COMPARISON BETWEEN WOODY INDEXING AND A RAPID HYBRIDIZATION ASSAY FORTHE DIAGNOSIS OF LITTLE CHERRY DISEASE IN CHERRY TREES

Sweet cherry (Prunus avium) and sour cherry (Prunus cerasus) trees fro m orchards in the Kootenay and Okanagan Valleys of British Columbia, C anada were assayed for the presence of little cherry disease by three different methods: Northern blot analysis of double-stranded RNA, wood y indexing for fruit symptoms on sweet cherry cv. Lambert, and woody i ndexing for foliar symptoms on cv. Canindex 1. Results of the three me thods were in agreement for 85% of the samples. Of the 78 orchard tree s tested, double-stranded RNA isolated from 48 trees hybridised with a radiolabelled cloned probe specific for little cherry disease. When t he 48 trees were tested by woody indexing, buds from 41 trees induced fruit symptoms on cv. Lambert, but only 32 yielded foliar symptoms on cv. Canindex 1 under the conditions of the experiment, Of the 30 orcha rd trees that did not yield a positive response to the Northern blot a nalysis, 26 samples were negative on cv. Lambert and 26 were negative on cv. Canindex 1. Northern blot analysis of the 78 cv. Lambert indica tor trees revealed that there was an absolute correlation between the presence of little cherry disease-associated double-stranded RNA and t he development of typical little cherry disease symptoms on the indica tor trees. Reliability of woody indexing of orchard samples was impair ed by poor transmission of the disease from the inoculating bud to the indicator tree. Woody indexing with cv, Canindex 1 was particularly p rone to a large number of apparently erroneous negative results. Of th e three protocols used, diagnosis of little cherry disease by Northern blot analysis was found to be the most reliable and offered a greatly accelerated means of diagnosis.