A simple case is often seen in Chrysanthemums, Roses, etc., chilled and wetted to danger point, but not frozen, during the nights of autumn. The lowered resistance of the chilled tissues enables fungi like Botrytis cinerea to gain a hold, and the peduncles die-back with all the symptoms of Necrosis, the fungus gaining power more and more as its mycelium spreads in the dead tissues.
Many other cases are known where wound-fungi, such as Nectria, Cucurbitaria, Phoma, etc., in themselves incapable of true parasitism, gain a hold on the necrotic tissue of a wounded twig, and having laboriously accumulated a vigorous mycelium saprophytically, extend into other parts. In many of these cases the dying-back of the twigs is expedited owing to the mycelium invading the medullary rays and wood vessels, and so obstructing the transpiration current. The much more rapid spread of the hyphae up into the parts thus killed sufficiently indicates the fundamentally saprophytic character of such fungi.
Dying-back in all its forms is a common result of defoliation by insects, e.g. caterpillars, especially if it occurs when the wood is depleted of reserve materials, and thus cannot supply the auxiliary buds and enable the twigs to clothe themselves with a new flush of foliage, a common danger in Conifers.
Any form of defoliation—e.g. excessive plucking of tea and mulberry leaves, browsing of animals, etc.—exposes the twigs to the dangers of dying-back, the accessory phenomena being similar to those already described.
Stag-head.—Old trees, though vigorous and in full foliage throughout the crown generally, frequently lose the power of bearing leaves on their topmost branches and twigs, which stand out bare and brown, and fancifully resemble the antlers of a stag: hence the forester's name "stag-head." This "top-dry" condition is frequently due to the removal of litter, or to excessive draining, or to the roots having gradually penetrated into unsuitable soil. The consequence is that some dry summer the drought causes the breakage of the water columns above, and the twigs die back.
Tropical trees may also become stag-headed owing to the attacks of Loranthus and other parasites, the portions above the point of attachment dying back from inanition.
Cases also occur in the tropics where the stag-head condition is due to the persistent roosting of frugiferous bats—"flying foxes"—which tear the bark and foliage with their claws, and befoul the twigs generally.
Notes to Chapter XXVI.
The principal literature as regards frost is given in the works of Frank, Sorauer, and Hartig already referred to. An excellent summary will be found in Hartig's Diseases of Trees, p. 282, and in Fisher "Forest Protection," Vol. IV. or Schlich's Manual, p. 423.