ABSTRACT
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.
ABSTRACT
ABSTRACT Sublethal enrichment was used to generate mutants of Pythium sylvaticum tolerant to kanamycin and tetracycline. Kanamycin tolerance was readily generated, and mutants had growth rates similar to wild-type isolates at antibiotic concentrations lethal to wild-type isolates. Based on crosses between wild-type and mutant isolates, two types of inheritance of tolerance were identified. A high level of kanamycin tolerance was inherited in progeny only when the maternal parent was drug tolerant and was correlated with the inheritance of maternal mitochondrial DNA. A lower level of tolerance was observed in some progeny when the paternal parent was tolerant to the antibiotic and, based on the lack of inheritance of paternal mitochondrial DNA, was presumably nuclear-encoded. Selection of mutants tolerant to tetracycline took longer to generate than kanamycin-tolerant mutants. Based on crosses between tolerant and wild-type parents, tolerance to tetracycline was nuclear-encoded. Tolerance to both antibiotics was stable, with cultures retaining tolerance following repeated transfers on nonamended medium and after storage for 7 years.
ABSTRACT
The Brazilian pepper-tree (Schinus terebinthifolius Raddi) native to Brazil, recently has become an aggressive perennial weed in southern Florida. During a survey in December 1995, a foliar disease was observed on several pepper-tree plants in Palm Beach County. Disease symptoms consisted of dark, reddish-purple necrotic lesions, either with or without dry necrotic centers, that were distributed randomly over the leaf surface. Infected leaf samples from two separate sites were plated on potato dextrose agar (PDA; Difco) and water agar and incubated at 25°C in the dark. A fungus resembling a Rhizoctonia sp. was consistently recovered. To prove Koch's postulates, the fungus was grown on PDA for 10 to 14 days, and the cultures blended in a Waring blender. Metamucil (Procter & Gamble) was added to the mixture at the rate of 0.5% wt/vol, and the suspension was used to spray and inoculate 2- and 3-month-old Brazilian pepper-tree seedlings. Seedlings were sprayed until the inoculum dripped off the foliage and after inoculation were maintained at 100% relative humidity. After 48 h in the dew chamber the inoculated seedlings were moved to a greenhouse bench and examined for infection 5 and 10 days later. Inoculation was completed three times with the leaf lesions occurring 94 to 100%. A Rhizoctonia sp. was recovered from the lesions that appeared on the challenged plants. A determination of the anastomosis group was performed by plating it against the tester isolates of R. solani, AG1-1A, AG2-2IV, AG-3, AG-4, and AG-5. In two separate tests anastomosis (imperfect fusion) (1) was observed between the recovered Rhizoctonia sp. and tester strain AG2-2IV of R. solani. The fungus was identified as R. solani, and this is the first report of R. solani causing a leaf lesion of Brazilian pepper-tree in Florida. The potential of this R. solani as a biological control agent of Brazilian pepper-tree remains to be tested. Reference: (1) B. Sneh et al. Identification of Rhizoctonia Species. American Phytopathological Society, 1991.
