This section is from the book "The Gardener's Monthly And Horticulturist V21", by Thomas Meehan. See also: Four-Season Harvest: Organic Vegetables from Your Home Garden All Year Long.
Read before the Illinois Horticultural Society.
This disease was more than ordinarily noticeable during the early part of the past Summer. Afflicted trees present a most pitiful appearance; their young leaves were distorted and swollen, to the touch rigid and brittle, to the eye discolored or miscolored. Sometimes particular branches alone showed the distroyers' attack, and often certain trees were affected in the midst of healthy ones of the same variety.
The cause of this malady has been assigned to many and widely different agencies; but insects of one kind or other have had most often to bear the obloquy of the mischief. Now it is an aphis, long-legged and long-beaked; now a thrips, running and jumping and sucking, by turns; now it is a mite too small to be noticed; now a plant-bug too ill-smelling to be unnoticed. Others have imagined the cause to be in the constitution of the tree, or the food it had or did not have, or in the conditions of training and culture. It is true a well-known plant-louse (Aphis Persicse) does cause the leaves of peaches and those of other stone-fruits to twist and curl by its punctures; but the appearance is distinct enough from that under discussion. The leaves infested by insects keep their green color, are shorter rolled and have not the swollen, shining appearance as in the disease of which we speak. The true cause of the latter has been known for some years to be a parasitic fungus, whose mycelium or vegetating threads penetrate and ramify through and through the affected leaves and produce its fruit (spores) in a thin, whitish, powdery stratum over the foliae surfaces.
If a little of the white coating of a diseased leaf is removed upon the point of a knife-blade and placed in a drop of water under a compound microscope, numerous thin-walled cells of divers forms may be seen. Among them some, often very many, larger than the others may be observed having just about the outline of a track made in the mud by a flat-footed boy, the toes not appearing. Through the transparent walls are to be seen several small oval bodies, which he who is acquainted with such things quickly pronounces spores. Our foot-shaped cells are spore-sacks or sporangia. There are plenty of them, and if the disease does not spread rapidly it is not from the want of fertility and productiveness of the parasite.
The name very appropriately given the fungus is Ascomyces deformans.
So far nothing new has been said, though botanists have not usually made out the penetrating mycelium in the leaf-tissues and have described the fruit production on the surface as an anomaly. Some investigations by the writer have now to be added and a remedy proposed. When the leaves are making their appearance in Spring-time close inspection shows that they are diseased when they issue from the buds, and careful examinations of their sections under the microscope reveal the presence of the parasitic filaments growing among and through the young leaf-cells as roots grow in soil. The young leaf is affected throughout or only in a given portion of its area. As it expands by growth the parasite is carried along and finally produces the observed deformities correspondingly over the whole or only a part of its surface. The disease then is in the buds, and as the stem or axis on which the leaves are borne constitutes a part of the bud it might be inferred that this, too, is subject to the invasion of the enemy. This is the case. The young bark of a diseased twig is as completely filled with the threads of the fungus as the leaves themselves.
It, too, is distorted in its growth, thrown into ridges and swellings and blister-like excresences, and when not prevented by the firmness of the wood curls like the leaves. Diseased leaves finally die, wither, and drop off; diseased twigs when very badly affected die also, otherwise they retain the marks of the destroyer throughout the season. After the time of the production of spores is passed, the fungus, though preserving its vitality, appears to become dormant, and the injured but surviving twig may now send out healthy leaves from newly-formed buds. Hence it is that at one time the leaves of a tree may be blistered and distorted, at a later period fresh and sound. The old leaves do not recover, new ones take their places.
If, however, during the latter part of the summer, or at any time through the Winter, the diseased bark of young limbs be properly examined the fungus threads in a living state may be found. This is a most important contribution to our knowledge of the natural history of the parasite, and at once suggests a feasible plan of warfare. You have no doubt already anticipated this plan and are wondering if it will succeed. Select by close examination in Winter the portions bearing the cause of the disease and prune them away. This can be done with much certainty as to the results. There is no necessity of the gift of prophecy to predict what trees and what particular branches will show the disease in the leaves to be put forth the next Spring. An examination now of the one-year-old limbs tells the story.
It is still possible that infection may come from a foreign source, as the germination and development of the spore has not been traced, but the slow spreading of the disease leads us to hope that danger from this source is not very great. If the perennial mycelium is destroyed there is much to indicate a sure victory over the disease. Sometimes the amount of necessary pruning would be very severe, but only in trees so badly infested with the preying fungus that one can cut without much compunction of conscience. More often very litle use of the knife will suffice, and this all upon limbs of the last season's growth. It would assuredly be well to burn the severed branches, but their simple removal, even though thrown upon the ground beneath the tree, may and most probably would be sufficient.