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Peach Rosette: the Morphology of an Associated Mycoplasmalike Organism and the Chemotherapy of the Disease. Hugh C. Kirkpatrick, Research Plant Pathologist, Agricultural Research Service, U.S. Department of Agriculture, Southeastern Fruit and Tree Nut Research Station, Byron, Georgia 31008; S. K. Lowe(2), and G. Nyland(3). (2)(3)Research Associate and Professor, respectively, Department of Plant Pathology, University of California, Davis 95616. Phytopathology 65:864-870. Accepted for publication 10 March 1975. DOI: 10.1094/Phyto-65-864.

Prunus tomentosa was the only cherry host susceptible to peach rosette. Almond (P. dulcis) cultivar Jordanolo was not superior to peach (P. persica) in symptom expression or duration of incubation period. Nevertheless, it did persist more vigorously as an indicator plant, and appeared to be more resistant to the lethal effect of rosette after dormancy. Mycoplasma-like organisms (MLO's) were found in the sieve elements of rosette-infected leaves of Vinca rosea, P. persica, and P. dulcis, but not in healthy leaves of those hosts. The dimensions of the pleomorphic organisms from electron micrographs were 80 × 1,000 nm. They were found most abundantly in V. rosea. The partial remission (37.5%) of rosette symptoms occurred after treatment with tetracycline hydrochloride and the lesser remission (9.1%) with chlorotetracycline hydrochloride and oxytetracycline dihydrate, support the belief that the MLO is the causal agent of peach rosette. Tetracycline hydrochloride inhibited rosette symptoms four-to-five times more effectively than the other compounds. Remission occurred when the chemicals were injected under the bark or into the wood, but not when sprayed on the foliage.