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Evaluation of copper alternatives and enhancers for managing almond bacterial spot caused by Xanthomonas arboricola pv. pruni in California

Stacey Haack: Department of Plant Pathology and Microbiology, University of California

<div>Bacterial spot of almond, caused by <em>Xanthomonas arboricola </em>pv. <em>pruni</em>, is a new and important disease in California, especially on cv. ‘Fritz’. In the spring, gumming mesocarp lesions may cause fruit drop and later penetrate into the endocarp, lowering crop quality and reducing yield. Copper compounds are registered and used prior to late petal fall, but effective rates can cause phytotoxicity on leaves. With additional rainfall, supplemental treatments may be required following leaf emergence and thus, new rotational bactericides or copper activity enhancers are needed. Strains (n=72) from six counties were all copper-sensitive, with no reduction in growth at 10 µg MCE/ml, reduced growth at 20 µg MCE/ml, and no growth at 25 µg MCE/ml. In-vitro mean minimum inhibitory concentrations (MICs) of mancozeb, mancozeb in the presence of copper (10 µg MCE/ml), and oxytetracycline were 2.02, 0.04, and 0.07 µg/ml, respectively. The experimental salicylaldehyde benzoylhydrazone (SBH) was not inhibitory at 80 µg/ml, but the mean MIC was 0.31 µg/ml in the presence of copper (10 µg MCE/ml). In field studies, reduced copper rates mixed with mancozeb were highly effective against the disease without causing phytotoxicity. Oxytetracycline efficacy was moderate to low, possibly due to UV degradation. Adjuvants to improve oxytetracycline efficacy and testing of SBH in the field may provide new treatments in managing bacterial spot over an extended growing season.</div>