ABSTRACT
Ritzema Bos studied host races of the plant parasitic nematode Ditylenchus dipsaci as early as 1888. The resistance-breaking races were quite stable. When the resistance-breaking races in West Virginia were tested on normally susceptible cultivars, five were found to be resistant. In this case the resistance-breaking race broke the resistance of a new host species, a resistant cultivar of a known host. Resistance-breaking races may also become the prevalent populations in situations where cultivar selection pressure is a factor. The impact of resistance-breaking races of the cyst nematodes. So far the sources of resistance to each of the cyst nematode species seem to be endless because as soon as a resistant cultivar is parasitized by a resistance-breaking race another source of resistance has been found. The effectiveness of Forrest decreased in succeeding years and, although some benefit is still being derived, the loss to resistance-breaking races.
