Rose flowers and buds are often infected with
the gray-brown fuzzy growth of the gray mold fungus Botrytis cinerea.
The fungus is most active when temperatures are 62 to 72 °F and conditions are
moist. Infected canes have discolored sunken areas (cankers) and dieback that
can extend down the stem from the flowers. Diseased flower petals have small,
light-colored spots surrounded by reddish halos, which can quickly expand into
large, irregular blotches. Buds fail to open and often droop. Thrips can cause
similar damage to half-open buds, so inspect plants carefully.
Keeping the area clean is more important than anything else.
Collect and discard all fading flower blossoms and leaves. Provide good air
circulation, and avoid wetting the leaves when watering. Disease easily
develops on canes that have been damaged, on canes that are kept too wet by the
use of manure mulch, or on wet leaves. If chemical control is necessary,
fungicides containing thiophanate methyl, chlorothalonil or neem oil are
available for homeowner use. Use neem oil on a trial basis, especially on open
blooms and during hot weather. Neem oil is a rather weak fungicide. On dormant
bushes copper fungicides can be used.
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