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Epidemiology and management of bacterial spot of almond, caused by Xanthomonas arboricola pv. pruni
S. HAACK (1), L. Wade (2), H. Forster (1), J. E. Adaskaveg (1). (1) University of California, Riverside, Riverside, CA, U.S.A.; (2) Arysta LifeScience, Cary, NC, U.S.A.

Bacterial spot of almond, caused by <i>Xanthomonas arboricola</i> pv. <i>pruni</i> (<i>Xap</i>), was first observed at high incidence in California’s Central Valley in the spring of 2013. Fruit symptoms are the most common and include amber gumming with internal brown corky lesions. Leaf spots are less common and cankers have not been observed to date. Studies were done to increase understanding and develop management strategies for this new disease. Viable <i>Xap</i> was consistently isolated from symptomatic mummified fruit collected through February, indicating their role in overwintering the pathogen and producing primary inoculum in the spring. In field trials, delayed-dormant (late January) treatments of copper-mancozeb significantly reduced disease incidence, whereas early dormant (November/December) treatments did not. Although copper-mancozeb is a standard in-season treatment for bacterial diseases, copper can be phytotoxic to almond, while mancozeb has label use restrictions. In evaluating alternative in-season treatments, the antibiotic kasugamycin significantly reduced disease incidence, and based on this work, kasugamycin has been accepted into the federal IR-4 program for registration on almonds for bacterial spot management. This research represents the first studies conducted to better understand the epidemiology and management of bacterial spot in California almond orchards.

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