Dying-back. - All that is true of the necrosis of cortical tissues in small patches also applies to cases where the whole of the outer tissues of thin twigs and branches die of inanition owing to a premature fall of leaves - e.g. after a severe attack of some insect or fungus pest. The consequent arrest of the transpiration current and the proper supply of nutriment to the cambium and cortex explain the phenomena. The younger branches of Coffee trees suffering from severe attacks of leaf-disease are often denuded of leaves and die back from the causes mentioned, the whole of the outer tissues becoming necrotic, and drying up tight on to the wood, because other branches with functionally active leaves on them divert the transpiration current, and drought and inanition supervene.
Dying-back is frequently also a direct effect of early frosts, which kill the thin twigs before the "wood is ripened," as gardeners say.
Dying-back is also a frequent result of direct frost action on thin watery shoots or "unripe wood," and is apt to occur every year in certain varieties of Roses, for instance, in particular situations, such as "frost-beds," or aspects exposed to cutting winds, and so forth. The necrosis which results may affect all the tissues, or only the cortex and cambium, and the frequent accompaniment of all kinds of saprophytic Ascomycetes and moulds or other fungi is in no way causal to the phenomenon.
Dying-back may also be caused by fungi, and not necessarily parasites, for cases are often observed where saprophytes only are to be found in the necrotic tissues of the cortex, having made their way in through minute cracks, lenticels, etc.
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 myceliun 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.