Alternaria leafspot and head rot on broccoli is commonly caused by a couple of fungi: Alternaria brassicicola and Alternaria brassicae. Disease development is favored by cool temperatures and long periods (more than 9 hours) of high moisture. Fungal spores can be moved in a variety of ways: with wind currents, with splashing water, on equipment, on animals—including people, and in/on infected seed.
Leafspot symptoms start off as small yellow spots on older leaves that develop into 1Ž2 inch diameter or larger “bull’s eyes” or “targets” with concentric rings of varying shades of tan/gray/black. Alternaria head rot also starts off as yellow spots that turn brown and black. You will see black spores on spots on leaves or heads.
There are some things you can do to minimize or avoid having this alternaria leafspot and head rot. First practice a 2-3 year crop rotation away from brassicas to limit build-up of these fungi. Another step is to manage weeds that are in the brassica family because they can be hosts for these fungi. These weeds include wild mustard, shepherd’s purse, hairy bittercress, field pepperweed, and field pennycress. Promote leaf drying to help create an environment that is less suitable for the disease to establish. Practices include choosing sites with good air and soil drainage, orienting rows with prevailing winds, using drip irrigation, and avoiding overcrowding plants through close spacing. Treat seed with hot water to kill the fungus on the seed surface. Page E39 of the Commercial Vegetable Production Recommendations guide for Pennsylvania has information on how to do this. You need to be very precise with this technique for it to work. Buying seed that is heat treated or treated with other alternatives to heat from seed companies is ideal. Soil incorporate crop residue immediately after harvest to minimize the amount of fungal spores that can be moved to healthy plants. And finally research shows that the use of straw mulch can help minimize this disease.
(Source – http://www.farms.com/news/control-leafspot-and-head-rot-on-broccoli-99920.aspx)