First Report of Target Spot Caused by Corynespora cassiicola on Strawberry in North America

Corynespora cassiicola has a wide geographic distribution and host range and is a threat to economically important crops such as cotton (Gossypium hirsutum), cucumber (Cucumis sativus), tomato (Solanum lycopersicum), and soybean (Glycine max) in the Southeastern United States. Recently, C. cassiicola was reported on blueberry (Vaccinium corymbosum) in Florida (Onofre et al. 2016). In October 2018, severe leaf spotting and defoliation of strawberry (Fragaria × ananassa Duchesne) plants of cultivar Florida Radiance were observed in a shipment of plug plants received by a grower in Hillsborough County, FL. Leaf lesions were 3 to 5 mm in diameter, circular to irregular, with dark brown borders and beige centers. These symptoms were similar to the frogeye-type leaf spot recently described in a first report of C. cassiicola on strawberry in China (Zhang et al. 2018). Dark brown petiole lesions were also observed. Pieces of tissue from the leading edges of leaf lesions were disinfested in 0.625% sodium hypochlorite for 1 min, rinsed twice in sterile distilled water, placed on potato dextrose agar, and incubated for 10 days at 23°C with a 12-h photoperiod. Colonies were gray with velvety to hairy aerial mycelium. Conidia were obclavate to cylindrical, multiseptate, pale olivaceous to dark brown, smooth, 27 to 319 µm long, and 5 to 13 µm wide (n = 150). Based on the morphological characteristics described by Ellis and Holliday (1971), the pathogen was identified as Corynespora cassiicola (Berk & MA Curtis) C. T. Wei. The internal transcribed spacer (ITS) region and β-tubulin genes of three isolates were sequenced (Dixon et al. 2009; Shimomoto et al. 2011), and the sequences were deposited in GenBank (accession nos. MK333283 to MK333285 and MK335367 to MK335369). The β-tubulin sequences were 100% identical to C. cassiicola accession numbers KY290564.1 and KY082896.1, and the ITS region sequences showed 100% identity to C. cassiicola accession numbers MF320532.1 and JQ717069.1. Three single-spore isolates, obtained from different plants, were added to the University of Florida Gulf Coast Research and Education Center culture collection as accessions 18-664, 18-666, and 18-667; they were used for subsequent pathogenicity tests. Each isolate was evaluated for pathogenicity on five 35-day-old plants each of cultivars Florida Radiance, Florida Beauty, and Florida127. Each plant was spray inoculated with a suspension of 2 × 10⁴ conidia/ml. As a control, five plants per cultivar were sprayed with sterile distilled water. Plants were covered with plastic bags immediately after inoculation to ensure high humidity and placed in the greenhouse. After 36 h, plants were removed from the plastic bags and maintained in the greenhouse. Initial symptoms were visible 2 days after inoculation; within 8 days, symptoms like those described above were observed on leaves, petioles, and fruit calyxes of all three cultivars. Control plants remained symptomless. C. cassiicola was readily isolated from lesions of inoculated tissues, fulfilling Koch’s postulates. The experiment was repeated once. To our knowledge, this is the first report of C. cassiicola on strawberry in North America. Diseased plants were from a greenhouse nursery in Virginia, located near a soybean field. This raises a concern for the production of strawberry plants near other hosts that are susceptible to C. cassiicola. The establishment of the disease in the area could pose a serious threat for the strawberry industry in Florida.