Plant pathology

“ The future problems of plant pathology are manifold. The period of growth must continue long before the work now undertaken is done. Many diseases of even the cultivated plants are not yet recognized. The diseases of wild plants, particularly the weeds, must too be studied to ascertain the possibility of intercommunication of diseases between weeds and crop plants. The life histories of all disease producing fungi must be closely studied, particularly to determine their hibernating condition. ”

“ Growth in popular appreciation of the importance of plant diseases and of the value of remedial and prophylactic measures is perhaps the most striking characteristic of plant pathology in the last twenty years. At the beginning of this period spraying was in no wise general. It was of rare occurrence. Man suffered unresistingly the attacks of the molds, mildews, rots and blights. ”

“ Some of the variations in grafted plants are found to predispose the plant to disease, or the reverse, and cases may be cited where the resulting shoots, foliage, or fruits, or seedlings more readily fall a prey to, or resist, parasitic fungi and insects than the ungrafted plants. ”

“ If the soil contain not the inorganic elements, which are absolutely indispensable for the tissues of the plant, or even if they be there and not in a state to be absorbed, a dwindling and degeneration ensue; if light be deficient in quantity, pallor, feebleness, and elongation of tissue follow, with more fluidity and general softness of texture. These conditions of plants have their analogues in the ill-fed and ill-nourished children in some of our manufacturing districts; they are stunted and diseased. ”

“ A plant may be diseased as a whole because all or practically all its tissues are in a morbid or pathological condition, such as occurs when some fungus invades all the parts or organs—e.g. seedlings when completely infested by Pythium, or a unicellular Alga when invaded by a minute parasite ”

“ Having now obtained some insight into the factors concerned in disease, let us enquire further into the bearing of variation on these. It is evident that pathological conditions may vary; indeed they are themselves symptoms of variation, as we have seen. The history of all our cultivated plants shows abundantly that many of the variations obtained by breeding in our gardens, orchards, fields, etc., involve differences of response on the part of the plant to the very agencies which induce disease. ”

“ Such plants, which always consist of single cells, are called one-celled; they are found among the lowest forms of the microscopic world, among the algae and the fungoids. The green coating that covers the rocks, the tree-trunks, and the shingles of the roof, is resolved by the microscope into innumerable green round cells; the brown scum that floats upon ponds and ditches exposed to the sun, the yeast-plant, the bacteria that produce putrefaction, are one-celled plants of this kind. ”

“ FROM the time men first had interest in plants, knowledge of their imperfections or premature death has existed, without, however, definite conception that the imperfections in question really constitute a condition of disease.
The Bible and the early writings of the Greeks and Romans contain references to what we now recognize as wheat rust, fig blight, insect galls and other of the more strikingly conspicuous plant ailments. ”

“ In Conifers the stoppage of the lumina is increased by deposition of resin, which also soaks into the cell-walls and the wounded wood becomes semi-translucent owing to the infiltration.
Every living cell in an active condition is irritable, and one of the commonest physiological reactions of growing tissues is that of responding to the touch of a resistant body, as is vividly shown by the movements of the Sensitive plant, Dionaea, etc., and by those of tendrils, growing root tips, etc., on careful observation. ”

“ The disease is seldom troublesome on plants grown thinly in the open air. If it makes its appearance, water thoroughly, but not too often, and sprinkle dry sand over the seed-bed among the plants. [D]
Black Leg or Mildew.—This is a disease which attacks the stems of young plants which are being wintered over. It is undoubtedly due to one or more species of parasitic fungi, but I do not find that the subject has been studied. Doubtless the rupture of the bark by alternate freezing and thawing gives the fungi an opportunity to attack the plant. ”

“ When we come to enquire into the causes of disease, it appears at first an obvious and easy plan to subdivide them into groups of factors which interfere with the normal physiology of the plant. Scientific experience shows, however, that the easy and the obvious are here, as elsewhere in nature, only apparent, for disease, like health, is an extremely complex phenomenon, involving many reactions and interactions between the plant and its environment. ”

“ Plants have their troubles as well. Perhaps you have noticed on wild cherry trees growing by the roadside, or on plumb trees, that sometimes almost every tree in a clump, and even every branch on a tree, will have a hard black lump, as large as your fist, growing out of the bark. The tree is sick; sick with a patching disease of trees called the black knot.
The disease itself is something like a mold. It breaks through at the growing tip of the branch where the bark is new and thin. Thence it grows down the branch sucking up the living substance of the plant and killing everything as it goes. ”

“ Light, heat, moisture, and soil are therefore to be considered as the agents required to exist in a certain balance, or proportion, in reference to the health or power of vitality of the plant. Within a certain amount of variation, health may persist in virtue of the power of selection, which appertains to the spongioles of the root in absorbing nutriment; and also as regards light, from the tendency which most plants have to accommodate themselves to any deficiency of this element, by presenting their leafy expansion in that direction where the most of its influence may be obtained. ”

“ Cell-contents vary—Citrus, Cinchona, Almonds, etc. Double ideals in selection—Cultivation of pest and host-plant—Variations of fungi—Bacteria—Specialised races—Difficulties—Experiment only will solve the problems.
The numerous and often expensive failures in the application of any prophylactic treatment, have proved an acute stimulus to the research for other ways of combating the ravages of plant diseases. It is a matter of every-day experience that particular varieties of cultivated plants may suffer less from a given disease than others in the same district ”

“ Bark-boring—Wood-boring—Wood fungi—Leaf-miners—Pith flecks—Erosions. Skeleton leaves—Irregular erosions—Shot holes. Frost cracks—Strangulations—Spiral grooving.
Natural wounds are produced in a variety of ways during the life of the plant, and, generally speaking, are easily healed over by the normal process if the area destroyed is not too large, and the parts remaining uninjured are sufficiently provided with foliage, or with supplies of food-materials stored up in the roots, rhizomes, medullary rays, etc., to feed a vigorous callus. ”

