| dc.description.abstract | Field trials were conducted in 2002/2003 and 2003/2004 rainy seasons in Chinyika
Resettlement Area (CRA) to determine seed inoculum threshold levels for Xanthomonas
axonopodis pv. phaseoli (Xap) the causative pathogen for common bean blight and to
quantify yield losses due to different seed infection levels. In addition, botanical extracts
of garlic (Alium sativum), ginger (Zingiber oficinale), and turmeric (Curcuma domestica)
were screened and evaluated in vitro for their effects as seed dressings on Xap seed
infection and viability of the seed. Five seed infection levels were established by blending
symptomless bean seeds with visually Xap-infected seeds. Certified bean seed was used
as a control. The resulting seedlots were directly plated on Milk Tween Agar and
infection levels were translated by the Most Probable Number tables resulting in 0.1, 0.7,
1.1, 3.5 and 9.2 seed infection percentages. The control, with an infection level of 0.1%
and a seedlot with 0.7% resulted in low common bean blight incidence and severity than
treatments of 1.1% and greater in both growing seasons. Early disease symptoms were
detected at two weeks after crop emergence (WACE) for the 1.1% and greater seed
infection level treatments, except at one site in 2003/2004 season where disease
symptoms were observed at 2 WACE for the 0.7% infection level. In 2002/2003 season,
disease incidence were significantly different (p<0.05) among infection levels at 3 sites at
8 WACE. No significant differences were observed for disease incidence at 2 sites.
Treatment effects on disease incidence were significant (p<0.05) in 2003/2004 season at
8 WACE only at 2 sites. Disease severity for 2002/2003 growing season was lower than
in 2003/2004 season, with significant differences among the seed infection levels for
disease severity being recorded at 6 WACE and 8 WACE. Disease severity and incidence
in both seasons generally increased with time. Low seed infection levels resulted in lower
Area Under Disease Progress Curves (AUDPC) than higher infection levels in both years.
Differences in AUDPC values between low and high infection level treatments were
greater in 2003/2004 season than in 2002/2003. Treatments with low seed infection
resulted in more seed yield than those with high infections. In all sites and seasons, the
mean pod number per plant was positively correlated with seed yield, whereas disease
severity and incidence were negatively correlated with mean pod number per plant and
seed yield. For the seed dressing experiment, the bean seedlot used had an initial
infection level of 5.4% and bacterial population of 4.0x105 colony forming units per
millilitre (cfu ml-1). The minimum inhibitory concentrations (MIC) for all botanical
extracts were determined using the test tube dilution test, whereas extract efficacy was
determined by plating 15mm filter discs saturated with botanical extracts on Nutrient
Agar inoculated Xap and then recording inhibition zones. Garlic extract had the highest
inhibition on the pathogen resulting in a significant reduction of the seed infection level
and bacterial population to 0.4% and 4.7x103cfu ml-1 respectively. Ginger extract
significantly reduced seed infection levels and bacterial population to 2.8% and
5.6x104cfu ml-1 respectively. No significant inhibitory effect on Xap was observed for
turmeric extract. Garlic demonstrated the greatest anti-microbial effect, as indicated by
the inhibition zone of 38mm, followed by ginger with 24mm and turmeric 17mm. There
were no significant differences in seed viability among the different botanical extract
treatments and the untreated control. | en_US |
