Roses have a reputation of being hard to grow, but in fact are no touchier than other garden plants. With proper attention to planting location and pruning, the beauty of the rose plant outweighs the attention they need. Make sure the roses purchased are certified stock, free of disease. When gardening, make sure the roses are free of weeds, lawn debris and any cuttings that may harbor disease.
Black spot can cause a rose bush to become bare by the end of the summer and so weakened that it may not survive the winter. The fungus Diplocarpon rosae causes black spot. The fungus is spread by splashing water and grows on foliage that is wet for several hours. Circular black spots will appear on the leaves. The spots are usually inside yellowish areas that have appeared on the leaves. The spots can be as small as 1/16th inch to ½ inch in diameter. The leaves will turn yellow and fall off.
Black spot is controlled by pruning the diseased canes in the fall. All of the diseased leaves and canes should be disposed of immediately to prevent the fungus from wintering in the garden. A protective coating of fungicide applied during rainy periods is helpful in keeping black spot at bay.
If the rose bud droops and refuses to open, and then turns brown and decays, it may be suffering from botrytis blight. Look for a grayish black lesion on the stem just behind the flower head. At an advanced state, a gray fungus is apparent.
Botrytis cinerea, a fungus that lives on dying tissue, causes botrytis blight. Any dead plant can harbor the fungus, and thousands of spores can be released into the area and attack leaves and buds, especially if they are wet. Control botrytis blight by immediately removing and destroying infected blooms.
Brown canker can attack greenhouse roses as well as outdoor roses. Any part of the rose that is above ground is susceptible. The fungus Cryptosporella umbrina causes brown canker, showing up as tiny reddish purple spots on new cane growth. It can take several years for the spots to develop into whitish lesions on the stem which will eventually spread into patches. Left untreated, the cankers can kill the entire plant.
The spores of the Cryptosporella umbrina can drop to the soil, and contaminate healthy tissue when spattered back up onto the rose in water. Wind and pruning tools can also transfer the spores, but brown canker disease is only able to attack the plant if it can enter through a wound. When pruning the plant, do not cut into the diseased wood, but rather well below it.