Spilocaea oleaginea

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Spilocaea oleaginea is a fungus that causes olive leaf spot, also called olive peacock spot or bird’s eye spot. It mainly attacks olive trees and can affect trees on every continent where olives are grown. All olive varieties can be infected, but some are more susceptible than others. Young leaves tend to show more symptoms than older ones.

What it looks like and how it spreads
- In late spring, small dark spots appear on the upper leaf surface, usually with a yellow halo. Spots grow to about 0.1–0.5 inches (0.25–1.27 cm) across and become the sites where the fungus produces spores.
- The disease can cause leaves to fall off, weaken twigs, reduce blooming, and lower oil yield. Sometimes stems or fruit are affected, but leaves are most often the problem.
- The fungus mainly spreads by conidia (spores) carried by wind and rain, and possibly by insects. It can survive through winter and resume infection in the spring. Young leaves are most vulnerable.

Life cycle and environment
- Spilocaea oleaginea is a deuteromycete, meaning no sexual stage has been observed (if one exists, it would belong to Venturia).
- The disease needs free moisture on leaves to germinate, and it grows best at about 58–75°F (14–24°C), though it can survive in a wider range (2–27°C / 35–80°F).
- It is most active in mild, humid conditions. Fog, dew, and shaded, low-lying areas help it spread. Hot, dry summers can slow it down, sometimes causing leaves to crust and fall, which can help control the disease naturally.

Management
- There is no cure once infection is seen in spring or when fruit is on the tree. The goal is to keep the level of the inoculum low with preventative practices.
- Copper-based sprays are the main management method: apply after fall harvest and again in late winter if the area is very wet. A high-pressure sprayer helps cover the entire leaf surface, including interior parts of the tree.
- Other copper formulations include copper hydroxide, copper oxychloride, tribasic copper sulfate, and copper oxide; some are allowed in organic farming.
- Non-copper fungicides exist, such as Spotless, which is used monthly between harvest and flowering.
- No olive variety is completely resistant, but some have partial resistance indicated by certain genetic markers.
- Estimated losses can reach 10–20% of fruiting wood in heavily infected trees, and in some orchards the economic impact can be significant.

Overall, olive leaf spot is a widespread, climate-dependent disease best managed with preventative copper sprays and good orchard practices to keep infection levels low.