Browsing by Author "Jennings, D.L."
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Item Further Studies In Breeding Cassava For Virus Resistance(1957) Jennings, D.L.The breeding of cassavas resistant to mosaic and brown-streak viruses was started at Amani in 1937. A preliminary report of the work up to 1946 has been given by Nichols [6], who continued to be responsible for cassava breeding until his accidental death in 1951. This account therefore covers the testing period of much of Nichols's later material.Item Manihot Melanobasis Müll. Arg. - A Useful Parent for Cassava Breeding(1959) Jennings, D.L.; East African Agriculture & Forestry Research OrganisationCrosses between Manihot melanobasis and cassava were very fertile and the fertility was maintained in the hybrids. The first and subsequent generations of these crosses were very high yielding. Undesirable features of the early hybrids included a straggling habit and thin stems which yielded poor planting material, a high hydrocyanic acid content in the roots, and low virus resistance: all these were rectified by further breeding. Although M. melanobasis has a high protein content in its roots it is uncertain whether this character can be transmitted to its progeny. It is concluded that M. melanobasis is a very valuable source of new genes for cassava improvement.Item Observations on Virus Diseases of Cassava in Resistant and Susceptible Varieties I. Mosaic Disease(1960) Jennings, D.L.; East African Agriculture and Forestry Research Organization.An account is given of the mosaic-disease resistance shown by varieties of cassava in field trials. Experiments showed that a high proportion of the plants of the moderately resistant varieties studied became infected in field trials in which they were exposed to infection by a very large vector population. When conditions favoured the resistance mechanism of the host, no symptoms of infection became evident, and the virus moved to the base of the plant, leaving the upper parts apparently free. Mosaic symptoms appeared more frequently if the growth of the host was interrupted at a time when there was a large vector population.Item Variation in Pollen and Ovule Fertility in Varieties of Cassava, and the Effect of Interspecific Crossing on Fertility(1963) Jennings, D.L.Records of the set of seeds obtained from intraspecific cassava crosses, from interspecific crosses between other Manihot species and cassava, and from backcrosses to cassava of three generations of interspecific hybrids are used to assess the variation in pollen and ovule fertility. It is concluded that the capacity of cassava varieties to set seeds has been reduced since they have evolved from non-tuberous wild forms and have been propagated vegetatively. Fertility was found to be very variable, and the capacity of the pollen of a variety to promote seed-set was not related to the fertility of the variety's female flowers. This situation would be expected if reduction in the fertilities of the two sexes were caused either by loss of balance in the genes controlling the functioning of the male gametophyte, or by unbalance of those concerned with the functioning of the female gametophyte or the tissues which nurture the embryo. Additional causes of sterility, probably including meiotic irregularities, appeared to operate in the Fl interspecific hybrids, but such factors were probably not important after the first backcross generation. Manihot melanobasis, a wild form which is normally propagated by seed and whose relationship to cassava should be regarded as subspecific, contributed factors which enhanced the fertility of its hybrids with cassava. This form could be used in cassava breeding as a "donor" of seed fertility, but use could also be made of some existing varieties which still possess a moderately high capacity to set seed. It is desirable to select these as one parent when making difficult crosses.
