The Balcony Gardening: Insect and Disease: Elm trees (Ulmus spp)

Elms are alternate-leaved, deciduous trees valued for their ornamental use as specimen or street trees. There was a time when virtually every town and city in the northeaster United States was dominated by the majestic, vase-shaped American Elm (ulmus american); unfortunately, with the appearance and spread of Dutch elm disease (DED), many of them have died.

Because most elms are susceptible to so many insects and diseases, you might want to avoid using them in the landscape, but they are perfectly suitable for beautiful bonsai (specially the Zelkova species). If you do decide to grow elms, consider planting lacebark elm (U. parvifolia), which is more resistant to DED and elm leaf beetles.

Plant elms in spring or fall as bare-root or balled-and-burlapped trees. Full sun and well-drained soils are best.

Problems:

Leaves with rectangular holes or skeletonized: Cause: Elm leaf beetles. this is a 1/4inch yellow-green beetle with a dark line on the outer edge of each wing cover. It lays its eggs in spring on the undersides of leaves; these eggs hatch in June. One to several generations may occur each year.

Both adults and the 1/2inch black-spotted, yellow larvae feed on the leaves, eating everything but the veins. Trees are often defoliated and so weakened that they are susceptible to other insect and disease problems. Control beetles by spraying leaves with BTSD, particularly in June.

Leaves wilted and yellow or brown, drop early, branches show symptoms one at a time: Cause: Dutch elm disease (DED).

Caused by a fungus, DED is spread by the feeding of elm barck beetles, and by natural root grafts between trees growing in the same area. Keep trees healthy with proper pruning, mulching, and watering (during drought). Quickly repair all wounds to help prevent insect attacks and subsequent infection. Once the disease is established, there is no effective remedy. Remove and immediately destroy all diseased or dying elms. Remove the stump if possible or peel the barck off to below the soil line to deter elm bark beetles from feeding there.

Bark tunneled: Cause: Elm bark beetles.
These 1/10inch dark reddish brown beetles attack weakened elm trees and serve as vectors of DED. The adult beetles bore small holes through the bark and lay eggs in the wood beneath. The eggs hatch into 1/4inch curved, white grubs that continue to mine underneath the bark, creating winding tunnels, known as galleries. See the opposite page for an illustration of their damage. The grubs transform into adult beetles, which emerge through small holes in the bark. If the tree they emerge from was infected with DED, the beetles will carry the disease spores as they fly away to feed on other elm trees.

Make trees less attractive to beetles by promoting vigorous, healthy growth with proper pruning, mulching, and watering (during drought). Eliminate breeding areas by removing and destroying weak and dying trees.

Leaves skeletonized: Cause: Canker-worms. Both spring and fall cankerworms are 1/2-1inch yellow or greenish caterpillars that feed on the foliage of birches, often defoliating the tree.

Japanese beetles. This 1/2inch long pest has a metallic blue-green body and bronze wing covers. Hand pick in early morning if there are only a few. Apply milky disease spores to your lawn for long-term control.

Leaves wilted and yellow or brown, drop early, entire crown of tree affected: Cause: Phloem necrosis. In later stages, the inner bark (phloem) is discolored and smells faintly of wintergreen. This disease, also known as elm yellows, kills plants quickly, often in single growing season. It is spread from tree to tree by leafhoppers.

Controlling leafhoppers by spraying infested plants with insecticidal soap or pyrethrin may reduce the spread of phloem necrosis. If trees are close together, however, the disease can also spread underground by means of natural root grafts. Once plants are infected, there is no control: remove and destroy the infected trees.

Another examples of pests affecting the Ulmus spp.: