Olive leaf spot and its control with fungicides
AuthorsE. E. Wilson
H. N. Miller
Authors AffiliationsE. E. Wilson was Professor of Plant Pathology and Plant Pathologist in the Experiment Station; H. N. Miller was Research Assistant in Plant Pathology.
Hilgardia 19(1):1-24. DOI:10.3733/hilg.v19n01p001. March 1949.
Olive leaf spot, caused by the fungus Cycloconium oleaginum Cast., has recently been prevalent in California. The first symptoms are inconspicuous sooty blotches on the leaf blade. As these enlarge, the rest of the leaf turns yellow. Severe defoliation follows leaf infection. In some orchards 10 to 20 per cent of the fruiting twigs died as a result of defoliation.
The fungus was isolated by transferring single ungerminated conidia to a solid medium. It proved to be extremely slow-growing. Only three of many isolates produced conidia in culture. Conidia germinated and mycelium grew between 9° and 30° C, the optimum being 16° to 20°.
New lesions developed in the fall one season in central California, but not until late January and February in three other seasons. Affected leaves became yellow and fell during May and early June.
Lesions on leaves that remained in the tree seem to be the chief if not the only source of inoculum. These bore numerous conidia in spring but few during the summer. In autumn the fungus grew from the margins of such lesions into adjacent leaf tissue and produced conidia; thus a supply of inoculum was insured at a time when moisture conditions were favorable to infection. The spores seem to be spread by rain more than by air currents.
Bordeaux mixture (8-8-100 or 10-10-100) applied in fall or early winter effectively prevented leaf infection in three out of four seasons. Later applications gave less satisfactory control. For two seasons one treatment of 8-8-100 bordeaux proved less efficacious than two treatments. In the next two seasons, however, one November treatment of 10-10-100 bordeaux gave adequate protection. On the whole, proper timing of the bordeaux treatment seemed more important than renewing the fungicide deposit.
Ferric dimethyl dithiocarbamate and zinc dimethyl dithiocarbamate sprays containing an oil supplement (“sticker”) reduced infection materially but not so effectively as bordeaux.
Yellow cuprous oxide in a two-treatment program reduced infection satisfactorily but injured the leaves.
Lime-sulfur (3 per cent) controlled leaf spot as effectively as 10-10-100 bordeaux.
Persistent effects of treatments were sometimes noted. In several orchards trees sprayed with bordeaux or lime-sulfur in only one season bore less leaf spot at the end of the next season than did unsprayed trees.
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