How to Manage Pests
UC Pest Management Guidelines
Pathogen: Mycocentrospora (=Cercospora) cladosporioides
(Reviewed 3/14, updated 3/14)
In this Guideline:
Symptoms and Signs
M. cladosporioides causes the leaves to appear slightly chlorotic (yellowing—some varieties show more chlorosis than others). The undersides of some leaves become discolored with the conidial stage of the fungus, which appear to be covered with black dust. These leaves may fall, causing some defoliation in some cases. Fruit can also develop small, brown lesion spots and not mature uniformly.
Comments on the Disease
This disease has been documented in coastal areas with humid growing conditions. The disease cycle seems to be similar to that of peacock spot.
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. Leaf lesions on these infected leaves have a white, crusty appearance. 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 winter. By summer, most diseased leaves have fallen from the trees, leaving partially defoliated shoots with mostly healthy foliage remaining at the tips. High temperatures restrict spore germination and growth, thus the disease is inactive during the warm, dry summers in California.
In coastal areas of Europe, where experience with this disease has been greater than in California, M. cladosporioides is more difficult to control than peacock spot and requires more stringent treatment. In cool, wet areas, apply preventive treatments to olive trees after harvest, but before winter rains begin and again in spring, if rainy weather persists.
UC IPM Pest Management Guidelines: Olive
L. Ferguson, Pomology, UC Davis