By:Jackie Mullen,Plant Pathologist
Cercospora leaf spot is characterized by the appearance of numerous tiny maroon to purple oval spots scattered randomly across the leaf surface Later,the center of the spot turns tan to almost gray in color while the margin of the spot remains maroon to dark purple. Heavily spotted leaves will turn yellow and prematurely shed. Because Cercospora-damaged roses typically lose most of their leaves,this disease can easily be confused with blackspot.
Very little specific information is available concerning the development of Cercospora leaf spot on roses. However,the same warm wet weather patterns,which favor the spread and development of blackspot,will also start outbreaks of this disease. Beginning in early spring,spores of the causal fungus Cercospora are spread by air currents to the newly expanded leaves. Free water on leaf surfaces triggers spore germination and infection of the leaves. Frequent rain showers or possibly overhead watering favors continued disease spread until the first hard frost in the fall. Control Currently,selected shrub and ground cover roses appear to be the main targets of Cercospora leaf spot. The reaction of these roses to Cercospora leaf spot is summarized in the following table.
Reaction of Selected Shrub and Ground Cover Roses to Cercospora Leaf Spot
- Susceptible:Petite Pink Scotch,The Fairy,Fushia Meidiland,Carefree Delight,White Flower Carpet,Happy Trails,and Flower Carpet
- Highly Resistant to Immune:Jeepers Creeper,Ralph’s Creeper,Royal Bonica,Nearly Wild,Betty Prior,Sevillana,Magic Carpet,Easy Livin’,Cherry Meidiland,Pearl Meidiland,Rosa mutabulis,Rosa wichuraiana,First Light,Bonica,Carefree Wonder,Sea Foam,Pink Grootendorst,Nozomi,Red Cascade,Mystic Meidiland,Hansa,Double Delight
Source :Hagan,A. K.,R. Akridge,and J. Olive. 1999. A rose is not just a rose. Al. Exp. Sta. Auburn University Highlights of Agriculture 46(3):18-20
Note:Many of the roses resistant to Cercospora leaf spot are susceptible to blackspot. Although no fungicides have been screened for the control of Cercospora leaf spot on rose,a number have activity against other Cercospora diseases on other woody ornamentals. Fungicides,such as Daconil,Immunox,Zyban,and Cleary’s 3336,which are recommended as weekly treatments for blackspot control,should also provide good protection from Cercospora leaf spot when applied on the same schedule. Follow the label on each container for recommended application rates and treatment intervals for the above fungicides.
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