Screening Cherry Germ Plasm for Resistance to Leaf Spot.

A detached leaf disk assay was developed for screening sour cherry (Prunus cerasus) cultivars for resistance to cherry leaf spot (Blumeriella jaapii). This assay was used to characterize the events occurring in cv. Montmorency (susceptible) and cv. GiSelA 6 [GI 148-1] (resistant) host-pathogen interactions, and to develop a disease scoring scheme to categorize disease severity in sour cherry trees. Forty-three seedlings grown from seeds collected in Russia were screened for resistance using the scoring scheme. Cross infection studies were also carried out with leaf spot isolates from four other Prunus species. These studies were made possible by the development of a new method of culturing B. jaapii isolates, using cherry fruit agar. This method enabled large amounts of inoculum to be produced within 2 weeks rather than a couple of months as required previously. In 'Montmorency', disease symptoms were first observed 4 days postinoculation (dpi), with the appearance of small white spots on the undersides of inoculated leaves. These spots gradually grew, producing erumpent spore masses in acervuli approximately 7 dpi. In 'GI 148-1', disease symptoms were also first observed 4 dpi, with the appearance of small transparent lesions. In the majority of cases, these lesions did not increase in size; however, acervuli occasionally were formed approximately 8 dpi. These contained small cirrhi and were often surrounded by an abscission zone. Of the sour cherry seedlings tested, only 'Almaz' open pollinated (o.p.) R1(1) was rated as resistant. 'Almaz' o.p. R1(1) and 'GI 148-1' were also rated as resistant to leaf spot isolates from other Prunus species. These results and the implications for breeding resistant sour cherry cultivars are discussed.