VARIATION IN FITNESS AMONG FIELD ISOLATES OF EXSEROHILUM-TURCICUM IN ISRAEL

Isolates of race 1 of Exserohilum turcicum collected from 15 fields in Israel varied considerably in three components of parasitic fitness when tested on the susceptible sweet corn hybrid cultivar Jubilee grown in growth chambers maintained at 20 C. Infection efficiency (percentage of sites inoculated with 10-mu-l droplets containing an average of 50 conidia each that developed lesions) ranged from 17 to 95%. Lesion size and sporulation 10 days after inoculation ranged from 0.3 to 9.3 cm2 and from 833 to 14,000 spores per square centimeter, respectively. These two variables were significantly correlated (r = 0.45). Isolates did not differ significantly in spore germination. Variation among isolates from different locations was significantly greater than that among isolates from the same location.