The time is rapidly approaching for gardeners with aspen trees to apply preventative measures to their trees if they have experienced problems with leaf spotting and premature leaf fall.
Marssonina leaf spot or blight is a fungal disease of aspen, cottonwoods, and poplars. But it is the aspen that seems to get the most problems with this disease. Infection begins in the spring at bud break when spores are blown or splashed on to the tender, newly-developing leaves. We have discussed the need in previous columns to remove all fallen aspen leaves in the fall. This is a prime overwintering site for this disease. If you haven't cleaned up under your aspens yet, do it now.
Irregular brown flecks and spots with yellow halos begin to appear on leaves in mid to late July. Many of these spots eventually merge together, creating large brown blotches. While this disease is mostly aesthetic, leaves often drop prematurely. This is not a good result.
Defoliation over several consecutive years can result in severe stress which in turn can lead to insect problems such as wood boring beetles. These repeated attacks can lead to excessive twig and branch dieback.
If there is a history of severe infestations, the fungicides containing Daconil® may be used in the spring beginning at bud break. Fungicides are of no value by the time symptoms appear. The fungicide provides a protective barrier against infection. Thus good coverage is paramount. I would use a spreader-sticker, either a commercial product or a very little dishwashing soap added to the spray mixture.
Three applications may be needed, 7-10 days apart. By this time all the leaves should be fully expanded, and protected. I will begin spraying my trees Easter weekend.
If you planted aspens and don't live where they prefer to live, then do them a favor and do all you can to baby them along. This usually includes proper watering, both during the growing season and winter period. And do not let sprinkler sprays strike leaves, enhancing disease infection conditions. These trees also succumb to diseases including Cytospora canker.
I know that aspens and spruces are two trees that are ubiquitous for Coloradans but most of us do not live in aspen territory. Maybe you need to skip planting this tree in favor of more adapted trees for your location. The aspens will thank you and you won't spend all your time trying to keep them healthy.
Jim Leser retired to Cedaredge in 2007 after a career with Texas A&M University Extension in entomology. He is a member of the Cedaredge Tree Board and a Colorado Master Gardener.