Larch Canker
In the following notes, only the most serious cases are dealt with, or, in other words, such fungi referred to as are commonly met with and cause most damage to our woodland trees.
Larch Disease.—In all probability the larch disease or larch canker owes its origin to the minute spores formed in the fructification of the now well-known fungus—Peziza Willkommii. It has been pretty conclusively proved that these spores can only find a footing where the rind of the bark has become in some way injured, such as might be occasioned by the puncture of an insect, by wind, frost or from many other causes. The spores send down their germ tubes into the cambium, between the bark and the wood, where the moisture and nourishment afforded causes rapid development of the fungus. This soon spreads to the cells of the wood, and the annual layers either entirely cease to grow, or become disorganized and crippled in growth, causing a hollow appearance of the stem at the point of attack. The surrounding bark, by its attempts to heal over the wound, causes a thickened or burly appearance of the trunk, thus imparting to affected trees the cankered, swollen and distorted look that is so distinguished a characteristic.
The disease appears in this country on the larch, both common and Tyrolese, at all stages of growth up to thirty years, but rarely after that age. I have examined a plantation of only four years’ growth sadly infested by the Peziza whereas, in other cases, the trees may be fully twenty years old before being attacked.
Cause.—Under what conditions of growth the larch is most susceptible to the Peziza is still a matter of vague uncertainty, but there can be little doubt that an enfeebled constitution, as fully explained in the article on the larch “Trees for Economic Planting”, aided by our peculiarly erratic climate, has much to answer for. The variableness of our spring weather is, no doubt, one of the predisposing causes of disease, for, although no degree of frost experienced in this country can injure the tree when leafless, yet few are more sensitive when in young foliage.
Bearing on the subject of the larch disease, I have communications from almost every part of the country, and have personally visited and examined many of the worst infested plantations, particularly in England and Wales. Soil, if we exclude peaty, would seem to have little or nothing to do with encouraging the disease, as I have found it equally virulent on dry, sandy and heavy damp soils, but worse on chalk. Curiously enough, the disease is hardly known in the peat bogs of Ireland, and there the rainfall is excessive. It is equally strange that it first made its appearance in the Eastern and drier parts of England, and gradually proceeded to the more humid West. I have, however, noticed it in certain low-lying still, and damp portions of some woodlands, and where frosts would be most prevalent, but such cases are not general.
Remedy.—Several remedies have been more or less successfully tried with a view to getting rid of the disease on trees, such as by cutting and scraping out the injured portions, and applying a suitable dressing; but such remedies, although suited for single specimens, cannot be applied to a whole area of infested trees.
Under exceptionally favourable conditions, I have found the larch to outgrow the disease, though the cankered, swollen stems are never afterwards of great value for constructive purposes. Prevention in the present case is undoubtedly the best measure, for when once it has made headway, the larch disease is most difficult to cope with. By planting only sound, healthy trees, with uninjured roots in the most suitable soils and situations, and retaining these in as healthy a condition as possible, can we guard against attacks of the disease. Hardwoods mixed with the larch are to be recommended.
Polyporus squamosus attacks the elm and other hard-wooded trees and rapidly brings about decay. It is one of our commonest species on diseased trees.
The Sycamore Fungus (Rhytisma acerinum).—Every one interested in trees must be familiar with the conspicuous black, pitch-like spots which so mar the appearance of sycamore leaves. These are due to the above-named fungus which, appearing as small yellowish spots on the undersides of the leaves towards the end of June, gradually increase in size and intensity of colour until they attain to fully half an inch in diameter and are inky black, with a margin of dirty yellow. The attacked portion of the leaf becomes wrinkled and much thickened in texture, while all the affected foliage drops off prematurely. After lying on the ground during the winter, the thread-like spores are produced in large quantities at the time when the foliage of the sycamore is appearing in May and June. The sycamore is not the only tree affected by this fungus, for the Norway Maple and our native Acer campestre are equally liable to attack, and have in some instances suffered very severely from this cause. The fungus is becoming more plentiful, and it disfigures the maple and sycamore leaves to a wide extent.