Cercospora Leaf Spot Disease

Cercospora leaf spot disease is caused by the fungus, cercospora. It is a common problem among farm crops and home garden plants that can cause severe damage, including reduced crop yield and defoliation. Cercospora leaf spot disease must be treated promptly and measures should be taken to prevent it from returning in later seasons.

Symptoms

Cercospora leaf spot initially presents itself as tiny circular or oval spots on plant leaves that enlarge with time. The spots can vary in color from dark green to brown with or without maroon or purple margins. In later stages of the disease, the leaves of flowers may become yellow and fall off the stems, and leaves on edible crops may become a brownish-gray color as they dry out and die completely.

Disease Cycle

Development of cercospora leaf spot disease begins when the fungus' spores are dispersed by rain, irrigation water and wind. Germination occurs in humid conditions, usually during late spring and summer, and fungus growth is encouraged by frequently damp leaves. Plants that mature in the fall may escape acute infection. In general, the more rain, the worse the disease spreads. Cercospora infests seeds and crop residue where it harbors over the winter.

Commonly Affected Plants

Several farm crops are affected by cercospora leaf spot disease, most notably sugar beets, carrots and soybeans. Other crops known to be vulnerable are spinach, coffee, corn, eggplant, pepper, tomato, rice and cotton. Garden plants and turf grasses can also contract cercospora leaf spot disease, including ground cover and shrub roses, hydrangea and St. Augustine grass.

Treatment of Existing Disease

Treating existing cercospora leaf spot disease on crops can be challenging due to the fungus' ability to live in ground residue and its dispersion method which is affected by uncontrollable weather conditions. In severe cases, professional chemical crop spraying may be necessary. Fungicides containing copper are effective, but may deposit offensive residue on the leaves, affecting marketability of the crop. Other chemical sprays that may treat cercospora leaf spot disease are azoxystrobin (Heritage) and myclobutanil (Systhane). These products cannot cure existing disease, but can prevent its spread to healthy foliage. Home gardeners may try applying fungicide products available at nurseries or home improvement stores, such as Cleary's 3336, Zyban or Immunox. Due to the expansiveness of lawns, turf grasses should be treated by a professional fungicide applicator.

Prevention

Three-year crop rotation can prevent reinfestation by the cercospora fungus. Overhead irrigation should be kept to a minimum, especially in humid conditions. Residue from harvested crops must be buried deeply to reduce winter harboring of cercospora. Home garden beds should be weeded frequently to cut down on hosts for cercospora, and water should be given to the plant roots in early morning hours, if possible. Cercospora leaf spot disease on turf grasses may be prevented by using a fertilizer blend balanced with nitrogen and potassium and by irrigating in the early morning hours on dry days only.

Resources

• University of Florida IFAS Extension
• Montana State University