Tree Infections: the Continuing Battle

Frank Williams
Adams County Master Gardener

While gathering some background for this piece, I was reminded of Winston Churchill's book entitled The Gathering Storm, one of seven volumes in a series published in 1948. Churchill was referring to the ever increasing danger of Hitler's invasions in Europe leading to World War II. The analogy, of course, refers to the increasing number of infectious diseases to the trees of Pennsylvania and elsewhere. In this article, I shall mention just three such in our own state and one on the West Coast. Bagworms, rhizosphaera needle casts, hemlock woolly adelgid, and sudden oak death syndrome will be the topics described briefly below.

BAGWORM

Description: The bagworm is a perennial insect of many evergreens and certain deciduous trees. It is most common in regions of southeastern Pennsylvania in our state. Dispersal here is mostly through movement of infected nursery stock and wind dispersal in early June. The identification of this disease is fairly simple; most of us have observed them in their silken shelters (bags) often covered by pieces of foliage, bits of bark, or other plant parts. Eggs laid in fall can overwinter in the bag until the following year. Treatment: Damage by bagworm larvae leads to desecration of evergreen needles and hole in deciduous tree leaves. Control can be achieved on small shrubs and trees by handpicking or cutting the bags from infected plants in late fall, winter, or early spring, disposing of the bags to prevent reinfestation. When bagworms are too numerous or out of reach, insecticide application may be indicated. Apply the material only as directed and solely to plants listed on the label. This is a controllable disease.

RHIZOSPHAERA NEEDLE CASTS

Description: This "needle cast" is a fungus which attacks various conifers and is a significant cause of death to spruces in North America. Damage in forests thus far is negligible, but, on home landscapes and in nurseries, it is devastating. I speak personally here since several trees on our property are suffering seriously from this disease. The spores are spread easily by splashing and dripping water from spring until autumn. The yellowing, browning, and dropping of needles in winter are often obvious signs of the disease. Normally, the destruction begins at the bottom of the tree and slowly extends upward. Christmas tree farms carry and extend the problem easily because of human movement among the trees and retention of moisture in the dense foliage of such farms.

Treatment: Much has been explored about treating rhizosphaera needle cast but, to my knowledge, no genuine cure has yet been discovered. The best bet, I believe, is to cut the lower branches which have lost their needles, and eventually, plant a second generation of other trees or shrubs underneath the spruce. Fungicides may be recommended by some professionals but seem to provide, at best, slowing of the disease's advancement. In my case, I planted hemlocks with the hope they will eventually replace the once majestic blue spruces.

HEMLOCK WOOLLY ADELGID

Description: It comes to my twisted mind that the definition of a Master Gardener may be one who is willing to share his or her own mistakes so that others might learn not to repeat them! Hemlock, I have come to learn, has its own disease problem, both in the East and in Oregon and California. So, our own state tree is threatened! Hemlock woolly adelgid is found particularly in mature hemlocks by examining the trees for masses of white spots and filaments among the branches of the tree. Such populations overwinter and cause premature needle drop, reduced twig growth, and death of trees when severe. Female eggs start to hatch in April and, under normal conditions, hatching is completed by late June.

Treatment: The good news about this pest is that on home landscapes it is treatable. Two methods are currently recommended. In the spring, soil injections of systemic insecticides may be applied by commercial applicators around large trees. Spring is recommended because adequate soil moisture is usually present, a prerequisite for successful application. I had this treatment applied to all our hemlocks and a recent inspection revealed no need for additional measures, perhaps not for a year to eighteen months. Spraying in fall is another option, perhaps especially for large, extensively infected hemlocks. Several caviats are in order as follows: 1. Spraying is too expensive to use in our large hemlock forests; 2. This disease spreads quickly in neighborhoods harboring many hemlocks; 3. Repeated treatments are likely necessary over the years.

SUDDEN OAK DEATH SYNDROME

Description: A fairly recent pestilence called sudden oak death syndrome has been found in California and Oregon. It may well spread eastward according to an early October New York Times article. The disease has already killed tens of thousands of trees in California and infested seventeen species of trees, especially, but not exclusively, varieties of oaks. The beloved redwood and douglas fir have also been infested. Scientists are rewriting the biology of this botanical scourge with each new finding. Much remains to be learned and no cure is at hand.

The famous quotation, sometimes attributed to Thomas Jefferson, may be paraphrased here: "The price of freedom (from disease) is eternal vigilance." Suffice it to say that we are not in control of our environment but we can preserve some of it by being vigilant. The "Gathering Storm" of gardening and landscape insects need not overwhelm us!

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