Cercospora leaf spot is a fungal genus that contains members that are diseases to a wide variety of plant species. When the fungus Cercospora rosicola infects roses, it may induce premature defoliation if the infection is severe enough. Cercospora leaf spot is sometimes confused with black spot disease, which is a bacterial infection.
Despite the fact that the symptoms are similar and Cercospora leaf spot is a lesser-known foliar disease of roses in Texas, it is a very prevalent foliar disease of roses in the state.
Cercospora leaf spot Symptoms
An early indication of Cercospora leaf spot is the emergence on the leaves of small maroon to purple spots or lesions (Fig. 1) that vary in size and color from one leaf to the next (approximately 1 centimeter). When compared to the black spot of roses, the borders of the lesions are smooth, as opposed to the fringed or feathered appearance generated by the disease.
As the condition advances, the centers of the spots become gray or tan in color, but the borders of the spots stay maroon to dark purple in color. Leaves that have been severely diseased frequently develop chlorosis (yellowing), which causes them to fall off prematurely. Cercospora leaf spot is a disease that often starts on the lower leaves and moves up the plant. Because each rose cultivar has a different level of tolerance to this disease, the severity of the symptoms might differ from one another.
Cause of Cercospora leaf spot
C. rosicola only affects the leaves of the plant; damaging lesions on other plant components are not often seen. The most sensitive leaves to infection are those that are just beginning to grow. For most of the year in Texas, Cercospora leaf spot appears a bit sooner or at the same time as black spot disease, but it may appear at any point during the growing season. Conditions that are wet, moderate to warm, and conducive to disease growth and the generation of spores that disseminate via the air are ideal.
The presence of free moisture (i.e., water on leaves) aids in the initiation of spore germination and subsequent infection. As the infection progresses, fruiting bodies (where new spores form) will eventually emerge within the lesions (Fig. 2).Under magnification, tufts of spores on the surface of these lesions may be seen when the environment is humid.
Cercospora Fungi Management
When methods to manage black spot and powdery mildew are utilized, research studies have demonstrated that Cercospora leaf spot is not a severe problem. In Alabama, twenty-five rose cultivars were evaluated, and variations in susceptibility to black spot and Cercospora leaf spot were detected amongst the cultivars studied. However, only two cultivars, Petite Pink Scotch and The Fairy, exhibited chronic and severe signs of Cercospora leaf spot, whilst all other cultivars were vulnerable to both diseases, with the majority of cases being black spot.
Taking preventative measures is the most effective method of dealing with Cercospora leaf spot. The following are some examples of cultural behaviors that may aid in the prevention and reduction of illness incidence:
- In order to limit free moisture, avoid overwatering or watering in the late afternoon.
- Avoid watering from above since the water might dislodge and disseminate spores to uninfected plants, which is dangerous.
- Plants should be spaced apart to promote air flow and prevent excessive humidity levels.
Cercospora leaf spot may be controlled with the use of fungicides. Most traditional treatments used to prevent black spot on roses will also provide protection against Cercospora leaf spot, which is caused by a fungus. The active ingredients in these fungicides are chlorothalonil (OrthoMax Garden Disease Control) and myclobutanil (OrthoMax Garden Disease Control) (Immunox).