Comprehensive Assessment of Alfalfa Cultivars for Resistance to Meloidogyne incognita Using Multiple Evaluation Indices

Root-knot nematodes (RKN), especially <i>Meloidogyne incognita</i>, threaten global alfalfa crops because of their broad host range and pathogenic nature. Despite its significance, research on resistance is limited. In this study, 24 varieties from China, the US, Canada, Australia, and France were assessed for resistance using the Disease Index (DI) and Egg Mass Index (EMI). Results identified 19 varieties with varying resistance levels and 5 that were susceptible. Chinese Gannong No. 9 was highly resistant (DI: 10) and achieved the highest composite score (91). The US varieties Dryland and Moste were classified as resistant (DI: 14.3% and 12.5%, respectively) and also ranked highly by composite score (65 and 62.5). A moderate correlation between DI and EMI (r = 0.68) led to some inconsistent classifications, including for 2295, Instict, and WL168HQ, highlighting the importance of using multiple complementary metrics for accurate resistance evaluation. Egg mass production was strongly correlated with galling severity (r = 0.70), while root biomass showed no correlation with galling (r = 0.09), indicating root weight is not a reliable resistance indicator. Preliminary infection dynamics showed similar nematode penetration rates at 2 days post-infection across resistant and susceptible varieties. At 7 days post-infection, both resistant and susceptible varieties retained predominantly J2 larvae (78-89%), with no statistically significant differences in developmental stage distributions. These preliminary observations suggest that resistance-associated effects on nematode development, if present, are not strongly expressed at early stages of infection. The mechanistic basis of resistance in alfalfa remains unresolved and warrants further investigation using additional timepoints, histological analyses of feeding-site development, and molecular characterization. Geographically, American varieties displayed broad performance variation, Chinese varieties showed a bimodal distribution, and Canadian varieties exhibited moderate, consistent resistance. These results offer valuable germplasm for breeding and highlight the importance of multiple resistance metrics. Resistant varieties such as Gannong No. 9 provide important genetic resources for developing durable nematode resistance in alfalfa and can guide variety selection in nematode-infested regions.