Corynespora cassiicola is a highly diverse fungal pathogen that can infect more than 500 species of plants, including many economically important crops such as cotton, soybean, tomato, and cucumber. In Florida, the number one vegetable crop by market value are fresh-market tomatoes, which generate nearly half a billion dollars annually. Florida’s subtropical to tropical climate is conducive to infection and development of the target spot pathogen on tomato caused by C. cassiicola. There is no varietal resistance available for target spot of tomato, and preventative fungicide treatments are the primary method for control. In the last decade, C. cassiicola has been more frequently reported by Florida tomato growers, appearing not only more aggressive but also increasingly insensitive to various fungicides. This review brings together the most recent C. cassiicola literature, providing a history and understanding of the immense pathogen diversity and its relevance to tomato. It also provides insight into fungicide resistance development and pathogen survivability, which are important factors in providing effective control recommendations and in understanding the epidemiology of this disease, respectively.

Target spot of tomato caused by Corynespora cassiicola is one of the most economically destructive diseases of tomato in Florida. A collection of 123 isolates from eight counties in Florida were evaluated for sensitivity to azoxystrobin and fenamidone based on mycelial growth inhibition (MGI), spore germination (SG), detached leaflet assays (DLAs), and sequence-based analysis of the cytochrome b gene (cytb). Cleavage of cytb by restriction enzyme (Fnu4HI) revealed the presence of a mutation conferring a glycine (G) to alanine (A) mutation at amino acid position 143 (G143A) in approximately 90% of the population, correlating with quinone outside inhibitor (QoI) resistance based on MGI (<40% at 5 μg/ml), SG (<50% at 1 and 10 μg/ml), and DLA (<10% severity reduction). The mutation conferring a phenylalanine (F) to leucine (L) substitution at position 129 (F129L) was confirmed in moderately resistant isolates (#9, #19, and #74) based on MGI (40 to 50% at 5 μg/ml), SG (<50% at 1 μg/ml and >50% at 10 μg/ml), and DLA (>10% and <43% severity reduction) for both QoI fungicides, whereas sensitive isolates (#1, #4, #7, #28, #29, #46, #61, #74, #75, #76, #91, #95, and #118) based on MGI (>50% at 5 μg/ml), SG (>50% at 1 μg/ml and 10 μg/ml), and DLA (>50% severity reduction) correlated to non-mutation-containing isolates or those with a silent mutation. This study indicates that QoI resistance among C. cassiicola isolates from tomato is widespread in Florida and validates rapid screening methods using MGI or molecular assays to identify resistant isolates in future studies.

Fresh-market tomatoes bring in $400-$500 million annually as the third most valuable crop in the state. Defoliation and fruit damage from target spot, caused by Corynespora cassiicola, can have serious economic implications for growers. This new 5-page publication of the UF/IFAS Plant Pathology Department, written by Keevan MacKenzie, Jessica Chitwood, Gary Vallad, and Sam Hutton, reviews symptoms and epidemiology of the disease and provides current management recommendations.https://edis.ifas.ufl.edu/pp351