Olive leaf spot facts for kids

Spilocaea oleaginea is a tiny fungus that causes a plant disease called olive peacock spot. This disease is also known as olive leaf spot or bird's eye spot. It mainly affects the leaves of olive trees all over the world.

This disease can harm olive trees throughout the year. It can cause problems like spots on the fruit and slower ripening. It also reduces how much olive oil can be made. If the disease is very bad, leaves can fall off, and small branches might even die. Over time, it can make the trees less healthy.

What Plants Get Sick?

The Spilocaea oleaginea fungus only infects olive trees. It can infect all types of olive trees, but some types get sicker than others. Younger leaves are more likely to show signs of the disease than older leaves.

What Does the Disease Look Like?

In late spring, you might see dark spots on the top of olive leaves. These spots are caused by the fungus. Later, the fungus will release its spores from these spots.

You might also see spots on the stems and fruit, but they are most common on the leaves. As the season goes on, the dark spots grow. They can become about 0.1 to 0.5 inches (0.25 to 1.25 cm) wide. A yellow ring, like a halo, often appears around each spot.

If the disease is severe, olive trees can lose many leaves. Small branches might even die. The trees might not produce many flowers, which means fewer olives for harvest.

The Fungus Life Cycle

Spilocaea oleaginea is a type of fungus that does not have a known sexual stage. If a sexual stage is ever found, it would belong to a group of fungi called Venturia.

The main body of the fungus, called mycelium, grows inside the olive leaf tissue. The spots you see on the leaves are where the fungus is growing.

The fungus spreads using tiny reproductive cells called conidia, which are a type of spore. These spores can travel in several ways. Insects and wind can carry them. Rainwater can also spread them from one leaf to another. A tiny insect called Ectopsocus briggsi, which is related to lice, is thought to help spread the disease.

Olive trees keep their leaves all year. The first time the trees get infected each year is usually in the fall. The fungus living in the leaf spots makes new spores. These spores then spread to healthy parts of the tree. Young leaves are easier for the fungus to infect than older ones. The fungus keeps making spores through winter and into spring.

During the hot, dry summer, the fungus becomes inactive, or "sleeps." It survives as mycelium inside the spots on living leaves. Leaves that have fallen to the ground can also spread the infection, but this is usually not a big problem.

Where Does the Fungus Grow Best?

Olive peacock spot disease is a problem for olive farms worldwide. It grows best in mild to cool temperatures and needs water on the leaves to start growing. This is why it usually infects trees in the fall, winter, and spring.

Hot and dry summer weather makes the fungus inactive. The leaf spots turn white and crusty. During summer, many diseased leaves fall off, leaving only healthy ones. This helps control the disease naturally. The disease also mainly infects young leaves in the spring.

Water on the leaves is very important for the spores to grow. This can happen in as little as 9 hours if the temperature is just right. It usually takes no more than 24 hours. Without water on the leaves, the spores cannot grow. The best temperature for the fungus is between 58 and 75°F (14 and 24°C). However, it can survive in temperatures from 35 to 80°F (1.5 to 27°C).

The surrounding environment can also affect how the disease spreads. It grows well in low areas or places that don't get much sunlight. Areas with dense tree canopies, fog, dew, and high humidity are also good for the fungus. In these conditions, the disease can even spread in summer.

Olive trees that don't get enough nutrients or have unbalanced nutrients in the soil might get sick more easily. Too much nitrogen or not enough calcium can make an olive tree weaker. However, trying to fix this with special plant food or compost tea has not worked well.

How to Manage the Disease

People try to control olive peacock spot disease by keeping the amount of fungus low. This means taking steps to prevent it. There is no way to cure the disease once it appears in spring or when the trees have fruit.

The most common way to manage it is to spray the leaves with a copper-based chemical. This is done after the fruit is picked in the fall. If the weather is very wet, it might be sprayed again in late winter. Using a powerful sprayer helps cover every part of each leaf, even inside the tree. If copper is sprayed on the fruit, it is very hard to wash off. So, if the harvest is late, the fruit might be lost to infection. The spray comes in different forms of copper, like copper hydroxide or copper oxide. Some of these are even approved for organic farming.

There are other chemicals available that don't contain copper, such as "Spotless." This is sprayed on the leaves every month between harvesting and when the trees flower.

No type of olive tree is completely safe from the fungus. However, some types are less likely to get sick than others. Scientists have found special genetic signs in partially resistant olive trees. This information can help growers choose new trees that are more resistant to the disease. Growers often list this information when they describe different olive tree types.

Why This Disease Matters

In olive trees that are very sick with olive peacock spot, about 10 to 20 percent of the branches that produce fruit can be lost. While the disease usually isn't deadly, it can cause ongoing problems. It can also lead to big money losses for some olive farms. This is important because olive farming covers a huge area of 8.5 million hectares (about 21 million acres) worldwide.