Don't worry about ash trees; fungus will go away
First things first: Don't worry about your ash trees!
I have been inundated with calls this week about ash trees losing their leaves. I want to reassure each of you that ash anthracnose, a fungal infection of ash trees, is widespread this year. This fungus affects the leaves on the lower crown of your ash tree and causes autumn-like leaf fall shortly after the new leaves emerge. If the ground under your tree is littered with green leaves, you've probably got ash anthracnose. Close examination of the leaves reveals brown spotting, and curled and cupped leaves.
Ash anthracnose will not continue in hot dry weather, so once summer hits, the fungus will go away, and your tree will have ample time to re-foliate.
Why are we seeing so much anthracnose this year? The weather conditions have been ideal. The fungus overwinters in the buds, stems and branches of the tree, and appears during the late spring- early summer. Cool wet April weather, followed by our recent warm weather, provided the ideal conditions in which this fungus likes to grow. By mid-June, we should see this infection dissipate and re-growth start.
The second biggest worry this week has been the massive re-emergence of the forest tent caterpillar (army worm). The good news: outbreaks of FTC are cyclical, lasting three to six years, and Minnesota has only had five major outbreaks since 1933. Although they are a nuisance, FTCs are rarely responsible for the death of trees. However, repeat infestations (three years or more often) can slow down a tree's growth.
Trees suffering from drought related stress or disease are much less tolerant of the defoliation caused by these worms, and will succumb much more rapidly to an infestation. Widespread spraying of forests and public lands may kill this year's FTC, but is an ineffective way to control future populations. (Moths from un-treated areas fly into treated areas and lay their eggs for next spring's generation.)
What can you do if you just can't live with these pests in your trees? Remove and destroy over-wintering egg masses from the branches of small trees (they look like brown bubble wrap). Use a stiff broom to brush caterpillars off infested areas, or use a strong spray of water to knock nests off structures and trees. Insecticides are effective early, when FTCs are small (no more than an inch). Additional early measures include using a product like Tanglefoot on the trunks of shrubs and trees, wrapping the trunk of the tree with oil coated plastic wrap (oil side out) or hand picking caterpillars off plants and putting them in soapy water to kill them. Remember, these measures will kill some of this year's worms, and will protect trees already under stress, but will do little to decrease next year's population.
Hope this offers you some reassurance about the health of your trees!
Until next time, happy gardening!
"Knowing trees, I understand the meaning of patience. Knowing grass, I can appreciate persistence."
- Hal Borland