Evaluation and verification of resistance in selected vegetable crops for sustainable root-knot nematode management in developing agriculture
Mothata, Tshiamo Shilla
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Root-knot nematodes, (Meloidogyne species) are a major constraint in vegetable production systems. These parasites cause high yield losses, particularly in subsistence farming systems. This study was conducted to establish i) whether monospecific populations of M. incognita race 2 and M. javanica were used in these trials by means of molecular identification, ii) determine whether root-knot nematode-resistance is present in commercially available tomato, green bean, pumpkin and Brassica genotypes and to iii) verify resistance found. M. incognita race 2 and M. javanica were identified as monospecific using molecular techniques. Host suitability trials for the relevant vegetable crops were conducted in a greenhouse. Although various nematode parameters were used (the number of eggs and 52 per root system and per gram of roots, number of egg masses and egg-laying females (ELF) per root system), reproduction factors values [RF = final egg and J2 numbers (Pf)/initial egg and J2 numbers (Pi)] were used as the main criteria to select for root-knot nematode resistance. Although substantial variation existed among the relevant vegetable genotypes with regard to all parameters used, none of these genotypes were immune to either M. incognita race 2 or M. javanica since these parasites reproduced on all vegetable genotypes used in this study. However, three tomato and a range of Brassica genotypes had W-values lower than 1, indicating resistance to M. incognita race 2. With regard to the verification of M. incognita race 2-resistance in tomato genotype Rhapsody relative to the susceptible Moneymaker in a microplot trial using a range of initial inoculation levels, strong relationship existed for both genotypes for the majority of nematode variables used. These relationships were best described by non-linear equations. Significantly lower numbers of eggs and J2 in roots, as well as J2 in soil were obtained for Rhapsody compared to Moneymaker. RF-values were inversely proportional to initial population density (Pi) for Rhapsody (r = -0.3), while it increased gradually to Pi for Moneymaker (r = 0.94). A range of Brassica genotypes were also identified resistance to M. incognita race 2 and M. javanica, respectively, but none of the green bean and pumpkin screened had RF-values 5 1, indicating susceptibility to both species.