Stemphylium leaf spot of parsley in California caused by Stemphylium vesicarium

Authors: KOIKE, STEVEN - University Of California - Cooperative Extension Service; Oneill, Nichole; Van Berkum, Peter; Wolf, Julie; DAUGOVISH, OLEG - University Of California - Cooperative Extension Service

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/4/2012
Publication Date: 5/1/2013
Citation: Koike, S.B., Oneill, N.R., Van Berkum, P.B., Wolf, J.E., Daugovish, O. 2013. Stemphylium leaf spot of parsley in California caused by Stemphylium vesicarium. Plant Disease. 97:315-322.

Interpretive Summary: Parsley is a familiar leafy plant that is grown both as a fresh market commodity for use as a vegetable, herb, and garnish and as a dehydrated product for various culinary uses. California is the number one producer of parsley in the USA with approximately half of the country’s production of both the curly and flat leaf varieties. A problem that appeared in 2009 was an unfamiliar symptom on the leaves of commercial conventional and organic flat leaf parsley grown in Ventura County, California. Because of the high quality standards for parsley, the problem resulted in yield loss because crops in diseased fields were not harvested. We cultured the fungus that was causing the disease symptoms on the leaves of parsley. Using traditional methods based on spore measurements and the analysis of a gene in the DNA, the fungus causing the disease was identified. From our research it was clear that this fungus had never been identified as a disease of parsley in California. The source of the disease was traced to the seeds used to produce parsley. Therefore, the disease possibly can be controlled by treatment of the seeds with fungicides, but none of the fungicides that are used to control other crop diseases are registered for use on parsley in California. Since our experiments indicated that at least one flat leaf parsley cultivar was resistant to this disease a search for other resistant parsley cultivars could assist growers. Also, these resistant varieties could be used by breeders in programs to produce more desirable varieties that would not succumb to this disease.

Technical Abstract: 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.