The role of seed-borne inoculum levels in the development of common bean blight disease (Xanhomonas Axonopodis PV. Phaseoli) on bean in Zimbabwe under smallholder farming systems and evaluation of alternative control methods based on Botanical extracts

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.