Characterization of Mycovellosiella Phaseoli (Drummond) deighton, the causal agent of floury leaf spot and assessment of bean (Phaseolus Vulgaris l.) genotypes for its resistance

Abstract

Floury leaf spot was not known to be an important disease of beans in Kenya. Widespread occurrence of the disease was noted in farmers field and experimental plots in Taita hills of Taita- Taveta district. This study was carried out with the objective of assessing the plasticity of adaptiveness of floury leaf spot of beans and its causal agent (Mycovellosiella phaseoli). Studies were therefore conducted to determine the floury leaf spot distribution and intensity, isolate and characterize the causal agent, determine factors affecting its development and to evaluate bean genotypes for resistance to the disease. To determine the prevalence, incidence and severity of floury leaf spot, a survey was carried out on floury leaf spot of beans in Taita hills and Kiambu district of Kenya during the short and long rain seasons of 1993 and 1994, respectively. Infected materials from Taita hills were used in laboratory experiments. Greenhouse experiments were carried out to determine the effect of temperature, =.p()sition and age of plant at inoculation on severity and symptoms development of floury leaf spot. Bean lines already known to have resistance to other foliar diseases were screened for resistance under both green house and field conditions. Floury leaf spot occurred at 1500m to 2000m above sea level in Taita hills. The intensity of infection and distribution of the diseases was higher during the long rainy season than the short rain season. However the disease was absent in Kiambu district. Laboratory studies showed that conidial concentration had no effect on germination. Light, temperature and period of incubation significantly (P=O.05) affected conidial germination. The highest level of germination occurred in darkness at 30°C. Mycelial radial growth was very slow in all xx media tested. The pathogen grew relatively fast in PDA enriched with bean leaves CEPDA) and bean leaf decoction agar (BLDA) at 20° to 25°C. Sporulation of M. phaseoli was best achieved in BLDA and EPDA. However, substantial sporulation occurred in all the other media tested. Field evaluation' of 56 bean genotypes for resistance to floury leaf spot revealed that 6 of them were highly resistant, 5 were resistant, 30 were intermediate, 10 were susceptible and 5 were highly susceptible. Field and greenhouse disease rating were positively correlated (r= 0.89, P= 0.01). For abundant growth and sporulation of M phaseoli, a concentration of 2x1 OS conidia ml' should be cultured in bean leaf decoction agar or potato dextrose agar enriched with bean leaves extract at 20-25°c and alternating 12 hours light! 12 hours darkness. Disease assessment is best achieved during the 261!!day after plant inoculation, on infected plants inoculated when 2 weeks old, with a spore load of 2x 105 conidia ml'.