Disease in plants

by H. Marshall Ward

EXUDATIONS AND ROTTING.

Tumescence—Rankness—Bursting of fruits, etc.—Root rot—Rot of fruits—Bulb diseases—Flux—Honey-dew—Slime flux—Resinosis—Gummosis—Manna.

I put together in one artificial class a varied group of diseases, the principal symptom of which is the escape of fluids from the tissues, under circumstances which betray an abnormal state of affairs, often obvious, but sometimes only to be inferred. In many of these cases bacteria abound in the putrefying mass, and some evidence exists for connecting these microbes causally with the disease in a few of the more thoroughly investigated cases, but in no case has this been sufficiently demonstrated; and considering the ease with which bacteria gain access via wounds caused by insects and fungi, as well as by other agents, the necessity for rigid proof must be insisted upon before we can accept such alleged examples of Bacteriosis.

Tumescence.—It occasionally happens that herbaceous parts of plants pass into a condition of over-turgescence from excess of water in the tissues, an abnormal state which indicates pathological changes resulting from various causes, often not evident and therefore regarded as internal. Such disease was formerly termed Œdema or Dropsy. This disease is frequently due to the excessive watering of pot plants with large root systems and deficient foliage, in hot-houses with a saturated atmosphere: it is, therefore, primarily referable to diminished transpiration. It can sometimes be brought about by covering potato plants, for instance, with a bell-jar in moist, hot weather; and this, and the prevalence of the disease in hot-houses as compared with plants grown out of doors, point to the above explanation. Similar phenomena do occasionally occur out of doors in hot, moist situations or during wet seasons, however, and the watery shoots of rank vegetation are merely particular cases of the same class. Moreover, the well-known tendency to succulence of sea-side varieties of plants which have thin herbaceous leaves when growing inland, points to the action of the environment in these matters, excess of salts being no doubt one factor in such cases.

Rankness affords another example where superfluity of water is concerned, though it does not involve simply this, because the plant may also contain excessive quantities of nitrogenous and mineral matters taken up by the roots.

Rankness is, in fact, in many respects analogous to etiolation in so far as the tissues are soft and surcharged with water, but it differs fundamentally in the deep green of the chlorophyll: this may lead to abundant assimilation if free access of air and drier conditions can be gradually brought about. Any sudden drying, however, may be fatal to the tender tissues.

Rankness commonly depends on excess of food materials, especially nitrogenous manures, as may be seen in meadows and cornfields where the manure heaps have remained on the ground and saturated it to excess as compared with the rest of the soil; this may often be observed with weeds, etc., in the neighbourhood of farm-buildings. If the period of rank growth is accompanied and followed by days of suitably bright sunshine and dry air, the increase of vegetative structures usually results in increased flowering, heavy crops, or strong wood; but if the rankness continues too long, or is accompanied by wet and dull weather, the watery tissues are peculiarly susceptible to attacks of fungi and insects, and to damage by sudden frosts or chilly winds. Rankness affords, in fact, a typical illustration of predisposition to disease.

Damping off.—When seedlings are too closely crowded in beds kept too damp, or in moist weather, they are very apt to rot away, with all the symptoms—spreading from a centre, contagious infection, mycelia on and in the tissues, etc.—of a fungus attack. The commonest agent concerned is one of the species of Pythium, the propagation of which is favoured by the rank, over-turgid, and etiolated conditions of the plants. Species of Mucor, Botrytis, and other fungi, may also be met with.

Bursting of fleshy fruits, such as Tomatoes, Grapes, etc., is due to over-turgescence in rainy weather or excessively moist air. But the phenomenon is by no means confined to such organs. Hot-house plants when oedematous not infrequently put out watery blisters from the cortex or leaves, which rupture; and the stems of fleshy fasciated (e.g. Asparagus) or blanched and forced plants (e.g. Celery, Rhubarb) are particularly apt to crack here and there from the pressure of the turgescent tissues on the strained epidermis. Beets, Turnips, and other fleshy roots show the same phenomena in wet seasons. That these ruptures and exposures of watery tissues afford dangerous points of entry for parasites and moulds will be obvious—e.g. Edelfäule, a rotten condition of the grapes in the Moselle district.

Root-rot is a common disease in damp, sour clay soils after a continuance of wet weather—e.g. Wheat, especially if root-drawn and exposed to thaw water.