The soilborne fungus Verticillium dahliae causes Verticillium wilt on more than 200 plant species and persists in soil as darkly melanized microsclerotia. Seed transmission of V. dahliae occurs in many plant species, including spinach. Since V. dahliae has a broad host range, planting infected spinach seed can introduce the pathogen into fields, where it causes disease in susceptible crops grown in rotation, such as lettuce. The diversity of Verticillium species and frequency of the three V. dahliae races have not been examined in commercial spinach seed lots. In this study, 75 Verticillium isolates were obtained from 75 individual commercial spinach seed lots and characterized based on morphology and PCR assays using species- and race-specific markers. All isolates exhibited morphological features consistent with V. dahliae, and species-specific PCR confirmed their identity as V. dahliae. Race typing of isolates based on the PCR amplification of avirulence gene markers showed that V. dahliae race 3 was predominant (57%), followed by those of unknown race (36%), and race 2 (7%), whereas race 1 was not detected. PCR analyses of DNA extracted from bulked, ground seed samples revealed a similar predominance of race 3 (56%). However, the frequencies of race 2 (14.7%) and unknown race (9.3%) differed, suggesting mixed race populations within some samples. In support of this, markers for two different races were detected in 20% of the samples. Collectively, these results demonstrate that commercial spinach seed lots harbor diverse V. dahliae populations, indicating the dissemination of diverse pathogenic genotypes of V. dahliae.
Nayak et al. (Fri,) studied this question.