Field experiments from 1959–60 to 1963–4 showed that when blackarm resistant strains of Egyptian type cotton were sown around the beginning of August and adequately protected from fleabeetle, the resultant crop was of higher yield and considerably better average quality than that from the normal mid-to-late August sowings. Effective length, maturity ratio, standard fibre weight, bundle strength and lea-count × strength product were all improved. Sowing in early July, tested in one season only, gave rather better quality but lower yield than sowing in early August. Sowing in late August or in September depressed both yield and quality. The importance of direct climatic effects and of blackarm, fleabeetle, bollworm, other insect pests and wilt in controlling the choice of, and response to, sowing date is briefly discussed.Thanks are due to the Sudan Gezira Board for kindly supplying the results of the large-scale sowing date trials and for the grading of experimental cotton, to the Shirley Institute of Manchester for the fibre and spinning tests and to the Chief, Agricultural Research Division, Ministry of Agriculture, Sudan, for permission to publish this paper.
SynopsisSitka spruce is a monoecious, wind-pollinated, cross-fertilising species showing wide genetic variation which suggests heterozygosity for many alleles and natural selection against self-fertilisation. Phenotypic selection for the important trait of vigour is ineffective, so testing progenies of selected individuals and clonal testing is an essential and time-consuming part of any improvement programme. Old trees can be vegetatively propagated by grafting and very young trees by rooted cuttings.The British tree improvement programme is based, in the short term, on the use of seed collected from superior plantation trees of desirable origins; in the mid term on seed derived from clonal orchards based on mixtures of clones previously tested for superiority in family tests; and in the longer term on highly superior seed or clones derived from a system of recurrent mating with family selection. Three populations are being developed. There is a small interspecific hybridisation programme.
317EXPERIMENTAL METHODS Experiments to investigate the relationship between the sowing date of cotton and the incidence and effects of insect pests were carried out in the last 3 years of a 5-year series of experiments on the interactions of date of sowing with environmental and cultural factors. In each experiment the cotton was grown on ridges 12 m long with 80 cm between ridges and 50 cm between plant holes on the ridge. Seed, sown at a rate of about ten seeds per hole was dressed with Abavit against Xanthomonas malvacearum and, as is customary in the Gezira, the seedlings were thinned to give a final stand of three plants per hole. Irrigation was at approximately fortnightly intervals and other cultural practices also conformed with those standard for the Gezira Research Station (Crowther, 1948). In each of the three experiments on the interactions between sowing dates and insect pest numbers the cotton was of the commercial blackarm resistant variety XL 1 (Knight, 1954) and was given the standard fertilizer dressing of 76-3 lb nitrogen per acre supplied as urea. Regular counts of the numbers of jassids (Empoasca lybica de Berg), whitefly (Bemisia tabaci Genn.), and in the first experiment only, fleabeetle (Podogrica puncticolis Weise) were made on all plots. Jassid nymph and whitefly adult counts were made, according to a routine method, on five leaves per plant-two at the top, one in the middle and two at the bottom, on thirty plants per plot in the first experiment and twenty plants per plot in the second and third. Fleabeetle adult numbers were counted on thirty plant holes per plot and fleabeetle damage assessment recorded on thirty separate plant holes. In the first two experiments the dry weights of the different aerial parts of the plants were determined and the numbers of flower buds
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