In California, broccoli head rot problems typically occur in the late winter through spring seasons. For broccoli head rot to occur, the conditions need to be humid and wet. The problem is therefore triggered by rain, persistent fog or dew, and overhead sprinkler irrigation that is applied during cold, wet months (note that sprinkler irrigation applied in the drier summer and fall do not usually result in head rot issues).
In California there are two types of head rot. (1) Bacterial head rot symptoms (Photo 1) consist of water-soaked, greasy looking, soft decays of the upper parts of flower buds. Diseased heads will have a very disagreeable odor. Unless other secondary organisms are growing on the diseased flower buds, bacterial head rot will not show the growth of fungi or molds. (2) Alternaria head rot symptoms (Photo 2) consist of relatively drier, firmer decays that tend to be dark green to black in color. The fuzzy growth of the Alternaria fungus can often be seen on the upper parts of the flower buds. Advanced Alternaria head rot cases, however, can become colonized by secondary bacteria and resemble bacterial head rot. Diagnosis of either type of head rot is tricky and requires laboratory and microscope analyses.
Photo 1. Early symptoms of bacterial head rot of broccoli
Photo 2. Early symptoms of Alternaria head rot of broccoli