dc.description.abstract | Build-up of pathogen inoculum in seed and soil due to recycling of seed and continuous
cropping by farmers contribute to high bean disease pressure and low yields. This study was
carried out to determine the effect of seed quality and soil borne pathogen inoculum on fungal and
bacterial disease pressure in common bean. Field experiments were carried out in two
agro-ecological zones (AEZs) LM1 (Busia) and LM4 (Bondo). Seeds of four bean varieties,
KATX56, KATX69, KK8 and GLP2, sourced from market, farmers and agro-chemical
outlets were evaluated for quality before planting and at harvest. During crop growth, data on
emergence, stand count, incidence and severity of root rots and foliar diseases was taken at
emergence, second, fourth, sixth, and eighth weeks after emergence. Inoculum of soil borne
pathogens and soil fertility levels were determined at planting and infections on bean stem bases
determined by isolation on agar medium. Plant biomass, number of pods per plant and grain yield
was determined at harvest.
Soil nutrient levels varied between AEZ, but all soil samples had below recommended levels
in N and C. The soil borne pathogens isolated from soil were Fusarium solani, F. oxysporum,
Rhizoctonia solani, Pythium ultimum and Macrophomina phaseolina. There were variations
in the level of root rots inoculum between the two AEZs and sites, with higher populations
in soils from LM1 of up to 24,000 CFU/g. Incidence of root rots diseases, infection on stem
bases and stand count varied significantly (p ≤ 0.05) across sites and among seed sources.
High disease incidence was in beans grown in LM4 (71%) compared to LM1 (37%). Bean
crops planted from certified GLP2 and KK8 had lower root rot disease intensity and
infections on stem bases of as low as 15% and 34%, respectively and higher stand count of
up to 97%. However, stand count of below 25% was recorded on bean crops raised from
farmer saved, market sourced GLP2 and KATX69 with higher disease intensities of up to
90% and infection levels up to 75%. Seed purity, germination and bacterial contamination
varied depending on seed source. All seeds except of certified GLP2 and KK8 had purity
below the recommended 95% and more than 15% discoloured and shrivelled seeds.
Germination of farm saved and market sourced seeds was less than 85%, with high
proportions of mouldy seeds and seedling infection of up to 70%. Higher bacterial pathogen
inoculum levels of up to 3,187 CFU/seed for Xanthomonas axonopodis pv. phaseoli and
1,634 CFU/seed for Pseudomonas savastanoi pv. phaseolicola were detected in farm saved
and market sourced seeds compared to certified seeds. Disease intensity and yield varied
significantly (P ≤ 0.05) among crops planted from seed from different sources. Crops raised
from farm saved and market sourced GLP2 had higher disease intensity of above 70% and
low yields of less than 400kg/Ha compared to30% disease intensity and 1100kg/Ha for
certified seeds. The study showed that high disease inoculum levels and poor quality seed are
the contributing factors to high disease levels and low yields. Therefore use of certified seeds
of tolerant varieties and agricultural practices that improve soil fertility and prevent build-up
of inoculum, should be promoted among farmers to achieve optimum bean yields. | en_US |