Cause Several different fungi have been reported causing leaf spots in the PNW including Cylindrosporium sp., Rhopalidium cercosporelloides and Septogloeum schizonoti. The later has been found a few times by the OSU Plant Clinic. Powdery mildew has also been reported from Washington and Idaho. Pseudomonas syrinagae and Xanthomonas sp. were bacteria found associated with a leaf spot on plants grown in a Washington nursery. Most leaf spots are favored by conditions that keep leaves wet for extended periods and plant debris that harbors the pathogen.
Symptoms Bacterial leaf spots were generally circular, purple and of various sizes. Spots tended to have a slightly lighter color purple halo.
Septogloeum schizonoti - Spots can be numerous, irregular in shape, and scattered on the leaves. Spots are dark-brown and small up to 0.25 inch in diameter. They occur in indefinite areas that become orange or reddish and often extending over the whole leaf.
Cultural control
- Irrigate plants such that leaves are not wet for extended periods or use drip irrigation.
- Remove debris from under plants and in production areas before spring growth.
Reference Dearness, J. 1917. New or noteworthy North American fungi. Mycologia 9:345-364.