Prevalent especially during cool, wet weather. The disease is caused by several strains of the fungus Botrytis cinerea and attack blooms and canes, but is rarely seen on rose leaves.
Blooms become spotted, then brown and finally completely rotten, in severe cases the stems can be affected and die back completely.
During the rainy season, the disease affects rose buds and petals – you may see pink/ red 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 ball and fail to open, or result in a 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 cinerea causes the still developing bloom to not fully open – moisture becomes trapped between the many petals and rot ensues. An entire crop of blooms can be lost this way. In severe cases – black or caramel-colored, sunken and elongated lesions, with a definite outline, appear on young soft stems causing it to be weakened, girdled, and collapsed at the point of infection followed by wilting of the foliage above the lesion – if favourable conditions continue the entire plant will die.
Rainy and high humidity climatic conditions create just the right mix to bring on an attack of botrytis on roses. Warmer and drier weather takes away the humidity and moisture that this fungus loves to exist in, and under such conditions this disease will usually discontinue its attack. Good ventilation through and around the rose bush helps keep the humidity buildup within the bush down, thus eliminating a favorable environment for the botrytis disease to get started.
The fungus over seasons on decayed plant material or in infested soil.
Sclerotia are the main structures for field survival, although conidia may over season in the field and can survive a temperature range of 4 to 54°C. The overwintering stage can be spread by anything that moves soil or plant debris and transports sclerotia, mycelium, or conidia.
Treatment and Prevention
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.
No home garden fungicides are available that controls Botrytis. But since this disease is only prevalent for a short period of time – it does not cause long term problems.