Black Spot of Rose (Diplocarbon
rosae) is caused by a fungus in the Ascomycetes phylum. It is frequently
found on rose bush leaves here in the Bay Area, especially during the winter or
in the fog belt. Symptoms include black spots and yellowing on the upper
surface of leaves. The fungus produces ethylene, which induces chlorosis (yellowing)
of the leaves, and eventually abscission (leaf drop). Ethylene is a growth
regulator that affects leaf drop, fruit ripening, and plant senescence; and can
cause vascular wilt diseases. The black spots may have a feathery margin and
may include fruiting bodies (best seen with a hand lens). Inside the fruiting body, microscopic sexual spores called ascospores are produced in sacs called ascus (pl. asci), and released upon maturity.
The fungus requires water to reproduce and grow, and can
survive in fallen leaves and infected stems. Overhead sprinklers may keep
leaves wet for long periods of time, and may splash the fungus from infected to
uninfected plant material.
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| Black Spot of Rose, with chlorosis. Photo courtesy of C. Ash, available by APSnet via License to Copy. |
For management tips, see the Integrated Pest Management
(IPM) site. Prevention is the best solution – plant resistant rose bushes in a
sunny location with plenty of room for air circulation; remove fallen leaves
and infected plant parts; and do not let moisture stay on the leaves for more
than seven hours. This means you should avoid watering with overhead sprinklers;
and, if you spray the leaves to remove dirt or pests, do so early in the day so
the leaves have a chance to dry. You can manage black spots on leaves using a
combination of bicarbonate of soda and horticultural oil, or using Neem oil or fungicides.
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