A RAPID, RELIABLE BIOASSAY FOR PATHOGENICITY OF COLLETOTRICHUM-MAGNA ON CUCURBITS AND ITS USE IN SCREENING FOR NONPATHOGENIC MUTANTS

A rapid, reliable bioassay for large-scale screening of pathogenicity of Colletotrichum magna, a causal agent of anthracnose in cucurbits, was developed. This method involved exposing whole seedlings and cuttings (plants with excised root systems) to conidial suspensions in scintillation vials. The results corresponded well to disease responses observed by the standard leaf inoculation procedures. This continuous dip method allowed for a rapid disease response in cuttings; mortality occurred within 48-72 hr after inoculation. The method was also reliable for determining susceptible and resistant cucurbit cultivars and enabled screening of more than 300 UV-irradiated potential pathogenicity mutants. Three nonpathogenic mutants, isolates HU 25, HU 36, and HU 43, were identified. Isolate HU 25 did not cause mortality in whole plants but did induce a hypersensitive response when conidia were assayed on cotyledons. The banana-specific pathogen, C. musae, did not cause disease in cuttings and cotyledon-inoculated seedlings. The proposed method enables rapid, reliable, and large-scale screening, and requires less time and greenhouse space than the standard leaf inoculation techniques. The continuous dip method may also be well-suited for screening pathogenicity in isolates of soilborne fungi and especially for evaluating resistance of cultivars to wilt pathogens.