Black spot—Diplocarpon rosae
Black spot causes black spots to develop on the upper surface of leaves and succulent stems. The spots
have feathery or fiberlike margins and no powdery growth on the undersides of leaves. Small black fruiting
bodies are often present in spots on upper sides of leaves. Yellow areas develop around the spots. Leaves
may drop.
Solutions
The black spot fungus requires free water
to reproduce and grow, so leaves should never be allowed
to remain wet for more than 7 hours. (When hosing off aphids,
do so in the morning so leaves have a chance
to dry by midday.) Provide good air circulation around bushes.
Remove fallen leaves and other infected material and prune
out infected stems during the dormant season. Black spot
is usually not a problem during
California's dry summers unless overhead sprinklers are used,
but the disease can be serious where rainy summers prevail
or in cooler areas. Miniature roses are more susceptible
than other types, although a
few varieties are reliably resistant to all strains of black
spot. A combination of bicarbonate
of soda plus horticultural
oil can be used
to manage black spot (as well as powdery mildew). Use about
4 teaspoons of baking soda per gallon of water with a 1%
solution of narrow-range oil. Avoid getting on open blossoms.
Neem oil can
also be effective. Preventive sprays of fungicides such as
triforine or chlorothalonil may
also be effective.
See the pest
note for more
information on treating diseases and disorders common to
roses. | 
Spots
on upper surfaces of rose leaves
Yellow areas develop around spots
|
