by Nanette Londeree, Master Rosarian
April showers bring May flowers, and along with it, the potential for Botrytis – the disease that can turn beautiful spring blooms into a brown, soggy mush. This ubiquitous disease is also know as Botrytis blight, bud rot, gray mold, wooly mold and when growing on grapes, the “noble rot” where it can either ruin a crop of grapes, or produce excellent dessert type wine. In our mild Mediterranean climate, we don’t see this disease too much during the dry summer months, but once the rains begin again in the fall, the wooly gray fungus returns with a vengeance.
The disease is caused by several strains of the fungus Botrytis cinerea and attack blooms and canes, but is rarely seen on rose leaves. It has a wide range of host plants in addition to roses and grapes – dahlias, gladiolus, tulips and marigolds are very susceptible to the disease.
During the growing season, the disease affects rose buds and petals – you may see spotted flower petals or the tips and edges of the petals turn soft and brown. The spots look like water spots on the petals, however, the spots are actually caused by the plants’ reaction to the invasion of the fungus at the spot where the petal has been damp. Other times, the flowers simply fail to open, or result in a shattered mess of brown petals. This can be followed by wooly gray fungal spores on decaying tissue. Twigs may die back and large, diffuse, target like splotches form on canes.
Botrytis is also seen on bareroot roses that are handled via mail order since the cool, moist conditions in the shipping container are just right for the fungus to grow. It is important to remove your bareroot roses from their shipping containers as soon as you can to reduce the potential for the disease. While it thrives in warm, humid conditions, it can overwinter even in cold climates. It can remain dormant on a plant until the weather conditions are right then spring to life.
Since it is such a prevalent fungus, prevention is the best approach – plant roses that are not susceptible to botrytis blight; reduce the humidity around plants by providing good air circulation, modifying irrigation and reducing ground cover; deadhead any infected flowers immediately and dispose of fallen leaves and petals; prune out infected canes, buds, and flowers and generally maintain good garden sanitation.
The disease can be controlled by spraying with fungicides like chlorothalonil (Daconil 2787, Fungi-gard), or mancozeb (Fore). This disease is notorious for rapidly developing resistant strains, so if you spray, it is important to alternate spray materials. Remember that fungicides are among the most toxic chemicals in the gardener’s pest control arsenal, so use caution when spraying and follow the directions on the product label.