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Answer: Alternaria brassicae (or Alternaria black spot). Canola pods and plants will often darken the longer they sit in the swath, especially if conditions are moist for any period of time. Alternaria black spot is one disease that can continue to develop and darken plants as the crop sits in the windrow. Alternaria rarely causes high yield loss in B. napus canola, but seed damage can occur when the lesion grows through the pod. Other fungi that can cause canola swaths to darken are the large group saprophytic “sooty molds,” which grow on dried down pods and can be found on seed surfaces after combining. Sooty mold is loose and crumbly and brushes off easily. Many fungi can participate in sooty mold, and some are closely related to disease causing fungi. Alternaria alternata, for example, is a common saprophytic fungus on canola and cereals. Swathers that are covered in dusty spores could be washed before you leave the field as a standard biosecurity measure to slow disease spread.