How to Manage Pests
UC Pest Management Guidelines
Pathogen: Spilocaea oleaginea
(Reviewed 3/14, updated 3/14)
In this Guideline:
Symptoms and Signs
Peacock spot appears on leaves as sooty blotches that develop into black, circular spots about 0.1 to 0.5 inch (2.5–12 mm) in diameter. There may be a yellow halo around the spot. Infection occurs in the fall, with temperatures between 35oF and 80oF. The optimum temperature is 58oF to 75oF. About 48 hours of free moisture allows the germinating spores to infect the leaf and cause significant disease. The warmer the temperature (up to about 75oF), the shorter the time necessary for infection to occur; it can be as little as 14 hours. The pathogen also infects fruit and fruit stems, but lesions are observed most often on upper leaf surfaces of leaves low in the tree canopy. Leaves fall prematurely. When significant defoliation occurs, strong bloom fails to develop and crop production is substantially reduced. Twig death may occur as a result of defoliation, and productivity is eventually further reduced.
Comments on the Disease
This disease occurs throughout California's olive-growing regions, but coastal areas with high rainfall suffer the most. Cultivars vary in their susceptibility to this disease, but all are subject to infection.
Outbreaks are sporadic, and the disease may take several years before it becomes serious enough to cause economic damage. Not all infected leaves fall from the tree, and the fungus survives in those that remain on the tree. The margins of these lesions enlarge in fall, and a new crop of spores develops there. New infections are associated with rainfall and mostly occur during fall and winter. By summer, most diseased leaves have fallen from the trees, leaving partially defoliated shoots with mostly healthy foliage remaining. High temperatures restrict spore germination and growth, thus the disease is inactive during the warm, dry summers in California.
If olive trees exhibit peacock spot leaf symptoms, it is important to treat for it yearly. Apply a preventive treatment in late October before winter rains begin, and again in the spring if wet weather persists.
UC IPM Pest Management Guidelines: Olive
L. Ferguson, Pomology, UC Davis