Cause Several Marssonina spp., fungi, have been reported from British Columbia and Oregon. Not a common problem in Idaho. These fungi survive in fallen leaves or twigs infected the past year. Spores develop in leaves on the ground and infect new leaves as they are flushing and expanding in the spring of the following year. The disease is most severe in wet weather and is common throughout the Pacific Northwest on aspen as well as cottonwood and poplar.
Symptoms Brown flecks with yellow margins form on leaves and later turn to black spots. Acervuli initially appear as tiny blisters in the centers of spots. Spots coalesce into brownish-black, vein-limited blotches. The fungus also may invade twigs. Severe infection may cause early defoliation (early August) and reduce growth. Trees do not die from this disease alone.
- Collect and burn leaves in fall.
- Avoid overhead irrigation that keeps trees wet for extended periods of time.
Chemical control Use in the spring when leaves begin to flush out and wet weather is forecasted. Applications work best before rainfall occurs.
- 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.
- Daconil Weather Stik at 1.4 pints/100 gal at budbreak. Repeat at 7- to 10-day intervals if necessary. Group M5 fungicide. 12-hr reentry.
- Heritage at 1 to 4 oz/100 gal water plus a non-silicone-based wetter sticker. Group 11 fungicide. 4-hr reentry.
- Protect DF at 1 to 2 lb/100 gal water plus 2 to 4 oz spreader-sticker. 24-hr reentry.
- Spectro 90 WDG at 1 to 2 lb/100 gal water. Group 1 + M5 fungicide. 12-hr reentry.
References Bates, J., Davies, K., Deboodt, T., Fitzgerald, S., Humphrey, A. Kaiser, J., Lowrie, J. Markus, A., Rodhouse, T., Sharp, R., Stringer, D., and Strong, N. 2010. Land Manager's Guide to Aspen Management in Oregon. OSU Extension Service. EM 9005.
Spiers, A.G. 1978. An agar leaf-disc technique for screening the effectiveness and persistence of fungicides for control of Marssonina on Populus. Plant Disease Reporter 62:148-152.