Botrytis cinerea: Botrytis blight is common in all parts of the world. This fungus is not a specific pathogen and can take advantage of many situations to produce a blight or rot condition on many hosts. It is an opportunist on cut or pruned rose canes and will infect flowers and buds.
The most common symptoms usually are seen on young flower buds which droop, turn black at the base and later produce the cottony grey-black mycelium of the fungus. Flowers can also be affected in the same way and cut ends will have the black canker like symptoms with presence of mycelium. Any time conditions are cool and wet a grey-black mycelial growth will indicate Botrytis.
This fungus is not specific to roses, but will grow on many different plants and plant debris. Under cool wet conditions profuse sporulation results and spores are moved to roses by air currents or blowing rain. A minor wound in a bud or flower, or perhaps a pruning cut will provide the initial point of entry. As the infection progresses more sporulation results and additional sites become infected. The fungus is a low level parasite and will colonize wound sites as well as dead plant materials.
Prevention is the best means of control. This can be accomplished through intense sanitation procedures. By elimination of opportunistic colonization on dead plant material the amount of sporulation can be reduced. Good ventilation is also essential in reducing disease incidence. Some sprays may give short term relief but the fungus usually becomes quickly resistant. In greenhouse conditions special covers are used to reduce the levels of ultraviolet light required by the fungus for sporulation. In most cases, removal of infected plant parts and protection of wounds by chemicals is all that can be done until warmer and dryer conditions prove too unfavorable for continuing disease.
Horst, R. K. 1983. Compendium of Rose Diseases. The American Phytopathological Society, St. Paul, Minnesota. Pp. 18-19 and Color Plates 30-35.