Plant disease

“ Whenever any crop shows signs of root disease, or soil disease, it is particularly important that another crop be grown on the place.
See that the disease or insect is not bred on weeds or other plants that are botanically related to the crop you grow. If the wild mallow, or plant known to children as “cheeses” (Malva rotundifolia) , is destroyed, there will be much less difficulty with hollyhock rust. Do not let the cabbage club-root disease breed on wild turnips and other mustards, or black-knot on plum sprouts and wild cherries, or tent-caterpillars on wild cherries and other trees. ”

“ 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. ”

“ Hollyhock-disease and the cotton-plant. The hollyhock-disease has been a bane to European gardeners for ten years past. It is one of the most destructive of plant-diseases; being able to kill young plants within a week from the time of its attack, and making sad havoc wherever it appears. ”

“ 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. ”

“ 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 ”

“ Weeds in and about gardens are a common cause of disease in cultivated plants. It often happens that a weed, being sturdy, is only slightly inconvenienced when attacked, whilst a cultivated plant will speedily succumb if attacked by the same fungus. This is the case in the Sempervivum disease. In this country the common House Leek is the nurse-plant, and is seldom much injured; but if the disease Endophyllum sempervivi gets among greenhouse species, every plant may be utterly destroyed. ”

“ But while this system may be adopted to some extent, it must also be borne in mind that the frequent repetition of some crops cannot be practised with impunity, for plants are liable to certain diseases which manifest themselves to the greatest extent when they have been too often cultivated in the same soil. Clover sickness, which affects the plant when frequently repeated on light soils, and the potatoe disease and finger and toe have been attributed to the same cause. ”

“ Our perceptions differentiate but imperfectly symptoms which are due to very different causes and reactions, probably because the organization of the plant is so much less highly specialized than that of higher animals. The yellowing and subsequent casting of leaves, for instance, is a very general symptom of disease in plants, and may be induced by drought, extremes of temperature, insufficient or excessive illumination, excess of water at the roots, the action of parasitic Fungi, insects, worms, &c., or of poisonous gases, and so forth ”

“ The breeding of plants resistant to specific diseases not readily amenable to other means of control must proceed. Such work is now in progress with cotton, melons, tomatoes, tobacco, grains, flax and other plants. The relation existing between many root fungi and bacteria and the roots they inhabit remains to be studied. Aside from parasitism there is also mutualism, a kind of beneficial disease falling to the province of plant pathology. ”

“ Not only may such insects weaken the infested plants, but they also cause the destruction of chlorophyll, interfere with the normal functioning of the stomata, and have a toxic effect upon the tissues; further, many serious plant diseases are carried and spread by sucking insects, whilst the punctures made when feeding may allow the entry of disease spores. ”

“ The guiding principle must be to allow to each plant ample room to spread, and absorb the air and sunshine, in accordance with the character of the sort and the condition of the soil. A considerable proportion of the losses from disease may be traced to overcrowding in the first instance; the tangled haulm being rendered weak through want of air, and then becoming loaded with water, and in contact with wet ground, the disease has made havoc where, had the management been founded on sound principles, there might have been a vigorous healthy growth. ”

“ 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. ”

“ In almost every administrative district there is a Consulting Pathologist, connected with the Bureau of Plant Pathology of the Department of Agriculture, who has charge of all work dealing with the eradication of tree diseases.
A tree disease is really any condition that interferes with the normal functioning of the tree, be this condition caused by fungi, mistletoe, fumes, smoke, frost, sunscald, drought or excess of water in the soil. Parasitic fungi and mistletoes cause most of the tree diseases. ”

“ Club-root or club-foot is a well-known disease. The parasite lives in the soil. Practice crop rotation. Set only healthy plants. Do not use manure containing cabbage refuse. If necessary to use infested land, apply good stone lime, 2 to 5 tons per acre. Apply at least as early as the autumn before planting; two to four years is better. Lime the seed-bed in same way.
Carnation rust.—This disease may be recognized by the brown, powdery pustules on the stem and leaves. Plant only the varieties least affected by it. Take cuttings only from healthy plants. ”

“ 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. ”

“ Division of Forest Pathology, Plant Industry Station, Beltsville,
Maryland.
Mass plantings of many trees of the same kind frequently result in an increase in the severity of insect pests and diseases. Leaf diseases, for instance, spread quickly through such plantings when weather conditions favor growth of the causal organisms. Plants on sites unfavorable to a specific tree species also are responsible for disease increases. Chinese chestnuts grown on a site where they are subject to early-fall and late-spring frosts will fail. ”

“ 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. ”

“ The bacterial diseases of the tomato, potato, and egg plant, and the club-root, brown rot, and wilt disease of the cabbage, all more or less widely distributed, are readily transmitted in the soil; while in the South and West there are the wilt diseases of cotton, melons, sweet potatoes, cowpeas, and flax, and various nematoid and root-rot diseases which might easily become a serious menace over areas much larger than they now occupy if deliberately spread by the careless use of soil for inoculation purposes. ”

“ 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. ”

“ 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. ”

“ TOMATO-LEAF RUST.—The leaves of the plant attacked by this disease rapidly become covered with a dull brownish velvety mould, or fungus, known as Cladosporium fulvum. From the mouldy spots and patches thousands of spores are readily carried by a slight current of air to the surrounding healthy crop, and unless prompt measures are taken to check the pest the whole house is rapidly involved. Excessive atmospheric moisture encourages the mould, and it is spread extensively if diseased plants are sprayed with water in the presence of healthy ones. ”

“ 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. ”

“ 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. ”

“ Hence the plants are entirely dependent for healthy growth upon an adequate supply of oxygen to their roots from the medium in which they are growing. A serious disease of the rice plant, characterised by the browning and dying off of the leaves, which was thought at first to be due to the attacks of fungi, was found to be the effect of the inadequate aeration of the roots, while the entry of the fungi was shown to be subsequent to the appearance of the physiological disease. ”

“ But his mind had now become definitely fixed on the problems presented by plant diseases, and they remained the principal occupation of his life. In their widest sense these resolve themselves into a consideration of the mode in which one organism obtains its nutriment at the expense of another. This ranges from a complete destruction of the host by the parasite to a harmless and even advantageous symbiosis. ”

“ PLANT ENEMIES AND INSECTICIDES The well-tended garden does not suffer materially from inroads of insect pests especially in favorable seasons; cool, damp weather, and hot, muggy weather are conducive to fungoid diseases which sap the strength of the plants and make them less resistant to any kind of assaults, whether of insects or disease, but with normal weather and bright dry air a part of each day at least, little trouble should be experienced from insect pests ”

“ That a parasite may be prevented from successfully attacking a particular plant is shown by the failure of Cuscuta to establish its haustoria in poisonous plants such as Euphorbia, Aloe, etc., and it has been pointed out that poisonous secretions in the cells of the plant protect them against the penetration of fungi. ”

“ That some of them do cause disease is indisputable, since bacteriologists have, by their watchful and careful methods, separated almost a single plant from its surroundings and congeners, planted it free from all contamination, and observed it produce an infinitesimal brood of its own kind. Animals and patients inoculated with the plants thus cultivated have rapidly become subjects of the special disease which the particular plant was supposed to produce. ”

“ In this country today plant diseases result in losses estimated at $3 billion a year. So far, the most economical means of reducing the ravages of plant diseases has been to breed resistant plant varieties. ”