Black Spot (Diplocarpon rosae)
Damage:
Shiny, lacquered-looking droplets with black fungus field,
frequently among canes that have been attacked by aphids or scale.
Appears as an irregular shaped black spot, usually with yellowing around
affected area. Severely infected plants may be almost completely
defoliated by mid- summer. The plant is weakened, becomes subject to
winter injury, dieback and stem cankers. Less noticeable black spots
can also occur on a plant‘s petioles, stipules, peduncles, fruit, and
sepals.
Flower petals may be distorted and red flecks may occur. Raised,
purple-red blotches that later blacken and blister develop on the
immature wood of first-year canes.
Cause: Expanding leaves between 6 and 14 days of age
are most susceptible to blackspot infection.
High humidity enables spores to germinate. Aphids and scale excrete sugar-containing
juice(honeydew),
which is colonized by the black spot fungus. Black spot is spread by water
that remains on leaves for
at least 6 hours before infection takes place. The fungus overwinters in infected
canes and in fallen leaves.
Prevention is the key!!!
Protective fungicidal sprays and planting
of resistant cultivars are the best means of blackspot control. Leaf litter
should be removed from the area since the spores will over-winter on infected
leaves.Spray the rose with cold water and soft soap solutions Weekly sprays
from the time the plant leafs out in the spring until autumn are advised.
Spray both
the upper and low leaf surfaces. Proper pruning will remove spores overwintered
in small cane lesions.