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Publication #PP267

Cercospora Leaf Spot of Rose1

Jozer Mangandi and Natalia A. Peres2

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 affects roses commonly, 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 http://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 (Fig. 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 (Fig. 1b). At this point, the center of the spots turns tan to almost gray as the cells become brown and die.

In advanced necrotic lesions, groups of small tufts of conidiophores can be found. Conidiophores develop from masses of fungal tissue called stroma (Fig. 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 (Fig. 2b).

Figure 1a. 

Leaves infected with Cercospora rosicola.


Credit:

J. Mangandi, UF-GCREC


[Click thumbnail to enlarge.]

Figure 1b. 

Cercospora leaf spot with typical circular lesion and a necrotic center, 10x.


Credit:

J. Mangandi, UF-GCREC


[Click thumbnail to enlarge.]

Figure 2a. 

Conidiophores


Credit:

J. Mangandi, UF-GCREC


[Click thumbnail to enlarge.]

Figure 2b. 

Conidia of Cercospora rosicola, 400x.


Credit:

J. Mangandi, UF-GCREC


[Click thumbnail to enlarge.]

Control

Research trials have shown that Cercospora leaf spot is not significant when programs to control black spot and powdery mildew are used. Of 25 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 and 2. 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 professional pesticide applicators for control of Cercospora leaf spot on roses

Active ingredient

Fungicide group

Trade name

Copper oxychloride

M1

Agri Star® COC DF, Agri Star® COC WP

Copper oxychloride + Copper sulfate

M1

C-O-C-S® WDG

Sulfur

M2

Arysta Sulfur 6L, Micro Sulf®, Microthiol® Disperss®

Mancozeb

M3

Dithane® 75DF Rainshield®, Fore™ 80WP Rainshield®, Koverall™, Penncozeb™ 75DF

Maneb

M3

Maneb 75DF, Maneb 80WP

Chlorothalonil

M5

Daconil Ultrex® Turf Care®, Daconil Weatherstik®, Echo® 720 T&O, Echo® Zn T&O, Ensign™ 720, Ensign® 82.5% T&O, Initiate® 720, Prokoz Mainsail™ 6.0 F, Prokoz Mainsail™ WDG

Chlorothalonil + thiophanate-methyl

M5+1

Spectro™ 90WDG T&O

Thiophanate-methyl

1

3336® F, Nufarm T-Methyl SPC 4.5 F, Nufarm T-Methyl SPC 50 WSB, Quali-Pro® TM 85 WDG

Propiconazole

3

Fitness™, Nufarm Propiconazole SPC 14.3 MEC, Procoz Mainsail™ 6.0 F, Procoz Mainsail™ WDG, Quali-Pro® Propiconazole 14.3

Thiophanate-methyl + Iprodine

1 + 2

Nufarm TM+IP SPC

Bacillus subtilis

NC

Cease®

Potassium bicarbonate

NC

Milstop®

Reynoutria sachalinensis

NC

Regalia®

Neem oil

NC

Trilogy®

Fungicide Group (FRAC Code): Numbers (1-37) and letters (M) are used to distinguish the fungicidal mode of action groups. All fungicides within the same group (with same number or letter) indicate same active ingredient or similar mode of action. This information must be considered in making decisions about how to manage fungicide resistance. M=Multi-site inhibitors, fungicide resistance is low; NC= not classified. Source: http://www.frac.info/ (Fungicide Resistance Action Committee, FRAC).

Always read a current product label before applying any chemicals.

Table 2. 

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

Active ingredient

Fungicide group

Trade name

Copper hydroxide

M1

Hi-Yield® Copper

Copper Sulfate

M1

Bonide® Copper Dust

Copper Octanoate

M1

Bonide® Liquid Copper, Natural Guard Copper Soap

Sulfur

M2

Bonide® Sulfur, Ferti-lome® Dusting Sulfur, Hi-Yield® Dusting Wettable Sulphur

Mancozeb

M3

Bonide® Mancozeb

Chlorothalonil

M5

Bonide® Fungonil, Ferti-lome® Broad Spectrum, Hi-Yield® Vegetable, Flower, Fruit and Ornamental Fungicide, Ortho® Disease B Gon™ Garden Fungicide, Monterey Fruit Tree, Vegetable & Ornamental Fungicide

Myclobutanil

3

Spectracide Immunox® Multi-Purpose Fungicide

Propiconazole

3

Ferti-lome® Liquid Systemic Fungicide, Bonide® Infuse

Neem Oil

NC

Monterey Neem Oil, Natural Guard Neem Concentrate, Green Light® Neem Concentrate, Green Light® Rose Defense®

Acetamiprid + Triticonazole

NC + 3

Ortho® Bug B Gon® Insect & Disease Control, Ortho® Rose & Flower insect & Disease Control

Fungicide Group (FRAC Code): Numbers (1-37) and letters (M) are used to distinguish the fungicidal mode of action groups. All fungicides within the same group (with same number or letter) indicate same active ingredient or similar mode of action. This information must be considered in making decisions about how to manage fungicide resistance. M=Multi-site inhibitors, fungicide resistance is low; NC= not classified. Source: http://www.frac.info/ (Fungicide Resistance Action Committee, FRAC).

Always read a current product label before applying any chemicals.

Footnotes

1.

This document is PP267, one of a series of the Plant Pathology Department, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida. Original publication date May 2009. Revised July 2012. Visit the EDIS website at http://edis.ifas.ufl.edu.

2.

Jozer Mangandi, graduate student, Department of Horticultural Sciences; Natalia A. Peres, associate professor, Department of Plant Pathology, Gulf Coast Research and Education Center (REC)--Balm; Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida, Gainesville, FL 32611.

The use of trade names in this publication is solely for the purpose of providing specific information. UF/IFAS do not guarantee or warranty the products named, and references to them in this publication do not signify our approval to the exclusion of other products of suitable composition. All chemicals should be used in accordance with directions on the manufacturer's label.


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U.S. Department of Agriculture, UF/IFAS Extension Service, University of Florida, IFAS, Florida A & M University Cooperative Extension Program, and Boards of County Commissioners Cooperating. Nick T. Place, dean for UF/IFAS Extension.