In hot, wet, humid weather, you notice tiny brown or purple spots on your hydrangeas. Or on your roses, spinach, and junipers. Vegetables, shrubs, flowers, citrus fruits, and turf can all catch Cercospora leaf spot (CLS). With over 1200 species, this is the largest genus of fungus in the world, and it attacks a wide variety of plants.
The name changes depending on the plant this disease is on: Cercospora needle blight on junipers, Cercospora melongenae on eggplant, cercospora rosicola on roses, and on soybeans it goes by the common name “frog-eye leaf spot.” Whatever it’s called, CLS manifests the same way under the same conditions on all plants.
As the disease progresses, the spots grow larger until the leaves are completely covered. Defoliation occurs, which can lead to the death of the plant.
For plant diseases to take hold, conditions need to be “right.” The disease (pathogen) needs to be present, there needs to be a susceptible host (your prized plant), and there has to be the proper environment. This is called the Disease Triangle – but there is one more element needed: time for the disease to establish itself.
Cercospora is favored by hot, wet weather and is frequently found in the United States in the great plains north of Texas, the midwest and northeast, and it abounds in the warm, humid southeast.
Cercospora overwinters in plant debris. It is spread from wet leaf-to-wet leaf and plant-to-plant by wind, splashing water, and even garden tools. When temperatures are between 75°F and 95°F during the day, over 61°F at night, and the humidity is 90-95%, conditions are right for this disease to infect your plants.
Cercospora needs 11 to 16 hours of moisture to establish itself, with symptoms taking 5 to 21 days to appear. So your plants can be infected weeks before you realize it.
Conidia (spores) form and spread throughout the growing season, from late spring through the summer.
Sometimes Cercospora can be on seeds, and some weeds can host the disease and then spread to your garden. These weeds include bindweed, mallow, lambsquarters, and pigweed. Another reason to pull them out!
Identification and Damage
Cercospora begins as small purple or brown spots with a yellow outer ring on the leaves of plants. As the spots mature, they grow to about 1/8th inch in diameter and develop a light gray or tan center. A look with a magnifying glass at that center will reveal the black spores.
The spots continue to grow and sometimes will develop a rectangular or elliptical shape. As the spots grow, they meet up with other spots, and soon the whole leaf is blighted and falls off.
This disease is usually on the leaves but can be on the leaf petioles, seeds, and seed pods, on the bracts of flowers, and on fruits.
Damage is first seen on the mature, inner leaves – areas of the plant that do not dry quickly – and the disease moves from the inside out and the bottom up.
Not only can Cercospora be unsightly on ornamental plants, but it also weakens them. As the infected leaves fall, the plant loses its ability to make food which stresses the plant and allows other insect and disease problems to develop. Death can occur after a few seasons if left unchecked.
This disease also makes the fruits and vegetables it attacks inedible. This is a big problem for commercial growers – they can’t sell a crop of spinach covered in purple spots.
Sometimes Cercospora is mistaken for black spot. Black spot occurs on roses and has a distinctive feathery border to the edges of the spots. Remember that Cercospora is found on many different plant species. Both these diseases occur under similar conditions and treatment for both is also similar.
CLS also resembles pesticide damage and sun scorch.
With so many species of this fungus affecting so many plants, it is not possible to prevent this disease completely. And while we can’t control the temperature, rainfall, or relative humidity, there are steps we can take to keep Cercospora at bay and lessen its severity.
Proper spacing of plants allows for good airflow through the garden.
Plant resistant varieties. Breeders have developed resistant varieties of sugar beets and Crape Myrtles.
Avoid overhead watering. Don’t set up a sprinkler and let it run – hand water at the base of plants, or use drip irrigation or a soaker hose.
A good scouting program helps you spot disease and insect problems early, so you can take action before they become serious. When weather conditions are right, start looking for Cercospora on the leaves of your leafy vegetables, eggplant, and carrots; your roses and hydrangeas: your junipers and shrubs; and your perennials, fruit trees, and turf. Really… just look for it on any and every plant!
Many plants can survive a light infection – always start with cultural controls before moving on to fungicides.
Cultural controls are habits and methods that we use to prevent a disease from establishing itself. We try to disrupt the Disease Triangle.
Because Cercospora overwinters in plant debris, take care to clean up your garden at the end of the growing season.
Rotate your crops. Plant beets, spinach, and chard in a different place each year, only returning to the original spot in your garden after three years.
Avoid overhead watering. If you do water from overhead, water early in the day so that several hours of sunshine will dry the leaves.
Remove and dispose of infected leaves, and clean up fallen leaves. And pull those weeds, too.
Practice good sanitation and clean your tools after working with an infected plant.
When you chose to use chemical controls, remember that fungicides work to protect your plants from infection. They do not cure the disease but are applied to prevent it from spreading to healthy plant tissue. A spotted leaf will always be spotted, even though the pathogen is dead.
Neem oil and fungicides containing liquid copper are most often recommended. You might want to check with your county extension service or farm bureau before using a fungicide on Cercospora. This fungus can mutate quickly and can become resistant to a specific type of chemical control.
Bacillus amyloliquefaciens (BAA) is not a chemical control, but a biological one. BAA is a bacteria that is mixed with water and applied to the soil. It is used to control botrytis, fusarium, pythium, and root rot and can be used to control Cercospora.
Chemical controls need to be applied every 7 to 14 days. Be sure that the product you are using is labeled for Cercospora and for the plants you intend to treat. Follow the instructions exactly and carefully. Always use the appropriate personal protection equipment (PPE) and respect re-entry times.