Disease in plants

by H. Marshall Ward

Atrophy of the grains of cereals is sometimes due to the direct attack of animals, e.g. eel-worms (Tylenchus) eat out the grains of Corn; weevils and other beetles (Curculio, Bruchus, etc.) similarly devour the contents of grain and nuts, the flowers of Peas and Apples, and so forth, inducing atrophy of the parts left. Still more striking cases are afforded by small insects which bore into the halms of cereals, and cause atrophy of the whole ear—e.g. Cephus in Wheat and Rye. Barley occasionally withers after flowering, the grain atrophying from no known cause, terms like consumption given to the disease conveying no information.

Atrophy of young fruits is commonly due to the flowers not setting—i.e. some agent has interfered with the normal transference of the pollen to the stigma. This may be due to excessive rain washing out the pollen (e.g. Vine), to a lack of the necessary insects which effect pollination, often seen in greenhouse plants; to the stamens being barren—e.g. certain varieties of Vine—or to the premature destruction of the stigmas by frost, as in Cherries, Pears, etc., or by insects, as in Apples, or fungi, e.g. the infection of bilberries with Sclerotinia; or even by poisonous gases, as is sometimes seen in Wheat, etc., growing near alkali works. Drought is also a common cause of atrophy of young Plums.

Shanking of Grapes is a particular case of atrophy and drooping of the immature fruits, due to the supplies being cut off by some agency. It may arise from very various causes which bring about disease in the leaves or roots, and should always be looked upon as a sign of weakness in the Vine, the structure of which is affected, e.g. poor wood—or the functions interfered with, e.g. water supplies deficient owing to paucity of roots.

Barren Apple, Pear, Plum, and other flowers are often found to have been bored through the petals while in bud, and the whole "heart" of the flower eaten out by the grubs of Anthonomus, leaving the unopened buds brown and dead, as if killed by frost or drought, and often erroneously supposed to be so.

The wilting and shrivelling of Clover is sometimes due to Sclerotinia, the mycelium of which pervades the roots and stock, on which the sclerotia may be found. Lucerne is similarly killed in Europe by the barren mycelium of Leptosphaeria, which may be found as a purple mat on the roots.

Dwarfing consists in partial atrophy of all the organs, and is a common result of starvation in poor, dry, shallow soils, as may often be seen in the case of weeds on walls or in stony places. Dwarfs which are thus developed in consequence of perennial drought are not, however, necessarily diseased, in the more specific sense of the word; their organs are reduced in size proportionally throughout in adaptation to the conditions, and simply carry out their functions on a smaller scale.

Dwarfing is frequently a consequence of the lack of food materials, or of some particular ingredient in the soil, and in such cases is a diseased condition of some danger; similar results may ensue in soils containing the necessary chemical elements, but in unavailable forms.

Dwarfing may also be brought about by repeated maiming, nipping off the buds, pruning, etc., as in the miniature trees of the Japanese; and the case of trees continually browsed down by cattle, or of moor plants perennially dwarfed by cutting winds, are further illustrations in the same category, as are also those of certain alpine and moraine plants, whose only chance of survival depends on their adapting themselves to the repeated prunings suffered by every young shoot which rises into the cutting winds, since there is no question of lack of food-materials in these cases.

The practice of the Japanese is to pinch out the growing tips of the shoots wherever they wish to prune back, and it is by the judicious use of this heading in, and suitable pot-culture, that the dwarfs are made, 6-20 inches high at from 30-80 years old.

Dwarfing is often brought about by grafting on a slow-growing stock, and this method is employed in practice, as are also heading in, pruning of roots, and confinement in pots.