Black spot—Diplocarpon rosae
This fungal disease of roses is usually a problem in California only in foggy or humid coastal areas. It can occur anywhere leaves commonly remain wet, such as where roses are sprinkler irrigated during the evening or night.
The fungus produces black spots on the upper surface of rose leaves and stems. Spots sometimes have feathery margins, and tiny, black, fungal fruiting bodies may be visible in the spots. Yellowed areas develop around spots. Infected foliage often drops prematurely.
The pathogen persists on living and dead leaves and on infected stems. Its spores are spread by splashing water. For spore production and infection, leaves must remain wet for more than about 7 hours.
To reduce black spot, irrigate and hose off aphids in the morning instead of the evening or night. Do not plant roses too close together. Prune canopies to increase air circulation. Prune off infected stems during the dormant season and dispose of fallen rose leaves and stems away from rose plants.
Avoid planting roses where they will not receive at least 6 to 8 hours of full sun. Choose disease-resistant varieties when planting. Miniature roses are more susceptible than other types, but a few varieties are reliably resistant to all strains of black spot.
Where weather favors severe disease, available preventive fungicides include neem oil, potassium bicarbonate, sulfur, and the synthetic fungicides triforine and chlorothalonil. For more information, see Pest Notes: Roses: Diseases and Abiotic Disorders.
Spots on upper surfaces of rose leaves
Yellow areas develop around spots