Use of eco-friendly strategies in suppression of root-knot nematodes in french bean (phaseolus vulgaris) in Kenya
Root-knot nematode (Meloidogyne spp.) is one of the major pests infecting French bean (Phaseolus vulgaris L.) which is an important vegetable crop for export in Kenya and a potential income earner to smallholder farmers. Essentially, the control of root-knot nematodes (RKN) involves the use of synthetic nematicides. However, due to high costs and increased safety concerns in regard to toxicity of these nematicides, research for ecofriendly and sustainable methods of controlling RKN have been on the rise. The objective of this study was to evaluate and compare the efficacy of different green manure crops, eco-friendly nematicides and conventional nematicides in suppressing RKN in French bean. Field and greenhouse experiments were carried out in a Randomized Complete Block Design and Complete Randomized Designs, respectively at Finlays Horticulture Kenya Limited, Kingfisher Farm in Naivasha to evaluate the efficacy of four green manure crops; Caliente mustard (Sinapis alba), Nemat (Eruca sativa), Sudan grass (Sorghum sudanese) and African Marigold (Tagetes minuta) compared with Metam sodium and also eco-friendly nematicides; Rigel-G (salicylic acid), Phyto protect (Sesame oil extract), Mytech (Paeciliomyces lilacinus) Neemraj (azidarachtin) compared to Vydate® (Oxamyl) used as a positive control in suppressing root-knot nematodes in French bean. In the field, the green manure crops were grown in micro-plots measuring 3 m by 4 m for a period of 12 weeks, then chopped and incorporated immediately back into the soil. The fields were infested with second stage juveniles (J2) of Meloidogyne spp. The blocks were watered and left for 14 days prior to planting with the French bean cultivar Serengeti. In the greenhouse experiment, 5 kg pots were filled with steamsterilized soil and mixed with known weights of macerated green manure crops harvested from the field. The pots were infested with 200 second stage juveniles (J2) of Meloidogyne spp. per pot. Trials to evaluate the efficacy of different eco-friendly nematicides and bio-control agents in suppressing RKN with the conventional nematicides in the market involved two seasons of trials conducted under field conditions. The treatments were administered according to recommended rates and stages. Damage on plants was assessed based on galling indices, crop biomass and yield whereas nematode reproductive potential was assessed based on the J2 counts. There was a significant (P ≤ 0.05) difference in RKN population densities, galling indices and yield between the green manure treatments and the control in both field and greenhouse trials. Root-knot nematode densities and galling indices were highest in the untreated control, a clear indication that the green manure crops and eco-friendly nematicides suppressed the RKN. There was however, no significant (P ≥ 0.05) difference between efficacy of green manure plants and Metam sodium in the greenhouse. The effective green manure crops reduced the population densities of RKN by over 90% in the greenhouse condition and by over 67% under field conditions. There was no significant difference (P ≥ 0.05) in the nematode population densities and galling indices observed between the eco-friendly nematicides and the conventional nematicide (Vydate®). The results of this study clearly indicate that the green manure plants and eco-friendly nematicides can be fully adopted to suppress RKN in French beans as alternatives to conventional nematicides.