In the late 1980s and early 1990s, Verticillium wilt was the most widespread

and important disease of cotton in Australia. The widespread adoption of

CSIRO cultivars with resistance to Verticillium wilt has effectively reduced the

incidence and importance of the disease in recent years'. In California, where

there are several different and more virulent strains of the pathogen, the

repeated use of resistant cultivars resulted in the selection of more virulent

strains and an effective breakdown in the level of cultivar resistance to the

disease. For this reason it is of great importance to monitor the incidence of

Verticillium wilt with repeated cultivation of a resistant cultivar.