Effectiveness of Pseudomonas Fluorescens and Neem Extract in Management of Fusarium Oxysporum on French Beans in Kirinyaga County, Kenya

Abstract

Fusarium wilt, Fusarium oxysporum f. sp phaseoli, is among the major diseases affecting French bean production in Kenya causing up to100% yield loss. Use of synthetic fungicides has caused environmental pollution, pesticide resistance and increased risk to human health. This study assessed use of Azadirachta indica and Pseudomonas fluorescens in managing Fusarium wilt disease on French beans. Twenty soil samples and diseased bean plant materials were randomly sampled from twenty farmer’s field each in Kirinyaga County in November-December 2017. In vitro studies were conducted to isolate and identify effective P. fluorescens and determine their mode of action against F. oxysporum. Efficacy of neem against F. oxysporum was assessed using the poison food technique. Data on colony diameter was recorded in millimetres after 7 days of incubation at 28±2℃. Percentage growth inhibition of mycelia was used to select the most effective isolates for further studies. Greenhouse and field trials were carried out for two seasons to assess the efficacy of the most effective isolates on Fusarium wilt disease. The treatments included Pseudomonas fluorescens- Pf1, Pf2, Pf3, Pf4, Bio cure- B, neem extract, Pf1+neem and control. The bacterial isolates were multiplied on 100g of sterile rice bran and neem extract was prepared by blending plant material in 70% ethanol and concentrated by evaporation under vacuum. The treatments were applied twice by incorporating in the soil prior to planting and at vegetative growth stage. Data on disease incidence and severity, plant height (cm), shoot and root length, biomass, population dynamics and yield of French beans were collected. Data was subjected to analysis of variance using SAS version 9.2. Means were separated using Fischer’s Least significant difference test (P≤0.05). A total of 12 P. fluorescens isolates were tested for antagonistic activity against F. oxysporum in dual culture technique. Isolate Pf1 was superior with 72.2% mycelial inhibition compared to the control. In addition, Pf1 isolate significantly (P˂0.001) inhibited F. oxysporum mycelial growth through antibiosis, production of siderophores and metabolites. Among the metabolites, production of volatile compounds was the most effective mode of action and attained 80.9% mycelial inhibition. Under greenhouse condition, isolate Pf1 was significantly (P˂0.001) superior than untreated control and recorded the least mean disease incidence and severity and promoted growth and yield of French bean. Results from the field study showed P. fluorescens- Pf1 and Pf2 recorded significantly (P˂0.001) higher weight of marketable pods than other treatments and scored 4.5±0.56 and 4.9±0.45t/ha during the first and second season, respectively. The control plots recorded 1.5±0.33 and 1.5±0.18t/ha during the first and second season, respectively. In addition, Pf1 isolate had the highest rhizosphere population with 2.0±0.84×109 cfu/g during the second season. Pseudomonas fluorescens produced antimicrobial substances that inhibit the pathogen and indirectly promote growth and yields of beans. The study recommends isolates Pf1 and Pf2 to be formulated and commercialized for use as alternatives to fungicides in managing Fusarium wilt disease on French beans.