by Jan Cashman
This is the time of the year when we often get asked “What’s wrong with my tree?” Many of the answers to this question are the same year after year. But occasionally a new insect or diseases finds its way to our valley.
Last year was the first year we saw the cottony psyllid insect, which is infecting the leaves of black ash (Fall Gold Ash). This year this insect is even more prevalent, especially on drought-stressed trees. In the spring the young psyllids, which resemble aphids, hatch and suck sap from the new leaves which shrivel and discolor. They produce a white, cottony material as protection. If not controlled, they can severely weaken the tree.
Insecticidal soap is an organic control that will kill the cottony psyllid on contact. Malathion insecticide, also a contact killer, is an effective chemical control. There are a couple of systemic insecticides which make the plant poisonous to the insect and also kill on contact. Bayer Tree and Shrub Insect Control (active ingredient:Imidacloprid) is a systemic insecticide that is mixed with water and poured under the tree so the roots take it up. The systemic insecticide spray, acephate, once called Isotox, also works on the cottony psyllid.
An abundance of aphids emerged earlier than usual this year, right after the warm spell we had in May, even on plants that don’t normally get aphids. Aphids are small (1/16”) soft-bodied insects that cause new leaves on a plant to yellow, curl and distort. They reproduce quickly and weaken the plant by sucking its juices, excrete a sticky “honeydew”, and can spread viruses. The controls for aphids are much the same as for the cottony psyllid above but do not use acephate systemic insecticides on any plant with edible parts or fruit. Environmentally friendly lady bugs can be released to eat aphids.
Spider mites continue to be a problem during our hot summers, especially on junipers, arborvitae, spruce, potentilla, and raspberries. This tiny insect sucks the sap from the needles or leaves of the plant, giving them a dull appearance. To detect the almost microscopic spider mites, look for grayish, pale needles, small webs, or shake a branch over a piece of white paper and look closely for movement of the tiny mites. Malathion or isotox spray should eliminate spider mites. More than one application may be necessary.
Scale is a large group of insects that harm many ornamental plants. Hard (armored) scale on the branches looks like brown bumps. Soft pine needle scale looks like small white specks on the needles. A severe infestation of scale can kill the branch. The dense branches of cotoneaster hedges are a place we see scale. If your hedge is severely infected, you might need to cut it down to the ground, destroy the infected branches, and let it grow back. Because of the waxy covering surrounding them, contact insecticides do not kill scale. Dormant oil spray is effective in early spring before the tree leafs out to smother the scales. Or use a systemic insecticide.
If you have spent any time in our national forests recently, you may have seen the spruce budworm outbreak on Douglas fir. Budworms, really not a worm but a caterpillar, will also eat the new needles of spruce in your yard. While still in the caterpillar stage, they can be killed with BT (Bacillus thuringiensis), a specific bacterial spray toxic to caterpillars but not other insects. Spruce budworm is often confused with white pine weevil, which kills the leaders of spruce in this area. They are different insects.
Tent caterpillars, which eat the leaves of our trees, are back this year. Pick the tents off and destroy them when the caterpillars are still contained. BT works on tent caterpillars, also.
Other insects which damage trees in this area include the aspen borer, bronze birch borer, and leaf miners. The last couple of years, windblown poplar budgall mites have damaged Canadian poplars. Harmful fungi such as Verticillium wilt, cedar apple rust, black knot, and black spot, can cause tree problems, some severe.
Fireblight is a serious bacterial disease that primarily strikes apples, but also can afflict mountain ash, cotoneaster, pears, and hawthorn. Prune out infected branches and plant varieties resistant to fireblight. Pseudomonas is another bacterial blight that produces similar symptoms in lilacs. Prune it out and allow adequate space between plants for air circulation.
Soil deficiencies can be the cause of tree problems. In this part of Montana, we see yellowing of leaves between the veins of mountain ash, plums, shrub roses, ginnala maples and even aspens because of iron deficiency (iron chlorosis). The plants are unable to absorb enough iron and other trace elements from our alkaline soils. Either acidify your soil or add chelated iron. If your soil has an extremely high pH, do not plant hydrangea, mountain ash, or other plants that need acid soil.
Too much TLC – by overwatering or overfertiziling – could be the cause of trees’ problems. The symptoms of overwatering often mimic those of underwatering—small, yellow leaves that wilt or drop. Roots can become waterlogged when trees are planted within a sprinkler system in heavy clay soils. Even wetter conditions are created when the tree is surrounded by weed barrier fabric and mulch. Be observant and use common sense when watering trees and shrubs in your yard. After a deep watering, let the roots dry out so air can reach them. Probe the soil to check moisture levels. You may need to reset your sprinkler system and/or remove the fabric.
Most of the time, trees should get all the nutrients they need from your soil. We advise against fertilizing young trees because of the danger of overdoing it. If you choose to use fertilizer spikes on your trees in the spring, they should be pounded into the ground at least 2 feet from the trunk of the tree. Never use more than the recommended dosage, and never fertilize trees after July 1.
Some tree maladies have been around for years, some are new to our area. Some need treatments, some are just cosmetic. Regardless, remember: insects and diseases prey on weak trees. Your best defense against insects and diseases is to keep your trees healthy.