Development of a protocol for screening cherry germplasm for resistance to cherry leaf spot

All sour cherry (Prunus cerasus) varieties grown in the U.S. are susceptible to cherry leaf spot caused by Blumeriella jaapii and currently the disease must be controlled chemically. Therefore, a major goal of the MSU sour cherry breeding program is the development of new varieties that have stable resistance to cherry leaf spot. Due to the long life of a cherry orchard, it is important that the leaf spot resistance bred into new varieties be durable, reducing the probability of a future breakdown in resistance. Previous studies have shown that the interspecific cherry hybrid GiSelA 6 [GI 148-1] (P. cerasus cv. Schattenmorelle x P. canescens) is resistant to leaf spot and may be a source of resistance. It is likely that other sources of resistance to cherry leaf spot can be found within the available cherry germplasm. The objectives of this study were to develop a rapid detached leaf screening assay to maximize the use of the limited plant material available and collect fungal isolates from various varieties of sour cherry and other Prunus species growing in Michigan for use in future screening experiments. Due to the extremely slow growth rate of B. jaapii isolates on conventional leaf spot growth media (Lima bean agar), alternative means of culturing the fungus in vitro had to be developed to produce enough inoculum for inoculation of leaf disks. Comparison of infected excised leaf disks and whole plants showed that there were no differences in infection or symptoms produced between the two. A new means of culturing B. jaapii on cherry fruit I agar also was developed, which allowed the production of up to 2.5 x 10(6) spores ml(-1) per petri dish. This culturing system produced more than enough inoculum for infection screening of sour cherry germplasm. This work and preliminary results from screening trials are discussed.