Proceedings of the South & East African combined agricultural, cotton, entomological and mycological conference held at Nairobi, August, 1926

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Monograph

Identifikator:: 1738588467

URN:: urn:nbn:de:zbw-retromon-115043

Document type:: Monograph

Title:: Proceedings of the South & East African combined agricultural, cotton, entomological and mycological conference held at Nairobi, August, 1926

Place of publication:: Nairobi

Publisher:: East African Standard

Year of publication:: 1926

Scope:: VI, 337 Seiten; Ill.

Digitisation:: 2020

Collection:: Economics Books

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Chapter

Document type:: Monograph

Structure type:: Chapter

Title:: Part IV. Etomology & Mycology

Collection:: Economics Books

PART IV. 3 follow as the aftermath of such attack. Any factor which puts a severe strain on a coffee bush may, therefore, cause dieback of the branches and, in practice, several influences, either single or in combination, are known to be able to do this. Overbearing, at least in Kenya, is the commonest cause of the malady and is the one chiefly responsible for the condition already mentioned in which there are upper and lower regions of healthy branches and an intermediate dessicated zone. The explanation of this appears to be somewhat as follows. The youngest primaries situated near the apex of the bush have not yet reached the bearing stage, and have, therefore, not been subjected to the strain involved in maturing a crop. Those at the base, although they may be carrying a heavy load of berries, have had time to produce a system of secondary and tertiary branches to assist in the work of providing the food materials necessary to the development of the crop. The intermediate branches below the apical zone, while just old enough to come into bearing, have not the advantage of such secondary and tertiary growth so that, if the crop is sufficiently heavy, the strain proves too great and dieback results. Two parasitic influences which are frequently important as indirect causes of dieback are severe attacks of leaf disease (Hemileia vastatriz) and thrips. These cause extensive defoliation, and unless special measures, such as heavy manuring, are taken to assist the recovery of the affected bush, dieback invariably follows. In districts where leaf disease is prevalent a bush which is bearing a heavy crop succumbs much more readily to the disease than one which is not, so that the strain of overbearing is considerably accentuated by attack by the fungus and subsequent dieback is rendered the more certain. Insufficient rainfall may in itself be a cause of dieback but it is more usually only a contributory factor, although frequently an important one. Neglect of proper cultivation has a similar effect by depleting the soil of moisture and also by preventing proper soil aeration. This is well seen in the extreme case of a derelict plantation, containing rank growths of weeds; in such places the bushes are invariably badly affected with dieback. Any circumstance which prevents the proper development of the root system, such as faulty planting or the presence of hard pan, may also lead to dieback. It has been stated in the foregoing that dieback is never directly due to parasitic agencies. Many diseases and pests may, of course, cause the death of branches which they attack but the effect of such direct parasitism is confined in general to the parts attacked and the phenomena observed belong to a different category from those characteristic of true dieback. Thrips has been mentioned as a cause of severe defoliation leading to dieback; when, however, these insects suck the juices from young branches the parts affected are killed as the direct result of such attack and their deaths are not due in any way to a general strain thrown on the bush as a whole. Certain fungi, notably Colletotrichum coffeanum and a species of Phoma, are almost invariably present on twigs which have succumbed to dieback; they become established on such twigs after the event, however, and are not the original cause of it. These fungi exist quite harmlessly and in considerable amount in every coffee plantation, as is shown by the fact that one or other may be confidently expected to develop on any perfectly healthy coffee twig brought into the laboratory and merely kept in a moist chamber for a few days. Only two cases have been observed in six vears in which it seemed at all 2()¢

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