Fungicide spray programs need to be started as soon as new leaves appear in the spring. Avoid wetting the leaves when watering and locate plants where there is good air circulation. Raking and removing these leaves as well as pruning out affected canes by spring before the buds swell may help provide some control. The fungus overwinters in fallen leaves and stem cankers. Some roses are less susceptible than others, so cultivar selection is important. Infection occurs after leaves are wet for several hours, making it more serious during rainy periods. Infections on canes are identified as reddish-purple spots. The fringed margin and black color distinguish this leaf spot from others. Infected leaves turn yellow and fall off prematurely. Blackspot is identified as circular black spots that appear on the upper surface of the leaves, starting at the bottom of the plant and moving upward. This fungal disease can cause almost complete defoliatiation of bushes by early fall, resulting in a weakened bush on which cane die-back and cankers become severe. That is why, when the term "resistance" is used as a blanket term and assigned to a variety or class, it may or may not apply depending on where you garden. Genotypic resistance is due to the presence of genes that are not affected by climate, location, or horticultural practice. Phenotypic resistance is when a cultivar is resistant to a disease in one location or part of the country but not in another. In fact, there are a number of cultivars that are very prone to severe disease injury.Īnother interesting fact about diseases is that plants can have two types of resistance: phenotypic or genotypic. All too often, many gardeners assume that shrub roses (the class) are very tolerant or resistant to disease and make their selection based solely on class. You want to select by cultivar, not by class. When selecting roses, note the resistance to disease of a particular cultivar or named variety within a class. These all foster healthy roses that are better able to withstand the pressure of disease and insects. A pest control program starts with proper site selection, good soil preparation, good drainage, proper spacing, cultivar selection, and plant maintenance. Roses may survive without a basic pest control program, but they may not be very attractive. Roses are susceptible to a number of disease and insect pests.
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