Rhizobium leguminosarum and Meloidogyne enterolobii interactive effects on growth and nodulation of retained cowpea varieties.

Abstract

Reports on the interactions of nematodes and nitrogen-fixing bacteria have been inconsistent, making nematode management decisions hard to make. A study was conducted at the University of Mpumalanga under greenhouse conditions to determine the relationship between the two rhizosphere-habiting organisms on the development of cowpea farmer retained varieties. The results obtained will clarify the relationship between the two species and develop a better management plan for nematodes, especially for the communal farmers. Interaction between root-knot nematode, Meloidogyne enterolobii and rhizobium, Rhizobium leguminosarum on six retained cowpea varieties, Cv17A, Cv17I, Cv17C, Cv17B, Cv17D and Cv17F, were evaluated in pot experiments under greenhouse conditions over two seasons, winter and summer, 2021. A 3 x 6 factorial experiment was laid out in a randomized complete block design (RCBD) with 5 replications (n = 90). The first factor consisted of the time of organism inoculation with levels of (ⅰ) R. leguminosarum applied a week before M. enterolobii, (ⅱ) M. enterolobii applied a week before R. leguminosarum inoculation, and (ⅲ) both microorganisms applied at the same time, whereas the second factor consisted of six farmer-retained cowpea varieties. Plastic pots of 25 cm diameter were filled with a mixture of pasteurized (250 ⁰C for 4hr) sandy and sandy-loam soils at a ratio of 3:1 (v/v). The pots were placed on greenhouse benches at an intra- and inter-row spacing of 0.6 and 0.5 m, respectively. Four seeds were sown in each pot and irrigated with 250 ml of tap water after every 48 hours. The seedlings were then thinned at a two-true-leaf stage to leave one seedling per pot. A week after thinning seedlings, the treatments stated above were applied. When required the inoculum of 5 000 M. enterolobii second-stage juveniles and (1× 1010 CFU/ml) of R. leguminosarum were inoculated on the cowpea seeds depending on the order described above. At 75 days after initiation of the experimental treatments, data on plant growth, number of nematodes, xi and rhizobia variables were collected. Meloidogyne enterolobii was able to reproduce in all six varieties as indicated by a reproductive potential of greater than one, making all the farmer retained cowpea varieties highly susceptible to the nematodes. Season had the greatest effect on varieties response to time of inoculation. Both the growth and nodulation of cowpea varieties and the population densities of nematodes infecting the plants differed greatly with season, with summer generally improving varietal growth, nodulation, and M. enterolobii populations and reproduction. However, the interaction of M. enterolobii and R. leguminosarum differed with each cowpea variety, Cv17A had higher numbers of active nodules when it was inoculated with R. leguminosarum first than the other two inoculations whereas, the inoculation time had no effect on the number of active nodules of all other varieties. In conclusion, the relationship between the nematode M. enterolobii and R. leguminosarum is much complex than previously assumed. The environment, time of inoculation and cultivar all seem to influence the interaction.