Browsing by Author "Nichols, R.F.W."
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Item Breeding Cassava for Virus Resistance(1947) Nichols, R.F.W.; East African Agricultural Research Institute, Amani, TanganyikaSo long as cassava (Manihot utilissima Pohl.) continues to rank as a staple food for a large proportion of the indigenous population of East Africa, the improvement of the crop must figure in the agricultural economics of the country. The comparative freedom of the crop from pests, and its drought resisting qualities, render it of particular value as an "insurance policy" against famine when it is less afflicted by drought and locusts than. Other crops. That it is, however, subject to great reduction in yield, due to infection by the mosaic virus, has been recognized for many years. In Zanzibar, Briant and Johns (1940), have shown that the loss in a non-tolerant variety, propagated from diseased cuttings, may be as high as 95 per cent. Experimental work at Amani demonstrated that this virus was transmitted by a white fly, Bemisia sp. (Storey and Nichols, 1938); field experiments showed that natural infection was mainly seasonal and in certain areas extremely high. These experiments also demonstrated that some varieties are less prone to infection than others. This disease occurs throughout East Africa.Item The Brown Streak Disease of Cassava Distribution, Climatic Effects and Diagnostic Symptoms(1950) Nichols, R.F.W.; Kenya Agricultural Research InstitiuteBrown streak disease of cassava(Manihot utilissima Pohl.)was first recorded and described by Storey in a progress report published in 1936. There appeat to have been no other published accounts. The Amani work up to 1936 had demonstrated that the disease was perpetuated through vegetative propagation and transmissible through grafting, and in the field, but confirmation of this had not been obtained before the war interrupted controlled experimental studies of the disease, and it has not been possible,up to the present, to resume them.Item Studies of the Mosaic Diseases of Cassava(1938) Storey, H.H.; Nichols, R.F.W.; East African Agricultural Research Station, Amani, Tanganyika TerritoryThe symptoms of mosaic disease in cassava, although generally typical of the mosaic group, show wide variations, due in part to the varietal reaction of the plant, to its stage of development and to the environment. The most important cause of variations however is differences in the strains of the virus, of which we have recognized, through the study of symptoms, two groups of severe and mild strains. The viruses are transmitted across a graft, but we have failed to obtain mechanical transmission by needle or hypodermic injection. A Bemisia sp. can transmit both groups of strains. It can inoculate the plant only through immature leaves, less than about one-quarter of their full length. The virus so inoculated does not pass out of the leaf until about 8 days have elapsed. On the basis of this knowledge a convenient and reliable single-leaf cage technique has been developed. After the virus has entered the stem it passes rapidly to the base of this stem, but odp slowly into side branches from it or into other stems arising from the same original cutting. Infection of a plant with a mild strain of virus failed entirely to confer immunity from infection by severe strains introduced by grafting. If the severe strains were inoculated by insects there was an indication of some conferred resistance but insufficient to make the procedure practically useful in control.Item Studies of the Mosaic Diseases of Cassava(1938) Nichols, R.F.W.; Storey, H.H.; East African Agricultural Research StationThe symptoms of mosaic disease in cassava, although generally typical of the mosaic group, show wide variations, due in part to the varietal reaction of the plant, to its stage of development and to the environment. The most important cause of variations however is differences in the strains of the virus, of which we have recognized, through the study of symptoms, two groups of severe and mild strains. The viruses are transmitted across a graft, but we have failed to obtain mechanical transmission by needle or hypodermic injection. A Bemisia sp. can transmit both groups of strains. It can inoculate the plant only through immature leaves, less than about one-quarter of their full length. The virus so inoculated does not pass out of the leaf until about 8 days have elapsed. On the basis of this knowledge a convenient and reliable single-leaf cage technique has been developed. After the virus has entered the stem it passes rapidly to the base of this stem, but odp slowly into side branches from it or into other stems arising from the same original cutting. Infection of a plant with a mild strain of virus failed entirely to confer immunity from infection by severe strains introduced by grafting. If the severe strains were inoculated by insects there was an indication of some conferred resistance but insufficient to make the procedure practically useful in control.Item Studies of the Mosaic Diseases of Cassava(1938) Storey, H.H.; Nichols, R.F.W.; AMANIThe symptoms of mosaic disease in cassava, although generally typical of the mosaic group, show wide variations, due in part to the varietal reaction of the plant, to its stage of development and to the environment. The most important cause of variations however is differences in the strains of the virus, of which we have recognized, through the study of symptoms, two groups of severe and mild strains. The viruses are transmitted across a g~aft, but we have failed to obtain mechanical transmission by needle or hypodermic injection. A Bemisia sp. can transmit both groups of strains. It can inoculate the plant only through immature leaves, less than about one-quarter of their full length. The virus so inoculated does not pass out of the leaf until about 8 days have elapsed. On the basis of this knowledge a convenient and reliable single-leaf cage technique has been developed. After the virus has entered the stem it passes rapidly to the base of this stem, but only slowly into side branches from it or into other stems arising from the same original cutting. Infection of a plant with a mild strain of virus failed entirely to confer immunity from infection by severe strains introduced by grafting. If the severe strains were inoculated by insects there was an indication of some conferred resistance but insufficient to make the procedure practically useful in control.Item Virus Diseases of East African Plants: VII—A Field Experiment in the Transmission of Cassava Mosaic.(1938) Storey, H.H.; Nichols, R.F.W.; East African Agricultural Research Station, Amani.General Field observations indicate that there may be seasonal differences in the rate of spread of the mosaic disease of cassava. It may be surmised that the age of the plant at the time that it becomes exposed to infection may influence its susceptibility. This paper describes an experiment in which these two points have been tested.
