Assessment Of The Level Of Bean (Fhaseolus Vulgaris L. - Rcsecoco-Glip-2) Seed Htfecn.toi And Contamination By Pseudomonas Syrinsae Pv. Phasbolicola And Its Implications On Disease Incidence And Severity

Abstract

Detection of Pseudomonas syringae pv. phaseolicola (Burkholder, 1926) Young et_ al., 1978 inside and outside the bean seeds was carried out using dilution plate method. The seeds used were collected from farmers in Kisii, Nyeri and Meru districts. A sample of commercial bean seeds from Kenya Seed Company was also included. Only 4 bean seed samples did not have the pathogen both on the surface and inside the seeds; 12 bean seed samples had the pathogen on the surface only; 12 bean samples had the pathogen both on the surface and inside the seeds; the pathogen was only detectable inside the commercial seeds but not outside. Subsequently determination of the level of infection of bean seeds by F\ syringae pv. phaseolicola was carried out on the 12 bean seed samples that had the pathogen both inside and outside the seeds plus the commercial seed stock using dilution plate method coupled with most probable number method. The infection levels in farmers' seeds varied between 0.1% and 1.1% as compared to 0.3% for commercially certified seeds. The confirmatory tests used in these experiments were flourescein production, oxidase, arginine dihydrolase, leval production, gelatin liquefaction, nitrate reductase, phaseolotoxin bioassay and pathogenicity tests. Relationship between seed infection and subsequent seedling infection was carried out in the greenhouse. The ratio of the level of seed infection to the level of seedling infection was found to be approximately 1:1 for surface sterilized seeds. The contaminant pathogenic bacteria on the seed surface was found to increase primary infections in the greenhouse. Field studies were carried out to determine the relationship between time of inoculation and halo-blight severity on the bean plants, VI and the resultant infection level of the harvested seeds. The disease severity was assessed both on the pods and the leaves. Disease severity on the leaves in the second week ranged between 0.000% - 5.033% while in the third week the range was 0.000% - 8.367%. The range for disease severity on the pods in the third week was 0.000% - 1.9333% and that of the fourth week was 0.000% - 4.700%. When the harvested seeds from various plots were assessed for the level of infection, there was a variation between 0.0-0.6%. In the greenhouse, some plants were inoculated with JP. syringae pv. phaseolicola and kept under fairly controlled conditions until they produced seeds that were eventually harvested. The harvested seeds were grouped based on visual assessment of severity of infection. Some infected seeds were found to show obvious symptoms such as wrinkling and loss of the normal colour while others appeared to have normal colour and shape except for the yellowish colourization of the hilar layer, hence difficult to distinguish from the healthy ones.