Cause A few fungi have been found in association with leaf spots on spirea (Spiraea spp.) in the PNW. Cylindrosporium filipendulae (now named Blumeriella filipendulae) has been reported from all PNW states, Cylindrosporium spiraeicola (now named Phloeospora spiraeicola) has been reported from ID and WA, Ascochyta salicifoliae (formerly Phloeospora salicifoliae) was reported in WA and Cercospora rubigo has been reported in Oregon. Of the few samples send into the OSU Plant Clinic, Phoma and Cylindrosporium have been found. Most fungal leaf spots are favored by wet weather and/or overcrowding that keeps leaves wet for extended periods of time. Airborne or water-splashed spores infect young leaves during wet weather, often in spring. They generally survive unfavorable conditions in plant debris.
Symptoms Cylindrosporium - Light yellow lesions that turn brown as the tissue dies. Spots occur on both leaf surfaces and vary in size. Upper surface lesions are dark brown and irregular in shape while the lower surface spots are smaller, brownish-orange and tend to be circular. Spots may coalesce and the entire leaf may die. Severe defoliation can occur.
Phloeospora - Grayish to blackish brown spots on leaves surrounded by a yellow halo. Lesions are usually 2mm in diameter but several may coalesce to kill large areas of the leaf. Acervuli may be observed on the lower leaf surface.
Cultural control
- Space plantings for good air circulation between plants.
- Remove and destroy affected leaves and fallen leaves.
- Avoid overhead irrigation, or water such that plants are not wet for extended periods of time.
Chemical control Tank-mix and/or alternate products from different groups with different modes of action to prevent the buildup of resistant fungi. Limit the use of any one group during the growing season.
- Armada 50 WDG at 3 to 9 oz/100 gal water. Do not use a silicone-based surfactant. Not for nursery or greenhouse use. Group 3 + 11 fungicide. 12-hr reentry.
- Broadform at 2 to 4 fl oz/100 gal water. Group 7 + 11 fungicide. 12-hr reentry.
- Eagle 20 EW at 6 to 12 fl oz/100 gal water. Group 3 fungicide. 24-hr reentry.
- Insignia SC at 3 to 6 fl oz/100 gal water. Do not use with organosilicate-based adjuvants. Use preventively only. Group 11 fungicide. 12-hr reentry.
- Myclobutanil 20 EW T&O at 6 to 12 fl oz/100 gal water plus spreading agent. May observe a PGR effect. Group 3 fungicide. 24-hr reentry.
- Ortho Rose & Flower Disease Control at 2 fl oz/gal water. Group 3 fungicide. H
- Pageant at 6 to 12 oz/100 gal water. Do not use with organosilicone-based adjuvants. Group 7 + 11 fungicide. 12-hr reentry.
- Tourney EZ at 1 to 4 oz/100 gal water. Group 3 fungicide. 12-hr reentry.
- Trinity at 4 to 12 fl oz/100 gal water. Group 3 fungicide. 12-hr reentry.
Reference Fergus, C.L. 1957. Cylindrosporium leafspot of Spiraea. Mycologia. 49:262-268.
Shin, H.D., Lee, H.T. and Oh, J.T. 1999. Phloeospora Leaf on Spiraea. The Plant Pathology Journal. 15:76-78.