Remove and destroy all parts of dead plants. They are a source of disease. When thinning, remove thinned plants immediately. Don't harvest vegetables when plants are wet. Avoid overhead watering.
Drip irrigation can efficiently deliver water and keep foliage dry.
Crucifer Disease Aerate the soil. Leave adequate space between plants and rows. Organic Controls Copper, sulfur and Bordeux mixes have been used with limited success on vegetables in eastern United States. Some organic producers will not use these as they feel the products leave a chemical residue. However, such residues are easily washed off.