Seed may not form within infected pods or those that do can be shriveled or blemished. Spore production on infected pods may not occur. The species of Alternaria causing disease on crucifer plants can survive on or in crop debris, seed and possibly weed species in the crucifer family. Spores are produced on old crop debris and leafspots. However, if leaf wetness is prolonged for 20 hours or more, A. Brassicicola is capable of producing many spores outside the optimum range of temperatures.
Spore production occurs predominately at night or during long overcast periods.
Spore production has been observed to be greater in yellowed leaves. Spores are released during the day as the relative humidity diminishes. Movement of equipment or work crews in the field and wind aid in spore release and spread. Spores have been trapped more than one mile down wind immediately after a crop with Alternaria leafspot was harvested. After landing on susceptible plant tissue, Alternaria spores are durable enough to remain there until free moisture in the form of dew, rain, or irrigation water occurs at which time the spore germinates.