Cause Several fungal leaf spots have been reported from the PNW. Cercospora leaf spot caused by Passalora rosicola (formerly Mycosphaerella rosicola) has been reported in Washington. It has become more important in the Southeastern United States as black spot resistant but Cercospora-susceptible roses become more widely planted. Damage appears to be greater on shrub and ground cover roses compared with hybrid tea and grandiflora roses. Spores produced in the spots are rain splashed to healthy foliage. Rainy weather favors the disease.
Spot anthracnose caused by Elsinoe rosarum has been reported from both Oregon and Washington. The fungus overwinters on infected leaves and stems with conidia continuously being formed in spring and early summer. Spore dispersal primarily occurs with the aid of water through rain and irrigation. There were 5 races of the fungus identified in Germany.
Cladosporium herbarum has also been reported from leaf spots in Washington.
Abiotic leaf spots due to many different situations, including chemicals, are a common problem sent into the OSU Plant Clinic. Some chemicals used to manage common disease, such as black spot, powdery mildew, or rust, may cause various leaf or petal spots or discoloration. Always watch for warnings on labels of various products.
Symptoms Cercospora leaf spot - Numerous tiny maroon-to-purple oval lesions develop randomly across the leaf surface. The center of these spots then turn tan-to-gray while the margin of the spot remains maroon-to-dark-purple. Heavily spotted leaves turn yellow and are prematurely shed. Typically, leaf loss begins at the base of the canes and gradually spreads upwards through the plant canopy towards the shoot tips. Leaves are most often infected but stems, pedicels, fruits, and bracts can also have symptoms.
Spot anthracnose - at first, the appearance of red spots that vary from brown or dark-purple on the upper leaf surfaces occurs. Small spots may be scattered or grouped and sometimes overlapping. Chlorosis or yellowing of the leaves may also occur. Later, spots become ashen-white with a dark-red margin. This tissue may fall off of the lower leaf surface, leaving a thin papery membrane or fall out entirely resulting in a shothole symptom. Symptoms occur on stems, hips, and pedicels as well as leaves.
Cultural control In general, tactics used to manage black spot, powdery mildew, and/or rust will be effective on many leaf spotting fungi.
- Space or prune bushes to allow for good airflow. Avoid dense plantings and shaded areas.
- Avoid overhead watering that keeps plants wet for extended periods of time.
- Rake up and burn all leaves at season's end.
- Remove and destroy infected plant parts.
Chemical control In general, tactics used to manage black spot, powdery mildew, and/or rust will be effective on many leaf spotting fungi. Always watch for warnings on labels of various products.
Reference Bagsic, I., Linde, M., and Debener, T. 2015. Genetic diversity and pathogenicity of Sphaceloma rosarum (teleomorph Elsinoë rosarum) causing spot anthracnose on roses. Plant Pathology. DOI: 10.1111/ppa.12478