Peach Yellow Leaf Roll Epidemic in Northern California: Effects of Peach Cultivar, Tree Age, and Proximity to Pear Orchards. A. H. Purcell, Assistant Entomologist, Department of Entomological Sciences, University of California, Berkeley 94720. G. Nyland, Professor, and B. C. Raju, Research Plant Pathologist, Department of Plant Pathology, University of California, Davis 95616; and M. R. Heringer, Director, Commodity Services Division, California Farm Bureau Federation, Sacramento 95815. Plant Dis. 65:365-368. Copyright 1981 American Phytopathological Society. DOI: 10.1094/PD-65-365.

An unprecedented epidemic of peach yellow leaf roll (PYLR) occurred in 1978 in commercial cling peach orchards in the “peach bowl” area of northern California. Disease incidence did not differ significantly among peach cultivars but was significantly lower in trees 4 yr old or younger than in older trees. PYLR incidence was highest in peach orchards adjacent to commercial pear orchards, and the incidence generally decreased with increasing distance from pears. There was no evidence of peach-to-peach spread of PYLR.

Keyword(s): disease gradients, epidemiology, mycoplasmalike organisms, MLO, resistance, spiroplasma, X-disease.