How to Treat Leaf Spot Fungus

Many kinds of plants, including trees, shrubs, vegetables and grass, are susceptible to leaf spot fungus. The presence of small round or oval dead spots scattered across a plant's leaves is usually an indication of leaf spot fungus. Treating the plant can help prevent further damage, which can cause leaves and flowers to die, and, if left unchecked, even stunted growth in trees and shrubs.

Leaf spot fungus is caused by dozens of fungi in genera such as Entomosporium, Gnomonia, Venturia, Discochora, Alternaria, Gloeosporium, Elsinoe and Coryneum. Although the main symptom of leaf spot fungus is the many dead spots that appear on some of a plant's leaves, each kind of infection may have a slightly different appearance. The spots may appear as scattered spots on only the tops of leaves, only the bottoms or on both sides. The spots may be yellow, tan, gray, dark brown, black or purple and may be shiny raised spots or uneven holes. Spots may show up on only a small number of leaves or take over much of the plant.

Leaf spots can change over time, not always looking the same as they did at first. Small spots may develop margins of red, yellow, brown, purple or black. The spots may get larger as the growing season progresses, and they may merge, creating large, uneven dead areas on leaves. Seriously infected leaves can turn brown, withering and dying, which reduces the amount of food the plant can make, weakening it and making it vulnerable to pests and other diseases.

Once leaf spot fungus appears on leaves, the infected leaves can’t be healed, but you can prevent the fungus from spreading. University of Connecticut’s Integrated Pest Management Program advises that it is not always necessary to treat leaf spot fungus, however, because it doesn’t cause much harm to healthy, established plants.

Plants that were recently moved, that are very young, that struggle with insect infestation or that are otherwise stressed may benefit from treatment.

A fungicide that has a copper base won’t kill existing spores, but it will stop the disease from spreading by keeping the fungal spores from germinating. Apply a fungicidal spray in a series, with the first treatment at bud break; then apply the treatment again in one to two weeks. Additional applications may be necessary in wet weather. Before choosing a fungicidal spray, get help in identifying the fungus you’re fighting because so many fungi can cause leaf spot and their treatments vary. Put a sample affected leaf in a plastic bag, and take it to a garden center for help.

Usually grown as annuals, tomato plants (Solanum lycopersicum) around the world are affected by leaf spot fungus caused by Septoria lycopersici. According to Cornell University, the disease can be treated with one of several fungicides approved for tomatoes, including fungicides with the active ingredient mancozeb.

Fungicides such as mancozeb can be toxic if inhaled and can cause skin and eye irritation. Follow all label directions, and wear protective gear when handling, mixing or applying a fungicide.

Protective gear, including goggles, breathing mask, a long-sleeved shirt, long pants, hat, rubber boots and rubber gloves

Liquid fungicide concentrate that is 37 percent mancozeb

Teaspoon that won't be used for food

Bucket

Handheld, pump-type garden sprayer

Put on protective gear, including goggles, a breathing mask, a long-sleeved shirt, long pants, a hat, rubber boots and rubber gloves.

Mix 3 to 5 teaspoons of a liquid fungicide concentrate that is 37 percent mancozeb with 1 gallon of water in a bucket. Use 3 teaspoons of the concentrate per 1 gallon of water to treat leaf spot fungus on small plants and up to 5 teaspoons of the concentrate per 1 gallon of water to treat larger plants or those with a severe infection.

Pour the mixture into a handheld, pump-type garden sprayer. Tighten the garden sprayer's lid in place. Pressurize the sprayer tank by pumping it.

Spray the mixture on leaf spot fungus-affected plants' leaves until the leaves are coated with the liquid. The treatment works best on newly emerged seedlings and newly established transplants.

Repeat the application once every seven to 10 days until a minimum of five days prior to harvest, depending on the plant variety. Follow your specific fungicide's label information regarding applications and harvest for your kinds of plants.

Rinse out the garden sprayer, including the tank, nozzle and hoses, with water. Allow all parts to air-dry. Never store chemicals in the sprayer.

Leaf spot fungus thrives in cool, damp conditions and is most prevalent in plants that don’t get a lot of air circulation, especially during periods when new leaves grow. Promote healthy trees and shrubs by thinning out dense crowns or canopies and removing all diseased, weak, dead or insect-infested branches. Also cut out branches that rub against or grow across other branches.

Sterilize your pruning implements before you begin pruning as well as between plants and when you are done pruning to avoid transferring plant diseases, including leaf spot fungus.

Wear rubber gloves to keep bleach and the bleach-water solution from getting on your skin. Avoid splashing either item because both can harm skin, eyes and clothing.

To sterilize your pruning equipment, brush them off, removing dirt and other debris. Mix 3 parts water with 1 part household bleach in a bucket. Place the pruning tool blades into the bleach-water mixture, and leave them there for five minutes or longer. Remove the implement, and either rinse it with clean water or allow it to air-dry.

Keep the area around your plants clean by frequently removing dead leaves and fallen branches from infected plants. Doing so will help to reduce the amount of fungus in the area and assist in minimizing future infections.