Assessment of bean (Phaseolus Vulgaris k.) cv-rosecodo glp-2 seed for infection/contamination by Macrophomina phaseoloina and its implication on disease incidence and severity

Abstract

Charcoal rot, caused by Macrophomna phaseolina has been reported to be a major constraint to bean production in Eastern parts of Kenya. Studies were carried out on M. phaseolina to determine factors influencing mycelial radial growth under the laboratory conditions to be able to come up with the most ideal incubation conditions for this pathogen. Further studies carried out to determine the most ideal seed health testing technique for seed born M. phaseolina for the purpose of assessing bean seeds obtained from small-scale farmers. Experiments were also conducted to determine the effect of seed borne M. phaseolina on incidence and severity of charcoal-rot under field conditions. The influence of bean growth stage on infection of bean plants by charcoal rot and on subsequent seed infection/containmination by M. phaseolina was also determined. PDA (Potato dextrose agar) and MEA (Malt extract agar) supported the fastest growth of M. phaseolina at 30-35oC. Lighting conditions did not have any significant (P=0.05) effect on radial mycelial growth. Direct planting of seeds on PDA revealed significantly (P=0.05) higher infection levels than all the other methods. There was positive and significant (P=0.05) correlation between seed infection levels obtained using dry seed examination and the incubation methods. There was non-significant (P=0.05) correlation between seed infection levels determined by dry seed inspection and incubation methods and the grow-on test. Seed samples from Kaitha in Machakos district with infection level of 6.5% was noted as they have the highest infection level. Highest losses in germination and post-emergence damping-off were recorded in the plots that were sown with the seed sample that had 6.5% infection/contamination. Incidences of charcoal rot at V3 and R8 were also significantly (0.05) high for plots sown with seed sample with infection level of 6.5% when compared to all the other plots. Under field condition, the seed sample with 6.5% of seedborne M. phaseolina had the highest charcoal rot severity. Removal of malformed, discoloured and shriveled seeds from the seed lot with infection level of 6.5% improved the planting value of the seed leading to low severity. Surface sterilization after hand sorting did not have any advantage on the subsequent disease development. Certified bean seeds had the lowest charcoal rot severity. Yield per hectare was not significantly (0.05) different seed samples.