Development of a rapid, reliable screening method for pathogenicity of Colletotrichum species on strawberry seedlings

A rapid in vitro bioassay was developed, enabling large scale screening and isolation of nonpathogenic mutants of Colletotrichum acutatum and C. gloeosporioides on strawberry seedlings. Inoculation was carried out on 12 week-old (young) and 15 week-old (older) strawberry seedlings. A comparison was made between a foliar dip and a root soak method at 19°C and 25°C. Mortality of young seedlings was observed 4 days after inoculation, reaching 50% within 10 days using both techniques at 25°C. Mortality of older seedlings was delayed by 4 days compared to that in young seedlings with the root soak method. Disease progress decreased at 19°C. More than 980 REMI mutants were screened, resulting in five reduced-virulence isolates.