A New Pathotype of Meloidogyne chitwoodi Race 1 from Washington State

Meloidogyne chitwoodi Golden et al. is a serious pest of potato (Solanum tuberosum L.), and is widespread in the Pacific Northwest United States. M. chitwoodi is currently reported to consist of two host races and one pathotype (2,3) that are not distinguished morphologically. Host race 1 reproduces on Chantenay carrot but not on Thor alfalfa and host race 2 reproduces on alfalfa but not on carrot. Both races fail to reproduce on roots of S. bulbocastanum, a wild potato species used as a source of resistance in our breeding program (1). The resistance to race 1 in S. bulbocastanum is attributed to R_mc1(blb) gene. Pathotype 1 of race 2 breaks resistance and reproduces on S. bulbocastanum (2). We have tested resistant breeding lines repeatedly in Prosser, WA field plots infested with MC race 1 and harvested tubers free from M. chitwoodi damage. In 2004 however, tubers of some resistant lines were damaged by the M. chitwoodi population that did not cause damage in the past. Populations of M. chitwoodi were established on tomato by adding peels obtained from the infected tubers of resistant lines. The reproductive factor, final number of eggs ÷ initial inoculum, of the new population was determined on five replications of 3-week-old Chantenay carrot and Thor alfalfa. Five thousand eggs were extracted from nematode cultures reared on tomatoes and then were added around the root system of the test plants. The plants were maintained in the greenhouse for 55 days before the nematode eggs were extracted and RF (reproductive factor = final/initial population) values determined. Like the MC race 1, new populations reproduced on Chantenay carrot (RF > 1) but failed to reproduce on Thor alfalfa (RF < 0.1). Unlike MC race 1, the new populations reproduced on roots of all breeding lines that carried R_mc1(blb) gene (RF > 1). These results suggest that the selected population of M. chitwoodi in the Prosser site is a new pathotype, which is designated pathotype 1 of MC race 1. References: (1) C. R. Brown et al. Am. J. Potato Res. 83:1, 2006. (2) H. Mojtahedi et al. J. Nematol. 30:506, 1998. (3) G. S. Santo et al. Plant Dis. 69:361, 1985.