The influence of temperature on infection of rhynchosporium secalis on resistant and susceptible barley varieties!

Abstract

The upsurge of barley scald has been of much concern to barley growers, breeders and pathologists in Kenya. Yield losses due to scald average 30% though losses upto 52% have been recorded. Varieties released in the past with resistance to scald have of late succumbed to Rhynchosporium secalis . In this study, infection by an isolate of Rhynchos porium s ecalis on two popularly grown barley varieties 'Bima' and 'Proctor' was investigated at different tenperatures . 'Bima' was resistant whilst 'Proctor' was highly susceptible to scald in the field. The two varieties were grcwn in plastic pots in the greenhouse and inoculated 21 days later. 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 isolate of R. Sdcalis . Results showed that infection of R. secalis started with germination of germ tubes which grew along the leaf surfaces before forming appressoria. The differential stain periodic acid-Schiffs reagent stained deep red 'haloes' on leaf surfaces around penetration sites . These factors of conidia germination, appressoria formation and penetration were similar in the two varieties. Optinum tenperatures for conidia germination, growth of 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. 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.