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Stemphylium Leaf Spot of Parsley in California Caused by Stemphylium vesicarium

Steven B Koike, Nichole R O'Neill, Peter B Van Berkum, Julie E Wolf, Oleg Daugovish
Plant disease 2013 v.97 no. pp. 315-322
leaf spot, foliar diseases, Stemphylium vesicarium, tomatoes, celery, spinach, leeks, pears, crop losses, carrots, pathogens, leaves, parsley, cultivars, host range, seeds, garlic, onions, Apiaceae, inoculum, crop quality, fruits, fungi, California
From 2009 through 2011, a previously undescribed disease occurred on commercial parsley in coastal (Ventura County) California. Symptoms of the disease consisted of circular to oval, tan to brown leaf spots and resulted in loss of crop quality and hence reduced yields. A fungus was consistently isolated from symptomatic parsley. Morphological and molecular data identified the fungus as Stemphylium vesicarium. When inoculated onto parsley, the isolates caused symptoms that were identical to those seen in the field; the same fungus was recovered from test plants, thus completing Koch’s postulates. Additional inoculation experiments demonstrated that ten out of eleven tested flat leaf and curled parsley cultivars were susceptible. The parsley isolates also caused small leaf spots on other Apiaceae plants (carrot, celery) but not on leek, onion, spinach, and tomato. Isolates caused brown lesions to form when inoculated onto pear fruit, but only when the fruit tissue was wounded. Using a freeze-blotter seedborne pathogen assay, parsley seed was found to have a very low incidence (0.25 %) of S. vesicarium. When inoculated onto parsley, three of four isolates from seed caused the same leaf spot disease. This is the first documentation of a foliar parsley disease caused by S. vesicarium. The low incidence of S. vesicarium on parsley seed indicates that infested seed may be one source of initial inoculum. Because of the negative results in the host range experiments, it appears that this parsley pathogen differs from the S. vesicarium that causes disease on leek, garlic, onion, and pear fruit.