Browsing by Author "Sheffield, F. M. L."
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Item Infectious Diseases of Sugar Cane(1962) Sheffield, F. M. L.Item Interim Report on Virus Diseases of Pawpaw in East Africa(1967) Kulkarni, H. Y.; Sheffield, F. M. L.In September 1964, experiments were begun to determine whether a virus might be one of a complex of factors causing debility in pawpaws in Tanzania [1]. Healthy seedlings of varieties "Kiru" and "Solo" were raised in insect-protected glasshouses at Muguga: when 18 in. 102ft. high these were grafted with scions from unthrifty trees. After four months a high proportion of these grafted seedlings showed varying degrees .of stunting when compared with ungrafted seedlings or seedlings grafted with healthy scions. As the varieties of the scions from the field were unknown, incompatibility seemed a possible explanation of the stunting. Scions were therefore taken from the original grafted seedlings and put on to seedlings of the same varie1y. Similar symptoms were produced and were obviously due to a graft-transmissible pathogen. Having confirmed Peregrine's failure to isolate a visible pathogen [1], we concluded that a virus was probably present.Item Plant Quarantine in East Africa(1955) Sheffield, F. M. L.In most territories of the world there are many restrictions governing the importation of plants from abroad which, although they may seem irksome, are designed solely for the protection of the growers of that territory. Certificates of freedom from certain pests and diseases issued by competent authorities in the exporting country are often required; importation of certain plants, or from certain territories, may be prohibited or plants may be admitted provided only that they pass through a period in quarantine upon arrival. All this is done in an attempt to prevent entry into a country of pests and diseases not already established there. The principal danger is from virus diseases, those caused by fungi, bacteria or insects being usually more easily recognized at source and more easily controlled or prevented.Item Protecting Local Crops from Foreign Diseases(1965) Sheffield, F. M. L.There is, grave risk of introducing new Pests and pathogen; whenever propagating material introduced to a country from oversea. Various ways of minimizing these risks are discusses. The precautions to be taken depend largely on a knowledge of the particular risk, involved in each instance. This information, should be more readily available.Item The Stem-Pitting Condition in Coffee(1961) Sheffield, F. M. L.This condition has attracted considerable attention amongst coffee growers and research workers in recent years in both Kenya and Tanganyika. Its first onset is visible only under the microscope and consists in the formation of pockets of small, thin-walled mis-shapen cells amongst the elongated, highly lignified, thick-walled vessels and tracheids characteristic of normal woody tissue. Continued production of such abnormal tissue usually results after a time in furrowing of the outside of the wood cylinder and corresponding ridging of the inside of the bark. At this stage, the abnormality is apparent if a small piece of bark is stripped from the stem or if the stem is cut into slices and the cut surfaces are polished.Item Studies of the Clove Tree Histology: With Special Reference to Sudden Death Disease(The Annals of Applied Biology , 1950) Sheffield, F. M. L.; Clove Research Scheme, ZanzibarA detailed study was made of the histology of apparently healthy clove trees and of trees expected to die from sudden death disease, in an attempt to find either a diagnostic character of the disease or some indication of a possible pathogen.The apparently healthy clove tree differs from a typical dicotyledon in a number of ways. This is especially true of the vascular system where, not only are the bundles bicollateral, but, in the stem, secondary thickening occurs in the internal phloem, necrosis of the first formed cells keeping pace with the production of new cells. A definite cambium is found only to the exterior of the xylem and this is exceptionally narrow and the cells have very delicate walls. Phloem necrosis may occur in any part of the tree. The development and lignification of the xylem is very uneven, but this seems to bear no relation to the season. Tyloses are formed in the vessels of the root and, to a less extent, in those of the stem. Starch reserves are usually small, but large quantities of tannins, oils and calcium oxalate are secreted in all tissues. The oil may be contained in special schizogenous glands which are abundant in the aerial parts of the tree. Much mechanical tissue, consisting of fibres and stone cells, is formed in the pith and cortex of the stem and in the phloem in all parts of the tree, but lignification of this tissue is very uneven. Occasional dead cells are found in all tissues, even in the primary meristem. In the leaf, the plastids develop very unevenly; this is followed by retarded development of the pigment in some parts, resulting in a flecked appearance. Stomata are exceptionally numerous; in the leaf, they are confined to the lower surface and always communicate with very small cavities. Air spaces in the mesophyll are very small and are almost non-existent in other tissues. Bodies, morphologically similar to the amorphous intracellular inclusions due to some viruses but differing chemically from them, are abundant in the leaves.Some of these characters are usually associated with a diseased condition, but they occur in both healthy and diseased cloves in the Zanzibar Protectorate. In the sudden death suspect, some may occur earlier or be more accentuated than in the healthy tree. This evidence alone might suggest that sudden death is merely the culmination of a long period of adverse conditions, but other evidence suggests very strongly that the disease is due to a pathogen. Of the groups of organisms which may cause disease, only fungi were found. These occurred in the absorbing roots but are unlikely to be the pathogens causing sudden death, for, until the tree has actually wilted, very few hyphae are present, and no more than are found in healthy trees. Nothing incompatible with the suggestion that sudden death is caused by a virus was foundItem The Sudden-Death Disease of Cloves and Its Economic and Agricultural Significance(1951) Nutman, F. J.; Sheffield, F. M. L.; Swainson, O. S.; Winter, D. W.I. As the result of epidemiological evidence on the rate of spread of the Sudden-death disease of cloves in Pemba, the situation was thought sufficiently serious to justify an attempt to control the disease in advance of scientific proof of its aetiology. Although this attempt was eventually abandoned, many data were collected in preparation for it. This article reviews the history of the abortive control scheme, and discusses the data acquired. 2. From the results of a partial survey made in 1948 and another in 1949-50, there is an estimated total of over 2,000 separate outbreaks of disease in Pemba. These vary in size but average 25 dead trees. New outbreaks are constantly arising and the present annual rate of increase in number of outbreaks would seem to be about 14 per cent. Outbreaks increase in size at a highly variable rate but, on the average, the radial rate of spread is approximately constant. An attempt has been made to forecast the probable future development of the disease. 3. The effects of the disease on the economy of the Protectorate is discussed, and on the assumption that direct control is a practical impossibility suggestions are made for rehabilitation in Zanzibar Island, where up to the present the disease has caused much greater destruction than in Pemba. These are based on elimination of residual sources of disease in already devastated areas, followed by planned replanting with cloves and with other crops in order to achieve diversity.
