LARGE-SCALE SCREENING FOR X-DISEASE PHYTOPLASMA INFECTION IN CHOKECHERRY

Chokecherry (Prunus virginiana L.) germplasm collected from North Dako ta, South Dakota, and Minnesota and established in Bismarck, N.Dak, wa s screened for infection by the X-disease phytoplasma and for putative X-disease-resistant plants. A total of 1792 chokecherry trees were fi rst screened by observation of characteristic disease symptoms and wer e given a visual disease rating. In 1996, 97% of the trees were severe ly diseased, or rated 3 or lower on a 5-point scale. The remaining 3% (55 trees) showed vigorous growth and had few or no X-disease symptoms , and were rated 4 or 5. Of these trees, 44 (80%) were found to contai n the X-disease phytoplasma as indicated by reactions with a monoclona l antibody that is specific for this phytoplasma. The other 11 trees ( 20%) were then found to contain the X-disease phytoplasma using the hi ghly sensitive, but more expensive and time-consuming, nested-polymera se chain reaction (PCR). Employment of a combination of visual screeni ng, monoclonal antibody assay, and nested-PCR provided an effective an d efficient means for examining a large number of plant materials for infection by the phytoplasma, The chokecherry trees that were infected but had few or no X-disease symptoms were considered to be resistant/ tolerant to X-disease.