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Etiology of Almond Leaf Scorch Disease and Transmission of the Causal Agent. Srecko M. Mircetich, Research Plant Pathologist, Agricultural Research Service, U.S. Department of Agriculture, Department of Plant Pathology, University of California, Davis 95616; S. K. Lowe(2), W. J. Moller(3), and G. Nyland(4). (2)(3)(4)Research Associate, Plant Pathology Specialist, Cooperative Extension Service, and Professor of Plant Pathology, respectively, Department of Plant Pathology, University of California, Davis 95616. Phytopathology 66:17-24. Accepted for publication 15 July 1975. DOI: 10.1094/Phyto-66-17.

Marginal scorching of leaves, appearing after mid-June, is one of the most diagnostic characteristics of the disease. Electron microscopic examination of ultrathin sections of the mid-veins of scorched leaves from naturally and experimentally infected trees revealed the presence of rod-shaped bacteria in the xylem vessels. The bacterial cells had an average diameter of 0.4 µm and length up to 1.9 µm and they exhibited multilayered, rippled, and convoluted walls. No organism was found in the xylem of healthy almond trees. Leafhoppers (Draeculacephala minerva) transmitted the bacterium from naturally infected almonds to healthy Mission almond seedlings and rooted cuttings of Carignane grapevines in the greenhouse. The almond and grape indicators developed typical leaf symptoms of almond leaf scorch and Pierce’s disease, respectively, within 2 months after exposure to infective leafhoppers. Control grape and almond plants exposed to leafhoppers that had fed on healthy almond shoots remained symptomless, and no bacteria were observed in the leaves of any control plants. The almond leaf scorch bacterium was readily graft-transmitted by buds, bud chips, or stems from naturally or experimentally infected to healthy 1- and 2-year-old almond trees.

Additional keywords: bacteria, xylem pathogen, leafhopper vector, Prunus amygdalus.