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Outbreak of Choanephora Blight Caused by Choanephora cucurbitarum on Green Bean and Pepper in Florida

Severe outbreaks of Choanephora blight on green bean (Phaseolus vulgaris cvs. Bronco, Shade, and Gold Mine) and bell pepper (Capsicum annuum cvs. Aristotle, Crusader, and Sentry) were widespread in southwestern (Hendry and Collier counties) and northern Florida (Alachua County) in October and November 2002. Disease incidence, estimated by inspecting 100 randomly selected bean plants in each of four fields, was 40 to 100% and infected fruit ranged from less than 10 to 100%. Incidence estimated similarly on pepper plants in three fields was 35 to 40% 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. Pathogencity tests consisted of spray inoculation (5,000 spores per ml) of five 6-week-old plants 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°C, and symptom development was observed as early as 3 days after inoculation. The percentage of infected plants after wounding was 40% for bell pepper (‘Enterprise’), 100% for green bean (‘Opus’), 0% for watermelon (Citrullus lanatus ‘Star Gazer’), 60% for cantaloupe (Cucumis melo ‘Vienna’), and 20% 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. curcurbitarum was observed and isolated from a few noninoculated bean flowers and two noninoculated bean pods indicating spread to noninoculated 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 both + and - strains occur 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.