Determining the damage threshold of root-knot nematode, Meloidogyne arenaria on Vigna unguiculata (L.) Walp

The pathogenicity of root-knot nematode, Meloidogyne arenaria on cowpea was studied by conducting a randomized completely block design pot experiment. The plants were inoculated with nematode inoculum levels of 500, 1000, 1500, 2000 and 2500 J2's/kg soil. Effects of different inoculum levels were observed on plant growth and biochemical parameters in terms of plant biomass, number of nodules, leghemoglobin content, seed protein content, proline and photosynthetic pigments. It was found that with an increase in the inoculum level of M. arenaria, there was a gradual decrease in the plant growth and physiological parameters as compared to control, being significant at 2000J2 and above (P ≤ 0.05). Scanning electron microscopy revealed that there was a significant suppression in stomatal aperture with increasing inoculum levels. Further, the histochemical analysis revealed that the oxidative stress increased with increasing inoculum levels. The leaf proline and number of galls, increased significantly with increasing inoculation level. The disease parameters like number of galls, egg masses and total nematode population increased while as reproduction factor decreased in a dose dependent manner. Furthermore, the PCA (principal component analysis) accounted for 90.4% and 8.5% of total-data variability in plants with and without nematodes, indicating evidence for a strong correlation between the numerous examined parameters. The threshold nematode inoculum of root-knot nematode, M. arenaria on cowpea was determined as 2000J2/kg soil.