SOME PROBLEMS IN THE TREATMENT OF DISEASED CHESTNUT TREES
By Roy G. Pierce
Tree Surgeon, Pennsylvania Chestnut Tree Blight Commission
The problems that present themselves to the growers of chestnut trees concerning the present disease may be summed up under three heads: first, what the disease is, how it is caused, and how it may be recognized; second, what is to be done with diseased trees to bring them to health or to prevent them from infecting other healthy trees nearby; third, what means in the future can be undertaken to keep a tree healthy, that is, to prevent reinfection.
First, what the disease is, how it is caused, and how it may be recognized. The disease known as the chestnut tree blight is caused by the fungus, Diaporthe parasitica, which usually finds entrance to the tree through wounds in the bark. The mycelium or mass of fungous filaments gradually spreads through the bark in much the same manner as mold spreads over and through a piece of bread, even penetrating the wood to a depth of sometimes five annual rings. The spread of the fungus, resulting in the cutting off of the sap flow, is the immediate cause of the wilting and dying of the leaves and branch above the point of girdling. This wilting of the leaves, followed later by the death of one branch after another as the fungus spreads, has given rise to the term "blight" of the chestnut trees.
The danger signals which the chestnut tree displays when diseased are not a few. In summer, when the tree is first affected, the leaves turn yellow-green and wilt, later turning brown. Small burs and withered leaves retained in winter are some signs of the diseased condition of the tree. At the base of the blighted part a lesion, or reddish brown canker, is usually found. This lesion may be a sunken area or, as is frequently the case, a greatly enlarged swelling, known as a hypertrophy. After a branch has become completely girdled sprouts or suckers are very apt to be found below the point of girdling. In old furrowed bark on the main trunk of the tree the presence of the disease is seen in the reddish brown spore-bearing pustules in the fissures. In determining the presence of the fungus in the furrowed bark of old trees, one must learn to recognize the difference between the light brown color characteristic of fissures in healthy growing bark, and the reddish brown color of the fungus. When the disease has been present several years the bark completely rots and shrinks away from the wood, and when the bark is struck with an axe a hollow sound is produced.
Many of the owners of chestnut trees throughout Pennsylvania do not acknowledge that a fungus is causing the death of the trees. They state that since they have found white grubs or the larvae of beetles in nearly every tree that dies, that it has been the larvae that killed the tree. It is acknowledged that generally white grubs are found in dying chestnut trees, and that in nearly all of the large cankers or lesions these grubs are present. However, if one will take the pains to examine the small twigs and branches or the new shoots rising from the stumps, that are diseased, he will not find the grubs present.
Second, what is to be done with diseased trees to bring them back to health or to prevent them from infecting other healthy trees nearby. To bring the trees back to health implies that the disease can be cured. This is not always true for the tree may be already nearly girdled, when the disease is first noticed. A tree taken in time, however, may have its life prolonged indefinitely though it may have the blight in some portion of it every year. More particularly does this apply to valuable ornamental and orchard trees.
Prof. J. Franklin Collins, Forest Pathologist in the Department of Agriculture in Farmer's Bulletin No. 467 on "The Control of the Chestnut Bark Disease" gives the following: "The essentials for the work are a gouge, a mallet, a pruning knife, a pot of coal tar, and a paint brush. In the case of a tall tree a ladder or rope, or both may be necessary but under no circumstances should tree climbers be used, as they cause wounds which are very favorable places for infection. Sometimes an axe, a saw, and a long-handled tree pruner are convenient auxiliary instruments, though practically all the cutting recommended can be done with a gouge with a cutting edge of 1 or 1½ inches. All cutting instruments should be kept very sharp, so that a clean smooth cut may be made at all times."
All of the discolored diseased areas in the tree should be removed. Small branches or twigs nearly girdled are best cut off. Cankers in the main trunk or on limbs should be gouged out. Carefulness is the prime requisite in this work. If the disease has completely killed the cambium, the bark should be entirely removed as well as several layers of wood beneath the canker. By frequent examination, however, diseased spots may be found on the tree where the mycelium of the fungus is still in the upper layers of the bark. It is not necessary then to cut clear to the wood, but the discolored outer bark may be removed and a layer of healthy inner bark left beneath the cut. The sap may still flow through this layer. The border of the diseased area is quite distinct, but cutting should not stop here but should be continued beyond the discolored portion into healthy bark, at least an inch. The tools should be thoroughly sterilized by immersion in a solution of 1.1000 bichloride of mercury, or 5 per cent solution of formaldehyde, before cutting into the bark outside of the diseased area. Experiments have shown that a gouge or knife may carry the spores into healthy bark and new infection take place. Experiments are being carried on in the laboratory to determine the length of time which spores will live in solutions of different strengths of fungicides.