“ We have seen that all the essential phenomena of disease concern only the living substance—the protoplasm—of the plant, and that however complex the symptoms of disease may be, the occurrence of discolorations, lesions, hypertrophies, and so forth are all secondary matters subsidiary to the fundamental alterations of structure and function constituting the disease. ”

“ In plants, however, symptoms of disease are apt to exhibit themselves in a very general manner, or at any rate it may be that our perceptions of them differentiate symptoms due to very different reactions imperfectly, probably because the organisation of the plant is less specialised than that of animals. ”

“ Now is the strength of the living wall of the plant tested: as long as it is unbroken, the assault is repelled; but the persistent enemy presses into the smallest opening. Woe to the tree from which the wind has broken a limb, or in which the careless gardener has made a bad cut! The microbes, whose spores are floating through the air in unwholesome clouds, and fall with the dust, settle upon the wounded surface, and soon its whole cell-structure is pervaded by their destructive webs. ”

“ If left in a foul state they spread the infection to all around. In the space at our command it is only possible to notice a few of the garden pests, and we begin with one of the most frequent and troublesome of plant foes. ”

“ It is equally important to have the right kind of light, as disastrous experiences with greenhouses glazed with glass which cut off certain rays of light have taught. Here, again, it is important to notice that the optimum intensity or quality of light may differ for different functions and organs of the plant, as is shown by many adaptations on the part of species growing in natural situations—e.g. bud protection, orientation of leaves, etc.—and it may be taken as a rule that etiolated plants are peculiarly susceptible to other diseases. ”

“ Nobody thinks of saying that red or striped roses are diseased because they are departures in color from the white flower of the type species; or that white, yellow, or striped roses are diseased when the color of the type species is red. Nobody thinks of saying that double flowers are evidences of disease in the plant, or that diminution in the size of leaves or variation in their form is a disease. Why then should it be said that because leaves may become of some other color than green, or become party-colored, therefore they are diseased? ”

“ Such a parasite—Plasmodiophora brassicæ—enters the young root cells of a cabbage, stimulates those cells to an unwonted degree of multiplication until great tumors are formed giving rise to the vegetable disease known as 'club-root.' The plant cells become storage reservoirs of the spores of Plasmodiophora, and when the plant dies down the spores are liberated in the soil. ”

“ These sap the vitality of the plants and check their progress. If you dig up the soil a few inches and mix in it the fresh clipped grass from the lawn or a bit of very old manure you can offset this evil.
“The minute you find mildew on a plant, fight it, or it will spread so rapidly to other plants that you will find it well nigh impossible to kill it. In a very short time, your most beautiful flowers will be nothing but a memory. Powder your diseased plants with soot and sulphur nor care for their looks as long as you save them in the end. ”

“ On the other hand, we have reason to believe that many species have died out gradually as the environment altered. These plants died because they did not vary sufficiently, or did not vary in the right directions; they became diseased with respect to the then prevailing conditions of normal physiology or health.
Disease, therefore, may be said to be variation of functions in directions, or to extents, which threaten the life of the plant, the normal in all cases being the state of the plant characteristic of the species. ”

“ There are two very good reasons why some classes of plants cannot be well grown continuously in the same piece of ground. One is the depletion of available plant food, the other the formation of injurious compounds by the plants, or the gradual increase of fungoid, bacterial or animate pests in the soil, which finally become abundant enough to seriously hinder growth. Different plants take the plant foods, as nitrogen, lime, potash, phosphates, etc., in different proportion. ”

“ In plants, from their greater simplicity of structure, organic affections are perhaps entirely limited to the two first forms of animal organic disease; viz. to undue development of tissue in natural situations, and to the formation of natural tissue in parts of a plant where they are not usually found in a state of nature. ”

“ In the first place, the enormous holdfast, ensured by the millions of points of adherence, enables the plant to withstand even powerful lever actions from above, and provides fixed points against which the root-tips can work as they drive deeper into the soil. In the second place, the intimate contact of the root-hairs and particles of soil, ensures that the films of water held by surface-action on the soil-particles and root-hairs shall be in continuity with the water saturating the cell-walls of the latter, and therefore with the protoplasm and cell-sap in their interior. ”

“ As the discoveries of the microscopists continued to build up our knowledge of the anatomy of plants and began to elucidate the biology of the fungi and other cryptogams, while the chemists and physiologists laid the foundations of our modern science of plant life, it gradually became possible to tabulate and classify plant diseases, and discuss their symptoms and causes in a more scientific manner. ”

“ Do you know that this is a serious disease of the strawberry? What damage does fruit mold do to peaches, plums, or strawberries?
Write to your experiment station for bulletins on plant diseases and methods for making and using spraying mixtures.
SECTION XXVIII. YEAST AND BACTERIA
Can you imagine a plant so small that it would take one hundred plants lying side by side to equal the thickness of a sheet of writing-paper? There are plants that are so small. Moreover, these same plants are of the utmost importance to man. Some of them do him great injury, while others aid him very much. ”

“ THE destruction and decomposition of organic substances, both animal and vegetable, are promoted by the lower fungoids, particularly by yeast-plants and molds, which we may for brevity call rots. During life, that is, as long as a lively circulation is kept up, plants are protected against the attack of these ever-present organisms, but during the periods of rest, when the life-activity of the plant is reduced to a minimum, defense by the vegetative process is suspended. The older parts of plants, also, in which the normal circulation has become very limited, are poorly fitted to resist decay. ”