Aspen are a native tree to the cooler parts of the United States. Cooler may be the north or higher up in elevation. But as man often does, we have brought these lovely trees down to warmer areas and have discovered they have a few problems where they are not native, but these can easily be overcome with a little preventive care every year. I have set this up in the order you would apply.
Dormant no leaves
Either in the fall or the spring, before the leaves come out, make an application of ferti-lome Horticultural Oil. This will kill overwintering insects, eggs and all including Scale. This is also a great time to remove as many leaves as possible to slow down this year’s issues with fungus.
Spring as leaves begin to emerge through full leaf
This is the right time to make an application of ferti-lome Tree & Shrub Drench. This systemic insecticide will be present for up to one year to help protect against; Borers, Scale, Aphids and more. Simply mix in a bucket and pour right next to the base of the trunk. (3 ft away may not work)
If your Aspen has yellow or light-green leaves throughout the year every year, this is the time to apply ferti-lome Chelated Liquid Iron. You can apply later in the year, but the best time is early in the season. This can be mixed and sprayed on the leaves or you can mix and pour, out away from the trunk, about straight down from the outer most branches.
Spring as leaves form
When the leaves are about the size of a quarter to fifty cent pieces, make your first application of ferti-lome Liquid Systemic Fungicide II. Make at least two more applications (for a total of 3), 10 to 14 days apart. These applications should help to prevent; Leaf Spot, Ink Spot and more.
Spring as after full leaf
After all the leaves have come out, apply ferti-lome Tree & Shrub Food. This is a full feeding with added Iron for greening. Apply this directly to the ground or turf 2 or 3 ft out and in, straight down from the outer most branches. NO need to dig holes, simply apply and water thoroughly.
Late Spring or Early Summer watch leaves for signs
Aspen Leafminer, if you see trails in the leaves, you can spray NG Spinosad. This natural product will absorb through the leaf and pools where Leafminers feed. Make 2 applications, 7 to 10 days apart. Also works on Tent Caterpillar and Webworm.
Summer watch leaves for signs
Two surface fungus that may infect the leaves of Aspen are Rust and Powdery Mildew. Powdery Mildew can show up during wet weather or dry periods, if it coats the leaves too heavily, control may be required. It is cutting down on Chlorophyll production hurting health.
Rust shows up as rust-colored spots on the underside of the leaf. For both apply ferti-lome Triple Action, a natural product from the Neem tress, make 2 or 3 applications 7 to 14 days apart.