Frogeye Leaf Spot

Frogeye leaf spot refers to three different fungal diseases. One attacks soybeans, one infects peppers, and the other prefers your apples. Let’s look at all three, shall we?

Frogeye leaf spot on tobacco (Clemson University - USDA CESS, Bugwood.org) CC BY 3.0

Frogeye leaf spot in apples

Frogeye leaf spot in apples is caused by Botryosphaeria obtusa. This pathogen is also responsible for cankers in apples, cranberries, and quince, and dead arm disease in grapevines. Apple leaves infected with frogeye leaf spot develop purple specks that expand into brown spots with purple margins, hence the name. Left to progress, frogeye leaf spot causes black rot in your apples.

Black rot in apple (University of Georgia Plant Pathology, University of Georgia, Bugwood.org) CC BY 3.0

The only treatment for frogeye leaf spot in apples is to prune out infected branches, sanitizing your pruners between each cut. Infected plant material should be thrown in the trash and not composted. To prevent this disease from continuing, all fruit should be removed from the tree. As a gardener who loves her fresh apples, this can be a sad fact. Hopefully, those apples are ripe enough to eat and use for applesauce. The fungus won’t hurt you, and you can cut out the bad bits, but this disease can cause significant leaf and crop loss.

Frogeye leaf spot in peppers

Frogeye leaf spot in peppers is caused by Cercospora capsici fungi. Also known as Cercospora leaf spot, this disease infects eggplant and tomatoes, along with peppers. It starts as tiny, grayish-brown spots with dark margins on leaves, petioles, and stems. These spots eventually grow to one-half inch in diameter. If you look at one of those lesions with a hand lens or microscope, you will see tiny black flecks. Those flecks are fungal spores.

Frogeye leaf spot symptoms on pepper leaf (David B. Langston, UGA, Bugwood.org) CC BY 3.0 us

Frogeye leaf spot in soybeans

Frogeye leaf spot in soybeans is caused by Cercospora sojina. This disease is found in many parts of the world and has been expanding its range northward from the southern US. Frogeye leaf spot in soybeans causes small, somewhat rounded, or angular reddish-brown to purple lesions on upper leaf surfaces. The interior of these lesions is often gray or tan. As the disease progresses, leaves become tattered and fall off. Pods and stems can also become infected and covered with dark-rimmed lesions with reddish-brown centers. The lesions also produce ethylene gas which increases leaf loss. And nobody wants rotting soybeans.

Frogeye leaf spot lesions on soybean leaf (Maggilautaro) Public Domain

This disease can occur at any time during a soybean plant’s lifetime and several different stages of the disease may occur at the same time on different plants. The pathogen overwinters as mycelium in the soil, seeds, and plant residue, and remains viable for up to two years.

In each case, frogeye leaf spot is most likely to occur in situations with warm, humid conditions or long rainy seasons. To reduce the chance of frogeye leaf spot occurring in your garden, use these tips:

• Invest in certified disease-free, resistant seeds.
• Quarantine new plants.
• Space plants with mature sizes in mind and prune for good airflow.
• Irrigate at ground level.
• Avoid working around wet plants.
• Remove infected plants and dispose of them in the garbage.
• Employ 3-year crop rotation with corn or some other cereal crop.
• Till infected areas after plants are removed to expose mycelium.

Fungicides can also be used, but the frogeye leaf spot pathogens have already developed resistance to some of those treatments (strobilurins). As temperatures continue to rise, frogeye leaf spot is expected to become a more common problem.