STUDIES ON FUNGUS PARASITES OF STRAWBERRY LEAVES IN ONTARIO

Abstract

Four fungi were isolated from lesions on strawberry foliage in Ontario during the growing seasons of 1948 and 1949. They are Mycosphaerella fragariae (Tul.) Lindau. Diplocarpon carliana (Ell. & Ev.) Wolf, Dendrophoma obscurans (Ell. & Ev.) H. W. And., and Gnomonia fructicola (Arnaud) n. comb. (Zythia fragariae Laibach), and they cause the diseases known as leaf spot, leaf scorch, leaf blight and leaf blotch respectively. Apparently this is the first report of Zythiae fragariae in North America and of Dentrophoma obscurans in Canada. From field surveys, strawberry varieties were found to vary in resistance, especially to spot and scorch, and market differences in seasonal distribution of the diseases were also evident. Spot was most prevalent in the spring and fall; scorch increased in severity during the warmer months; and blight became prominent after the cropping period. As a result of inoculation with M. fragariae only the middle-aged leaves of vigorously growing strawberry plants developed lesions. Premier plants, although resistant to spot in the field, proved susceptible when artificially inoculated. Penetration by M. fragariae may occur without lesion formation if the temperature is below 15 °C. Symptom expression was best within the 20°–25 °C range. Adding strips of strawberry leaves to a suspension of conidia of Diplocarpon earliana greatly enhanced spore germination. Only leaves which are old or middle-aged develop scorch lesions, and these may be blotchy or diffuse depending on the variety. Dendrophoma obscurans appears to attack mainly the older leaves near the margins. The conidiophores are long and branched, the conidia slightly pointed, and the pyenidia are dark, leathery, and clumped in culture. Zythia fragariae has simple conidiophores, biguttulate conidia with rounded ends, and soft light brown pyenidia. The name Gnomonia fructicola is proposed for the fungus Gnomonia fragariae Klebahn form fructicola Arnaud because it is considered to be a different species from Gnomonia fragariae Klebahn.