduce yields, and can be controlled through intensive removal of infected leaves.

Non-Chemical
Surface sterilization of seeds infected with A. raphani (3 min. in 4% sodium hypochlorite) reduced infection by 30%.

Seed treatment with hot water is one method of controlling spores on the seed coat. However, this treatment sometimes depresses germination.
 * Hot-water treatment:

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.
 * Crop rotations:

Chemical
The following fungicides can be used to control A. raphani:
 * 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.