The last couple of weeks we have been getting many calls and samples of cabbage, cauliflower, and broccoli plants wilting. The incidence has been higher on Imperial valley. The lower leaves turn yellow and the whole plants wilt. In all the samples we have observed so far, the plants seem to have black fungal growth in the crown (see picture), which might confuse the PCAs as black leg disease. However, when those spores were observed under the microscope, we identified it as Alternaria spp.
Upon surface sterilization and isolating pathogen under selective media, we isolated Rhizoctonia from each sample. Alternaria is a facultative parasite, meaning it can live on both living and dead plant tissue. So Alternaria in this particular situation is a secondary opportunist pathogen taking advantage of the weak plants and Rhizoctonia and possibly other damping off fungi are the main culprit.
Damping off is a common problem on overwatered fields and fields with low drainage (thus more of an issue in Imperial Valley). Overwatering, high salts from over fertilizing and cool soil temperatures are all associated with damping off. Damping off pathogens are divided into two main groups. The “true fungi” that include Rhizoctonia, Fusarium, Thielaviopsis genera. And the second group are oomycetes that include Pythium and Phytopthora.
Water management is crucial for disease management. Avoiding wet feet, watering the plants early in the day leaving enough time for the soil to dry up, use of soil amendments to improve soil health and soil aggregate can be beneficial. Ridomil, Endura, Fontelis will help, but the key is to not create the environment not conducible for pathogen