Cause The fungus, Venturia oleaginea (anamorph: Spilocaea oleagina), occurs across the world where olives are grown. The fungus is thought to primarily reproduce through the production of conidia (asexual spores) and survives as infected leaves that remain on the tree. Conidia are moved by water and wind. Disease is promoted by wet conditions and moderate temperatures (41 to 77°F). After infection, a latency period of 120+ days may occur during cold or hot, dry conditions. Olive leaf spot can cause severe reductions in yield.
Symptoms Affected leaves initially exhibit brownish round spots that are less than a half inch in diameter. Leaf spots eventually develop olive-green, gray, and dark brown concentric rings with a yellow halo surrounding the leaf spot. Leaf spots can coalesce when disease pressure is severe. A brown coloration may develop on the central vein on the underside of leaves. Trees can defoliate, especially in the lower canopy, with branch dieback. Spots can develop on the stems of fruit (pedicel) or the developing fruit themselves, leading to fruit drop.
Cultural control
- Select sites with good air movement and lower relative humidity.
- Plant resistant varieties such as Arbequina and others listed in the Buonaurio et al. review at https://doi.org/10.3389/fpls.2022.1061136 and at http://www.oleadb.it.
- Employ pruning and irrigation practices that decrease the relative humidity within the tree canopy.
- Avoid over-fertilization with nitrogen. Avoid potassium deficiencies.
Chemical control
- Copper formulations (Group M1) are registered for use.
- Hidro Cup at 5 to 12 lb/A on 30-day intervals. Make first application before fall rains commence. 48-hr reentry. O
Reference Buonaurio, R., Almadi, L., Famiani, F., Moretti, C., Agosteo, G.E., and Schena, L. 2023. Olive leaf spot caused by Venturia oleaginea: An updated review. Frontiers in Plant Science 13: https://www.frontiersin.org/journals/plant-science/articles/10.3389/fpls.2022.1061136.