Note the distinct purple margin and concentric circles of the lesion of this Alternaria leaf spot disease.
Alternaria leaf spots and blights are common leaf pathogens on many crops. Sometimes the Alternaria species is specific to a particular crop (such as A. gomphrenae); often Alternaria alternata is the pathogen involved; this species is common, has a wide host range, and can occur as a secondary invader of senescent or injured tissue. Cultural control measures include rotation to non-susceptible crops for two years, control of susceptible weeds,and removal of crop debris from the planting area or prompt incorporation of refuse after harvest. Careful attention to the timing of irrigation (avoid late afternoon or evening) and proper plant spacing to reduce the amount of time during which the plants remain wet can reduce disease spread. Treat with protective fungicides at regular intervals. Among the compounds registered for use on ornamentals are chlorothalonil, thiophanate methyl, copper compounds,and mancozeb. The effectiveness and number of sprays required will vary withweather conditions.
Fact Sheet: Leaf Spot Diseases of Ornamentals