Conceptual and Practical Aspects of Variability in Root-Knot Nematodes Related to Host Plant Resistance

Root-knot nematodes are the leading cause of crop loss due to plant parasitic nematodes. A few species (Meloidogyne armaria, M. chitwoodi, M. hapla, M. incognita, M. javanica) have very wide host ranges that limit nonhost rotation options. Numerous host resistance genes in diverse crops are identified and could be used in cropping sequences to manage root-knot. Approaches to their combined use that maximize effectiveness and preserve durability, including pyramiding genes in a crop and in a cropping system, are considered with resistance gene specificity to nematode species and populations. (A)virulence specificity and selection in Meloidogyne for matching host resistance genes is examined, with particular reference to tomato and cowpea. Analogies are made with bacterial and fungal pathogen-host gene-for-gene interactions. A biotype scheme is offered for characterizing, categorizing, and communicating Meloi­ dogyne (a)virulence.