This section is from the book "Commercial Gardening Vol1", by John Weathers (the Editor). Also available from Amazon: Commercial Gardening, A Practical & Scientific Treatise For Market Gardeners.
Notwithstanding the enormous amount of mischief done by insect pests to the various crops grown in the open air and under glass, that caused by fungoid diseases is if anything more considerable, and necessitates a large outlay every year to keep the diseases in check. While one may by clean cultivation and attention to natural laws keep insect pests largely in check, the very best cultivators may be caught napping when a fungoid disease begins to ravage his crops. Slugs, snails, caterpillars, moths, butterflies, beetles, etc, after all are enemies that can be detected by an observant cultivator, and measures for their suppression can be taken in good time. Not so, however, with the various fungoid diseases that cause so much mischief. In the early stages these are hidden from the eye, being of microscopic proportions, and it is not until plants or fruits have been in a measure destroyed that the disease is observable. The tiny speck or blotch on a leaf or fruit to-day may develop into a large and putrid mass of vegetable tissue to-morrow, filled with thousands of spores which will be blown about by the wind, thus to carry disease and death to other plants.
The true fungi consist of a large number of stemless cryptogamic plants, the chief feature of which is that, unlike the higher Cryptogams (such as Ferns), and the flowering plants proper or Phanerogams, they lack green colouring matter or chlorophyll. Owing to this absence of green colouring matter in the tissues, fungi are unable to utilize sunlight as a source of energy and food assimilation. They cannot take in carbonic acid gas from the atmosphere, nor can they absorb nitrogen. Indeed fungi give off carbonic acid gas as a waste product. They must therefore obtain their nourishment in a form already prepared for their reception either by living plants or by dead ones. Hence there are two distinct groups of fungi: (1) those that exist on or derive their food from dead plants or organic material are known as "saprophytes", and (2) those that obtain their food from living plants are called "parasites". As a rule the parasitic fungi are most dangerous to the cultivator, because they attack his living plants in various stages, and unless checked or eradicated are likely to inflict serious losses. Intermediate between the true parasitic fungi and the true saprophytic ones comes a class that first of all causes portions of living plants to die by secretions or ferments of some kind, and then gains an entrance into the living tissues, and eventually causes their death.
According to their mode of attack the parasitic fungi are divided into two groups: (1) the epiphytic and (2) the endophytic. The latter penetrate the tissues of the plant and there develop their mycelium; the former vegetate on the surface and spread their mycelium over it.
The mycelium constitutes the typical vegetative structure of a fungus, and is of a thread-like and much-branched character. The threads of which the mycelium consists are called "hyphse". In some forms of fungi, however, there are no threads, but separate cells, as in the Yeast Plant. The mycelium (or its hyphae) may be one-celled, or it may be divided transversly into separate compartments, each of which may contain several nuclei from which in due course spores may develop. When the hyphae of some fungi branch between the cells of plants, they often develop. Generally speaking the visible part of a fungus is the " fruiting" part, from which fresh spores are distributed when ripe, and are carried from one place to another by the wind and water. In such fungi as the Common Mushroom, the so-called Toadstools, the Beef-steak Fungus, and many other conspicuous plants, the spores may be readily collected if the "caps" of the fungi are placed on sheets of paper in a warm room, and may be easily seen with the naked eye. In the case of the fungi that attack our plants, however, the spores and even the entire fungus pro ducing them are very minute, and require powerful microscopes to distinguish some of their special feeding or absorbing organs, called "haustoria", which penetrate the cell walls and absorb the nourishment from the cells.
It is thought that fungi originally came from the Algae, and have undergone various changes in the process of evolution. A fungus begins life from some kind of a spore of a very simple character, quite distinct in character and structure from the seed of a flowering plant. When the spore of a fungus germinates on a suitable plant tissue it swells up by absorbing moisture and sends out a germ tube, which penetrates the host plant and as it lengthens and branches becomes the mycelium or spawn. After a time the mycelium begins to give off fresh spores, varying in form, development, numbers, etc, according to the different kinds, and known by specialists under different names. The various spores of fungi may arise from the sexual union of two distinct cells, or they may originate asexually by means of "swarm" spores. Under certain unfavourable conditions the spores of fungi may remain dormant for a considerable time, but possess the power of germinating afterwards under favourable conditions. It is the resting power of the spores of many fungi that constitutes the chief danger to the cultivator, as he never knows how many thousands of them may be sleeping in his soil. For certain pot plants, like Ferns, it is possible to sterilize small quantities of soil by pouring boiling water over it, or by roasting it in a furnace; but such operations are quite beyond the range of practicability with large quantities of soil. The nearest approach to sterilizing the soil in the open air, and preventing the spread of fungoid diseases (as well as insect pests), is to cultivate frequently and strew powdered sulphur over any areas known to be badly infected. Sulphur seems to be the great and most reliable antidote to fungoid attack, and this fact is so well recognized that it appears in one form or another in many of the best fungicides.
Another peculiar feature about some fungi is that they exist in two or three different forms or stages, and on two or three different plants. It is now well known that the "Smut" of corn exists in one stage on Barberry bushes (Berberis), from which it passes into another stage and then becomes injurious to corn crops. A somewhat similar state of affairs exists with the Apple Rust and Pear Rust. The fungus that exists on Junipers as Gymnosporangium clavaricœforme passes from the Junipers and infests Apples and Pears in the form known as Roestelia aurantiaca, causing orange-yellow or almost crimson patches on the leaves. Farmers now know that the proximity of Barberry bushes may lead to an attack of "Smut" on their corn, and fruit-growers know that Junipers may lead to the Rust of Apple and Pear leaves.
These cases are mentioned with the object of showing that there is cause and effect with fungoid attacks as with insect attacks, and once the real cause is known it becomes easier to check the spread of the disease.
In the following table some of the chief fungoid diseases afflicting fruits and vegetables are given, so that the grower may see at a glance the enemies he has to face. Further details will be found in Vol. III, dealing with Fruit Crops, and in Vol. IV, dealing with Vegetable Crops. The principal fungoid diseases afflicting Flowering Plants are referred to in Vol. II, under the plants attacked.