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Cercospora Leaf Spot of Rose

Jozer Mangandi and Natalia A. Peres

Introduction

The primary foliar diseases of roses are black spot (caused by Diplocarpon rosae), powdery mildew (caused by Podosphaera pannosa), and Cercospora leaf spot (caused by Cercospora rosicola). Cercospora leaf spot has been little investigated, especially on varieties that belong to the groups of shrubs and ground cover roses. Although C. rosicola commonly affects roses, its impact is reduced when control measures for diseases such as black spot and powdery mildew is conducted. Other fungi such as Alternaria alternata, Colletotrichum capsici, and Glomerella cingulata can also cause leaf spots on roses.

Causal Agent and Geographical Distribution

Fungi of the genus Cercospora are parasitic and infect a broad range of herbaceous plants. The main species affecting roses is Cercospora rosicola (Mycosphaerella rosicola, sexual stage). C. rosicola is distributed worldwide and was first reported on rose leaves in Florida in 1885.

Symptoms

Cercospora leaf spot is a disease often confused with black spot. Both diseases cause severe defoliation in heavily infected plants. The infection starts from the bottom of the canopy and progresses towards the tips where new growth is present. Lesions are primarily found in leaves but also in pedicels, stems, fruits, and bracts. (See EDIS publication Black Spot of Rose at https://edis.ifas.ufl.edu/PP268).

Symptoms of Cercospora leaf spot are circular spots usually 2–4 mm in diameter, but single spots can be as large as 10 mm in diameter (Figures 1a, 1b). The size is variable depending on the species or cultivar on which the lesions occur. When symptoms begin to appear, a small purplish area becomes apparent. In older lesions a small necrotic area develops and increases in size as the disease progress (Figure 1b). At this point, the center of the spots turns tan to almost gray as the cells become brown and die.

 

Figure 1a. Leaves infected with Cercospora rosicola.
Figure 1a.  Leaves infected with Cercospora rosicola.
Credit: J. Mangandi, UF/IFAS

 

 

Figure 1b. Cercospora leaf spot with typical circular lesion and a necrotic center, 10x.
Figure 1b.  Cercospora leaf spot with typical circular lesion and a necrotic center, 10x.
Credit: J. Mangandi, UF/IFAS

 

In advanced necrotic lesions, groups of small tufts of conidiophores can be found. Conidiophores develop from masses of fungal tissue called stroma (Figure 2a). Stromata are dark brown and appear as black dots over the necrotic area of the leaves. Under the microscope, cylindrical, almost straight, septate conidia can be observed (Figure 2b).

 

Figure 2a. Conidiophores.
Figure 2a.  Conidiophores.
Credit: J. Mangandi, UF/IFAS

 

 

Figure 2b. Conidia of Cercospora rosicola, 400x.
Figure 2b.  Conidia of Cercospora rosicola, 400x.
Credit: J. Mangandi, UF/IFAS

 

Control

Research trials have shown that Cercospora leaf spot is not significant when programs to control black spot and powdery mildew are used. Of twenty-five rose cultivars tested in Alabama, differences in susceptibility to black spot and Cercospora leaf spot were observed. All cultivars were susceptible to both diseases, predominantly black spot, but only two cultivars, Petite Pink Scotch and The Fairy, showed persistent, severe symptoms of Cercospora leaf spot.

The shrub rose 'Fuchsia Meidiland'® was reported as a susceptible cultivar in Alabama and North Carolina. In an experiment conducted to evaluate commercial fungicides for the control of Cercospora leaf spot in this cultivar, it was concluded that products such as Compass™ and Daconil Ultrex® applied weekly as well as Eagle® and Heritage® applied twice monthly reduced severity of this disease to just few spots on the lower leaves.

Scheduled applications used to control black spot with fungicides such as Daconil Weather Stik®, Immunox®, and Halt® also provide control of Cercospora leaf spot. Fungicides labeled for control of Cercospora leaf spot of roses in Florida are listed in Tables 1, 2, and 3. For managing fungicide resistance, products with different modes of action should be used in rotations. All fungicides within the same group (with same number or letter) indicate the same active ingredient or similar mode of action. Fungicide resistance is usually low with multi-site inhibitor fungicides (group M).

Tables

Table 1. 

Fungicide products marketed for use by professional pesticide applicators for control of Cercospora leaf spot on roses.

Table 2. 

Fungicide products marketed toward homeowners for control of Cercospora leaf spot on roses.

Table 3. 

Biopesticides registered to control Cercospora spot on roses.

 

 

Publication #PP267

Date: 10/1/2018

  • Program Area: Plant Systems
Fact Sheet

About this Publication

This document is PP267, one of a series of the Plant Pathology Department, UF/IFAS Extension. Original publication date May 2009. Revised July 2012 and December 2015. Reviewed September 2018. Visit the EDIS website at https://edis.ifas.ufl.edu for the currently supported version of this publication.

About the Authors

Jozer Mangandi, Ph.D., former graduate student, Department of Horticultural Sciences; and Natalia A. Peres, professor, Department of Plant Pathology, UF/IFAS Gulf Coast Research and Education Center; UF/IFAS Extension, Gainesville, FL 32611.

Contacts

  • Natalia Peres