By: Kevin Ong and Ashley Brake
Members of the fungal genus Cercospora are pathogens of many types of plants. On roses, the fungus Cercospora rosicola can cause premature defoliation when infection is severe. Cercospora leaf spot is often mistakenly identified as black spot disease.
Although the symptoms are similar and Cercospora leaf spot is not as well known, it is a fairly common foliar disease of roses in Texas.
An early symptom of Cercospora leaf spot is the appearance on the leaves of tiny maroon to purple spots or lesions (Fig. 1) that vary in size (approximately 1 centimeter). The edges of the lesions are smooth, as opposed to the fringed or feathered look caused by black spot of roses.
As the disease progresses, the center of the spots turns gray or tan while the margins remain maroon to dark purple. Severely infected leaves usually develop chlorosis (yellowing), which leads to premature defoliation. Cercospora leaf spot often begins on the lower leaves and progresses upwards. The severity of the symptoms can vary depending on the resistance of the rose cultivar to this disease.
C. rosicola affects the plant leaves; damage lesions are not often found on other plant parts. Newly developing leaves are the most susceptible to infection. In Texas, Cercospora leaf spot tends to begin in the spring a little earlier or about the same time as black spot disease, although it can occur anytime during the growing season. Wet, mild to warm conditions encourage disease development and the formation of spores that spread through the air. Free moisture (presence of water) on foliage helps initiate spore germination and subsequent infection. Eventually, fruiting bodies (where new spores develop) will appear within the lesions (Fig. 2). Under humid conditions, tufts of spores on these lesions are visible under magnification.
Preventive measures are the best approach to managing Cercospora leaf spot. Cultural practices that help prevent and reduce disease incidence include:
- Avoid overwatering or watering in the late evening to reduce free moisture.
- Avoid overhead watering where the water can dislodge and disperse spores to uninfected plants.
- Space plants to encourage air movement and reduce high humidity levels.
Fungicides are available to manage Cercospora leaf spot. Many of the conventional products used to prevent black spot of roses will also protect against Cercospora leaf spot. These fungicides contain the active ingredient chlorothalonil (OrthoMax Garden Disease Control) and myclobutanil (Immunox).
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