August 22, 2003
From: American Phytopathological Society PLANT DISEASE [edited]

Outbreak of Choanephora blight caused by Choanephora cucurbitarum on green bean and pepper in Florida

P. D. Roberts and R. R. Urs, Department of Plant Pathology, University of Florida, Southwest Florida Research and Education Center, Immokalee 34142-33975; T. A. Kucharek, C. R. Semer, and G. L. Benny, Department of Plant Pathology, University of Florida, 1453 Fifield Hall, Gainesville 32611-0680; and K. Pernezny, Department of Plant Pathology, University of Florida, Everglades Research and Education, 3200 E. Palm Beach Rd, Belle Glade, 33430-4702. Florida Agric. Exp. Sta. J. Series R-09544. Plant Dis. 87:1149, 2003; published on-line as D-2003-0714-01N, 2003. Accepted for publication 23 Jun 2003.

Severe outbreaks of Choanephora blight on green bean (_Phaseolus vulgaris_ cvs. Bronco, Shade, & Gold Mine) and bell pepper (_Capsicum annuum_ cvs. Aristotle, Crusader, & Sentry) were widespread in southwestern (Hendry and Collier counties) & northern Florida (Alachua County) in October & November 2002.

Disease incidence, estimated by inspecting 100 randomly selected bean plants in each of 4 fields, was 40-100 percent and infected fruit ranged from less than 10 to 100 percent. Incidence estimated similarly on pepper plants in three fields was 35-40 percent with substantial fruit infection observed predominantly around the calyx. Zucchini fruit and a pigweed plant (_Amaranthus_ sp.) were observed with sporulating lesions of _Choanephora_, indicating that other hosts were affected during the outbreak. Symptoms were blighted leaves, dieback of shoot tips, blighted blossoms, and black, soft-rot lesions on fruit. _Choanephora_ sp. was sporulating in abundance on diseased tissue.

Isolates of _Choanephora_ sp. grew readily as pure cultures on acidified potato dextrose agar and malt yeast extract (MYE) agar. _C. cucurbitarum_ (Berk. & Rav.) Thaxter was identified on the basis of shape and ornamentation of the sporangiola (1). The sporangiola of _C. cucurbitarum_ are ellipsoid to broadly ellipsoid, and the wall is usually longitudinally striate.

Pathogenicity tests consisted of spray inoculation (5000 spores per ml) of 5 plants 6 weeks of age, each with and without wounding made by lightly scratching the leaf surface with a needle. Plants were placed in the greenhouse with temperatures ranging from 21 to 26 deg C, and symptom development was observed as early as 3 days after inoculation.

The percentage of infected plants after wounding was 40 percent for bell pepper ('Enterprise'), 100 percent for green bean ('Opus'), zero percent for watermelon (_Citrus lanatus_ 'Star Gazer'), 60 percent for cantaloupe (_Cucumis melo_ 'Vienna'), and 20 percent for cucumber (_Cucumis sativus_ 'Thunder CY'). Lesions on inoculated leaves were similar to those seen in the field on bean and pepper, and sporulation of _C. cucurbitarum_ was present in the necrotic areas on all symptomatic plants.

Pure cultures of _C. cucurbitarum_ were reisolated. _C. cucurbitarum_ was observed and isolated from a few uninoculated bean flowers and 2 uninoculated bean pods, indicating spread to uninoculated plants; otherwise control plants were asymptomatic. Unwounded plants did not develop lesions, indicating that wounding was necessary for infection by this inoculation technique.

The mating type was determined by juxtaposing several isolates on MYE agar, and zygospore formation was observed, indicating that both positive and negative strains are present in Florida. These outbreaks show that under the proper environmental conditions, such as long periods of high rainfall, high humidity, and high temperatures, crops like bean and pepper that are not usually affected by the disease may experience significant damage.

Reference: (1) P. M. Kirk. Mycol. Pap. 152:1-61, 1984.