This section is from the book "Commercial Gardening Vol3", by John Weathers (the Editor). Also available from Amazon: Commercial Gardening, A Practical & Scientific Treatise For Market Gardeners.
It may be stated at the outset that the various preventive or remedial measures suggested in this article are only intended for use against parasitic fungi, and would not answer for insects, or in many instances might be diametrically opposed to the most approved means by which insects are successfully controlled. There is a very marked difference in the mode of procedure of the person who undertakes to destroy insects and the one who attempts to hold fungi in check; and I think that perhaps the chances of success are greater with the former, for the following reasons. The majority of insects are superficial; for example, hordes of caterpillars, etc. - such can be readily reached and killed. There is no such parallel in the case of the fungi. When an epidemic due to fungi is present, in the great majority the fungus is safely located in the tissues of the leaf, or other part of the plant, and absolutely free from treatment of any kind; hence a case of a cure against a fungus parasite that has once entered the tissues of a plant is unknown, whereas an epidemic resulting from insects may be, and is often, cured.
Coming to preventive methods, or in other words checking the extension of an outbreak, or in preventing its commencement, conditions still favour the destroyer of insects. It is a well-known fact that in the field, as spraying is done on a large scale, it is a very difficult matter to cover every portion of every leaf with the fungicide used. Well, this point is not of such vital importance in the case of dealing with insects as with fungi. The caterpillar, in obtaining its food, moves at a comparatively rapid rate, and sooner or later is certain to eat a portion of the leaf coated with the insecticide, which seals its doom. On the other hand, if the spore of a fungus alights on a portion of the leaf not protected by the fungicide, under favourable conditions it germinates and enters the tissues, and an infection which may spread is set up. If the spore alights on a portion of the leaf covered with the fungicide, of course it is killed at the moment of germination.
A point of great practical importance to cultivators of plants is a clear discrimination between primary and secondary causes of disease. In a great many instances the secondary cause of a disease is by far the most obvious, and in many instances in reality is responsible for the injury done; yet but for a primary cause, often of a very trivial nature, the secondary cause could not have come into being.
The following are illustrations of this condition of things. All the large toadstools and bracket-like fungi so destructive to our orchard and timber trees are known as wound parasites; that is, they can only gain a foothold on the trunk or branch of a tree through a wound. Such wounds may be due to the wind breaking a branch, to a heavy weight of snow, to pruning, etc. In all such cases a wound is produced, and if the scar made by the breaking away of a branch by wind, or by pruning, is left to take care of itself, in all probability some fungus will gain an entrance. On the other hand, if such wounds, as far as practicable, are properly trimmed at once, and thoroughly covered with gas tar, the tree is saved from the attacks of fungi which, if allowed to gain a foothold, speedily work its ruin.
Hundreds of trees die annually in this country for no obvious reason. From an apparently perfectly healthy condition the leaves commence to wilt and turn yellow early in the season. This condition of things may continue for two or three seasons, gradually becoming worse, until eventually the tree dies. In ninety-nine cases out of a hundred, when a tree shows the symptoms described above, death is due to the presence of a fungus that has entered the tree at the collar. Most of the fungi that cause such diseases are wound parasites, as defined above; but, whether this is the case or not, all fungi are ever ready to take advantage of a wound, rather than have to work their way through an unbroken surface to reach the living portion of the plant, from which they can alone obtain food. The spawn or mycelium of such root- or collar-infesting fungi is present in the soil practically everywhere, and rarely loses an opportunity of entering the tissues of a living plant when conditions are favourable for so doing.
The wounds enabling such fungi to gain an entrance into the collar of a tree are due to very varied causes. Over some of these causes we have complete control, if a certain amount of intelligence and care is exercised. In many instances it is certain that hundreds of valuable trees are annually injured, and many eventually killed, by the careless manner in which lawn mowers or grass-cutting machines are handled. A piece of bark from a projecting root, or from the base of the trunk, is torn away. The wound, if noticed at all, is usually covered with soil, so as to hide the scar, and left at that. Fungi present in the soil do the rest. If an accident of this kind does happen, the wound should at once be coated with gas tar. The actual proof that a tree has been, or is being, killed by a fungus in the root or collar is the presence of a white film of spawn or mycelium just under the bark. If a portion of bark is cut away, the white mycelium can be clearly seen. If the disease has not proceeded too far, that is. if it is examined on the first suggestion of wilting or yellowing of the foliage, it may be saved. The soil should be cleared away from the root as far as practicable. The root and base of the trunk should then be thoroughly well dusted over with a mixture of powdered quicklime and sulphur, using twice as much of the former as the latter. Fresh soil should be used in filling up the hole formed, and it should be well sprinkled at intervals with the same mixture of quicklime and sulphur. On examining a tree for the presence of a fungus, if the white mycelium is found between the bark and the wood above the collar, it may be taken for granted that the tree is doomed, and the sooner it is removed the better. The soil should be thoroughly sterilized, by using quicklime and sulphur, before another tree is planted in the same site.
Another very serious source of loss, especially when planting young conifers, is due to the common practice termed " heeling in" the young trees. A hole is made, the young tree placed in position, the earth pushed into the hole and stamped down, the final touches in the way of fixing the young tree being effected by the boot heel, which is stamped down round the stem. In too many instances the heel " barks" the tender young stem, producing a wound, which is promptly utilized either by the "canker fungus" (Dasyscypha calycina), if the tree happens to be a larch; or by the root fungus (Fomes annosus), which is less fastidious in its choice, so long as its victim is a conifer. Many thousands of young conifers die from the above causes, whereas by the exercise of a certain amount of care the loss could be reduced to a minimum.
The most important factor in combating the many diseases caused by fungi, and the one which invariably yields the best results, is all-round cleanliness. So far as fungi are concerned the significance of this advice cannot be grasped by the cultivator of plants, because the habits and mode of life of the fungi are so different from those of the plants he has to deal with. However, he must accept the facts that when a diseased plant is thrown on the manure heap, and allowed to decay and form manure, the spores of the fungus present in such diseased plant are not destroyed, but are carried back to the land, sooner or later, in a living condition, and ready to infect a future crop. Again, when portions of plants injured by a fungus - potatoes, foliage, etc. - are eaten by animals, the spores are not destroyed, but in many instances are all the better able to germinate after having passed through the alimentary canal of an animal, and when returned to the land in the manure are in a fit condition for infecting any suitable plant. The obvious moral is, burn all diseased plants promptly, and do not pile them in a heap to rot for manure, neither give such as food for cattle or pigs, otherwise the certain result will be an outbreak of the same disease at some later date.
The whole of the space allowed me for discoursing on the diseases of fruit trees generally could easily be occupied in suggesting and indicating weak points which favour the spread of disease. This, however, cannot be; therefore I advise that, in the case of a disease, promptitude, combined with the removal of infected plants from the neighbourhood of healthy ones of the same kind, whenever practicable, will go far towards preventing the outbreak of an epidemic. [g. m.]