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Browsing Books, Book Reviews and Book Chapters by Author "Arama, Peter"

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    Effects of cultivar, isolate and environment on resistance of wheat to septoria tritici blotch in Kenya
    (1996) Arama, Peter
    The research described in this thesis focused on the characterization of some of the factors that influence disease assessment, development and expression of resistance in wheat cultivars to septoria tritici blotch. Earliness appeared to have a strong effect and tallness a small effect on disease severity (DS). A regression equation derived was used to correct the DS in the entries. Another method that gave good disease assessment was to group the cultivars according to their earliness. It appeared that the disease developed in each earliness group at the same rate. The importance of interpiot interference in assessing septoria resistance in wheat was studied. There was no indication of interpiot interference. The Nitrogen (N) level is another factor that may affect disease assessment. In Kenya there was an increase in DS on cultivars exposed to more N while in The Netherlands there was no similar increase in DS. When an inoculum mixture or single isolates are used for inoculations, the ranking of the cultivars was essentially not affected, indicating that inoculum mixtures can be used effectively in screening wheat genotypes. The correlation coefficients between the DS at the seedling and the adult plant stages was low. Thus resistance assessed at the seedling stage could not fully explain adult plant resistance. Isolates from Kenya and The Netherlands were tested on wheat seedlings. It was concluded that there was variation in virulence (and so in race-specific resistance in the host) of Septoria tritici populations within both countries. The strong cultivar x isolate interactions observed on wheat seedlings was also observed on adult plants in the field. F6 lines of 36 crosses were evaluated in the field. Transgressive segregation towards more resistance and or more susceptibility occurred in most crosses. It can be said that a fair number of genes operating in an additive manner and epistasis are involved.
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    The influence of temperature on infection of rhynchosporium secalis on resistant and susceptible barley varietie
    (1987) Arama, Peter;
    The upsurge o f b a rle y s c a ld has been o f much con cern to barley g row ers, breed ers and p a th o lo g is ts in Kenya. Y ield lo s s e s due to s c a ld average 30% though lo s s e s upto 52% have been r e co r d e d . V a r ie tie s re le a se d in the p a st w ith r e s is ta n c e to s ca ld have o f la t e succumbed to Rhynchosporium s e c a lis . In th is s tu d y , in f e c t io n by an i s o l a t e o f Rhynchos porium s e c a lis on two p o p u la rly grown b a rle y v a r ie t ie s 'B im a' and 'P r o c t o r ' was in v e s tig a te d at d ifferen t tenperatures . 'Bima' was resistant whilst 'Proctor' was highly susceptible to scald in the fie ld . The two varieties were grcwn in p lastic pots in the greenhouse and inoculated 21 days la ter. Inoculated plants were placed in a Vindon growth chanber whose inside tenperature was set at 10°C, 15°C, 20°C, 25°C and 30°C respectively. Barley varieties in the gernplasm were also tested for scald resistance at two-leaf stage in the greenhouse using Nj oro isola te o f R. Sdcalis . Results showed that infection o f R. secalis started with germination o f germ tubes which grew along the leaf surfaces before forming appressoria. The d iffe re n tia l stain periodic acid-Schiffs reagent stained deep red 'haloes' on lea f surfaces around penetration sites . These factors o f conidia germination, appressoria formation and penetration were sim ilar in the two varieties. Optinum tenperatures for conidia germination, growth o f germ tubes and scald lesion development were found to be 22.0°C, 23.5°C and 20.0°C respectively. Higher tenperatures than optinum were not favourable and there was no infection. (ix) occuring at 30°C. After successful penetration, sub-cuticular hyphae were formed thereafter branching profusely within 'Proctor' leaves and sparsely within 'Bima' leaves. Scald symptoms appeared earlier in 'Proctor' plants than 'Bima' between 10 and 20°C. Scalded leaf areas and sporulation within scald lesions were significantly less in 'Bima' than in 'Proctor' between 10 and 25°C. The factors of infection involving suppression of germ tube growth, sparce mycelial development, longer incubation period, chlorotic zones preceeding scald lesions, less percent scalded l_^f areas and less sporulation within lesions of 'Bima' leaves than in 'Proctor' leaves were indicative of partial resistance in 'Bima' whereby the host allowed infection to occur but the rate of disease progress was lowered. The study indicated that testing for resistance at two-leaf stage in the green house gave distinct reaction types. The techniques were recommended for future use in identifying resistance sources to barley scald as well as evaluating segregating populations. Complete resistance to scald was found in the varieties Bey, Forrajera and Abyssinian (Cl. 3940). Resistance in one or two of these varieties should be incorporated into the agronomically well adapted and high yielding commercial barley variety 'Bima' to make it even more resistant.