PRO/PL> Choanephora blight, green bean, pepper - USA (FL)

CHOANEPHORA BLIGHT, GREEN BEAN, PEPPER - USA (FL)
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Date: 22 Aug 2003
From: ProMED-mail <[EMAIL PROTECTED]>
Source: Amer Phytopathol Soc, PLANT DISEASE [edited]


Outbreak of Choanephora Blight Caused by _Choanephora cucurbitarum_ on 
Green Bean and Pepper in Florida.
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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.

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[_Choanephora cucurbitarum_ is primarily a pathogen of melons, squashes, 
and pumpkins. Infection of blossoms and fruits can be high (30-40 percent). 
Disease development is favored by warm temperatures (greater than 25C) and 
wet weather. The fungus is considered a weak pathogen, and its occurrence on 
bean and pepper was probably facilitated by the exceptionally moist 
conditions in Florida. For disease management, provide for well-drained 
soils, avoid overseeding, and apply fungicides thoroughly in the leaf 
canopy. - Mod.DH]
.......................................dh/pg/jw

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