The citrus nematode (Tylenchulus semipenetrans) has host races or biotypes that can be distinguished by their capacity to infect and reproduce on Troyer citrange, certain selections of Poncirus trifoliata, and other noncitrus hosts. Thus, in California, there are populations of citrus nematode that can damage and reproduce on these resistant rootstocks and lines. There are also nematode populations to which host resistance is effective; these do not reproduce and cause damage. The widescale planting of resistant citrus rootstocks has no doubt selected for these aggressive populations that can circumvent resistance. This is a vivid example of the need for continuing breeding programs for host resistance.
Important host races also occur within root knot nematode species. Currently four host races within Meloidogyne incognita can reproduce on a variety of hosts, including cotton, tobacco, peppers, melons, and strawberries, as shown in a host differential test. An examle of how these differences relate to California agriculture is that M. incognita races 3 and 4 will reproduce on cotton, whereas races 1 and 2 will not. Comparisons of 10 separately collected populations of M. incognita from California showed differences in ability to reproduce on resistant tomato cultivar VFN8; eight populations did not reproduce on this resistant plant, but two other populations reproduced to some extent.
A similar consideration to host races is the difference in host specificity of the common root knot nematode species themselves. For example, cowpea cv. Blackeye 5 is resistant to Meloidogyne incognita populations, but it is a host for the other Meloidogyne species. Resistance in a number of tomato cultivars is effective against M. incognita, M. javanica, and M. arenaria, but no resistance in tomato cultivars is effective against M. hapla. Lima bean cultivar White Ventura N is resistant to M. incognita, but not to other Meloidogyne species. Cotton cultivars are hosts for certain M. incognita races, but are nonhosts for M. javanica, M. arenaria, M. hapla, and other root knot nematode species. Wheat cultivars are hosts for M. incognita and M. javanica, but are nonhosts for M. hapla. Because of these differences, it is important, when planning a resistance implementation program or a rotation program, to select a cultivar or crop with the differences in host preference that these more important nematodes exhibit.
From: McKenry, M.V. and P. A. Roberts. 1985. Phytonematology Study Guide. University of California, Division of Agriculture and Natural Resources. Publication Number 4045.