Variation in Pathogenicity Among Isolates of Diaporthe Phaseolorum F. Sp. Caulivora

ABSTRACT

Monoascosporic isolates of Diaporthe phaseolorum f. sp. caulivora (DPC) from Maryland were found to exhibit a dimorphism associated with their degree of pathogenicity to soybean seedlings. Isolates that were “fast-growing” on acidified potato dextrose agar (APDA) were largely non-pathogenic to seedlings of the soybean cultivars Blackhawk and Zane, whereas “slow-growing” isolates were largely pathogenic. Pathogenicity was often accompanied by a variation in disease response among seedlings inoculated with a given isolate. Both “fast-growing” and “slow-growing” isolates recovered from inoculated seedlings often exhibited a marked change in pathogenicity when they were re-inoculated into other seedlings of the same cultivars from which they had originally been isolated. In contrast to the Maryland isolates, Alabama and Florida isolates did not exhibit a sharply defined separation into two groups based on their culture morphology. The clear association of pathogenicity with colony diameter seen in the Maryland isolates was not as evident in the Alabama isolates. The variable response observed in seedlings inoculated with the Maryland isolates, however, was also observed in seedlings inoculated with the Alabama isolates. Inoculation of Blackhawk seedlings with Alabama isolates failed to produce cankers whereas inoculation of seedlings of this cultivar with Maryland isolates produced many cankers.

Key Words:

• dimorphism
• soybean stem canker