 Rose black spot disease, caused by the fungus Diplocarpon rosae, is quite common this year, predictably so given the many days of rainy weather this growing season. Infections by the pathogen generally occur when leaves are wet for seven hours or longer, and of course these conditions have been common this spring, and now into early summer. Many different types of roses are affected, even those, such as Knockout roses, that sail through in many cases. Why do roses sometime seem to be resistant and sometimes not? It is because disease pressure may vary from one location to another in addition to the possibility that new types of the pathogen may evolve and overcome resistance in time. Rose types with generally adequate resistance may still develop symptoms under high disease pressure, but they may survive black spot without much damage to the plant. This is one of those high disease pressure years, with multiple infection periods due to numerous periods of leaf wetness exceeding seven consecutive hours. Clean up black-spot infested leaf, cane, and flower debris; improve foliar drying conditions; use protective fungicide spray programs - and do a reverse rain dance (though beware what you wish for!). For more information, see:
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Last Updated ( Friday, 04 July 2008 11:35 )
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BYGL 
