Be Aware: Cercospora Leaf Spot of Vegetables

Published: June 15, 2021

Figure 1 Circular leaf spots with tan or brown necrotic centers surrouded by chlorotic areas of a cumumber leaf.

As the temperature goes up, we may finally feel relieved from the damages of some plant pathogens that are favored by cool and wet weather, such as anthracnose or downy mildew, but the incidence of another foliar disease, cercospora leaf spot, is rising recently. This disease is caused by a big group of fungal pathogens that belong to a genus, Cercospora. At present, the MU Plant Diagnostic Clinic staff has identified this disease from Juniper, red clover and cucumber samples. Many people may have heard about cercospora diseases of soybean because there are two major cercospora diseases: Cercospora leaf blight of soybean caused by Cercospora kukuchii, and frogeye leaf spot caused by Cercospora sojina. However, with current warm and wet weather condition, we found that this disease was causing more problems to many other plant hosts, especially vegetables such as cucurbit, beans, beet and tomatos.

Symptoms of cercospora leaf spot vary among different vegetable hosts. On cucurbits such as watermelon, melon and cucumber, this disease primarily affect the leaves and cause circular leaf spots with tan or brown centers (Figure 1). The spots may first show up in the older leaves with small specks with yellow rings and as the disease progresses, the lesions grow gradually in size and the areas surounding the lesions turn to yellow and chlorotic. The lesions then spread to other areas of the leaf, leading the yellowing and ultimately death of the entire leaf. Cercospora spp. can also cause frogeye leaf spot on peppers, which are circular and watersoaked spots with red or purple border. These spots develop into bigger lesions over time and can spread to petioles and stems, leading to both defoliation and girdling of the stems. In contrast to leaf spots on cucubits and peppers, the cercospora disease on tomato is called cercopora leaf mold, which is not very common in North America. It can cause interveinal lesions with light-gray to black centers. The lower side of the leaves are where this disease first show up. Eventually, the infected leaves start to bend and curl as the disease progresses. The symptoms are easy to be confused with downy mildew as the they all show up in the lower side, but the color of patches of downy mildew is much lighter than leaf mold.

Figure 2 Cecospora conida (translucent,elongated and segmented) and conidiophores (melanized and elongated) under microscope 400×.

Cercospora spp. produces conidia and conidophores in the center of the leaf spots. Conidiophores are straight or slightly bent and melanized in color. The mophorlogy of conidia is very characteristic as they are hyaline, straight and segamented (Figure 2). They were produced on the conidiophores in a way like a cluster of needles, which can be observed by a hand len or microscope. The conidia are disseminated by wind, sprinkler water, splashing rain, pruning tools and human activity. The temperature of 25°C to 35°C with high humidity favors the infection and the newly produced conidia re-infect the plants very rapidly.

Similar with the control practices for Anthracnose disease, sanitation is the major method. The removal of the infected leaves and stems will effectively help to reduce the population of inoculums. Crop rotation is also recommended if the disase pressure is very high and it can also help to eradicate the source of inoculmns. Please use the resistant varieties when they are available. When using fungicides for the preventative programs in controlling this diseases, please make sure to follow label directions and use fungicides that can also control other leaf diseases, such Alternaria leaf spots, anthracnose and Bipolaris leaf spots.

For appropriate diagnosis, the MU Plant Diagnostic Clinic can help you confirm if your plant has this disease. We encourage you to visit our website (https://extension.missouri.edu/programs/plant-diagnostic-clinic) and review submission guidelines before submitting your sample. If possible, you may take photos and send them to plantclinic@missouri.edu.

We just uploaded a new webinar about sample submission guidelines on YouTube to help you submit your sample step by step.
Please click here: https://www.youtube.com/watch?v=4dUcYKKFwaI

For sample submission and fee payment, you can either:

1) Visit our new online submission system at https://extension.missouri.edu/services/plant-disease-sample. Fill out the submission form online using your computer or mobile device and make payment online securely with a credit card.

2) Download the submission form at https://extension.missouri.edu/programs/plant-diagnostic-clinic/sample-submission. Fill it out and send to us together with your sample and payment. Check or money order. No cash please.

Contact Information:

University of Missouri-Plant Diagnostic Clinic
28 Mumford Hall
Columbia, MO 65211
Phone: 573-882-3019
Email: plantclinic@missouri.edu