Chemical Control of Prune Leaf Rust (Tranzschelia discolor f. sp. domesticae) in California. Themis J. Michailides, Former Graduate Research Assistant, Department of Plant Pathology, University of California, Davis 95616. Joseph M. Ogawa, Professor, Department of Plant Pathology, University of California, Davis 95616. Plant Dis. 70:307-309. Accepted for publication 15 October 1985. Copyright 1986 The American Phytopathological Society. DOI: 10.1094/PD-70-307.

The effects of wettable sulfur (WS), mancozeb, and timing of mancozeb applications in controlling prune rust (Tranzschelia discolor f. sp. domesticae) of French prunes were evaluated. In an experimental plot of prunes, unsprayed trees had 44% and WS-and mancozeb-sprayed trees had 21 and < 1% rusted leaves, respectively. In vitro urediniospore germination on acidified potato-dextrose agar amended with 5, 10, or 15 μg a.i./ml of WS or mancozeb was reduced 11, 33, and 38% with WS and 97, 97, and 97% with mancozeb. Results of three mancozeb sprays (in June, July, and August 1983) proved that the last application alone was sufficient for controlling prune rust. Sprayed trees had fewer infected leaves and also fewer rust pustules per leaf. On unsprayed trees, most of the infected leaves had ≥ 16 pustules per leaf. In a commercial prune orchard already infected by rust, two mancozeb sprays (in July and August) protected the sprayed trees for 3 mo from further rust infections, whereas 100% of the leaves on unsprayed trees were infected.