CHARACTERIZATION OF WESTERN X-DISEASE MYCOPLASMA-LIKE ORGANISMS

The causal agent of western X-disease, an important disease of cherry (Prunus avium) and peach (Prunus persica) in the western United States, was shown to be a non-culturable, mycoplasma-like organism (WX-MLO). Procedures were developed to purify WX-MLOs from celery (Apium graveolens) and Colladonus montanus leafhoppers infected with a greenhouse-maintained isolate of the peach yellow leaf roll (ghPYLR) strain of western X-disease.; WX-MLOs, purified from ghPYLR-infected leafhoppers, elicited the production of specific antisera (WX antisera) when injected into rabbits. When used in an enzyme-linked immunosorbent assay (ELISA), WX antisera quantitatively detected WX-MLOs in celery, periwinkle (Catharanthus roseus), and C. montanus leafhoppers experimentally infected with either ghPYLR or the Green Valley (GVX) strain of western X-disease. ELISA detected high titers of WX-MLOs in the roots and young leaves of symptomatic celery and periwinkle. The severity of symptom expression was directly correlated with titers of WX-MLOs. WX-MLOs were detected in individual leafhoppers 16 days after they fed on ghPYLR-infected celery. Pathogen titers increased in infected leafhoppers during the 40 days they were monitored by ELISA. F(ab){dollar}sb 2{dollar}/Protein A ELISA provided the most sensitive detection of WX-MLOs in field-collected cherry and peach with symptoms of GVX. High titers of WX-MLOs were detected in cherry peduncles at fruit maturity and in cherry leaves during August and September.; DNA was purified from WX-MLO enriched extracts and cloned in Escherichia coli. Recombinant clones were screened by colony, dot and southern hybridizations using {dollar}sp{lcub}32{rcub}{dollar}P-nick translated DNA extracted from healthy and ghPYLR-infected celery and leafhoppers. Twenty-four clones were identified which hybridized with DNA from diseased but not healthy hosts. DNA hybridization (dot blot) assays, using radioactively-labelled, cloned WX-MLO DNA, readily detected WX-MLOs in celery, periwinkle, and leafhoppers infected with either GVX or ghPYLR and in field-collected cherry and peach with symptoms of GVX. DNA hybridization assays were more sensitive than ELISA for detecting WX-MLOs in plants and leafhoppers.; ELISA and DNA hybridization assays indicated that WX-MLOs were antigenically and genetically unrelated to three strains of western aster yellows, four strains of Spiroplasma citri, and the corn stunt spiroplasma (Strain I747). Field-collected pear leaves from trees with symptoms of pear decline tested negatively by both diagnostic assays.