Colonization of spinach by Verticillium dahliae and effects of pathogen localization on the efficacy of seed treatments.

Verticillium wilt on spinach (Spinacia oleracea) is caused by the soilborne fungus Verticillium dahliae. The pathogen is seedborne and transmission through seed is a major concern because of the dispersal of the pathogen to areas where fresh and processing spinach crops are grown in rotation with susceptible crops. Reduction in seedborne inoculum minimizes pathogen spread; therefore, knowledge of pathogen localization in seed is critical to develop methods to reduce seedborne inoculum. Spinach seedlings were inoculated with conidial suspensions of a green fluorescent protein-tagged strain of V. dahliae and colonization events were followed through seed production by confocal laser-scanning microscopy. Between 24 to 96 h postinoculation (PI), conidia germinated and formed hyphal colonies on root tips and in root elongation zones. Hyphae colonized root cortical tissues both intra and intercellularly by 2 weeks, and colonized the taproot xylem with abundant mycelia and conidia that led to vascular discoloration coincident with foliar symptom expression by 8 weeks PI. At 10 weeks PI, the xylem of the upper stem, inflorescence, and spinach seed parts, including the pericarp, seed coat, cotyledons, and radicle, had been colonized by the pathogen but not the perisperm (the diploid maternal tissue). Maximum concentration of the fungus was in the seed coat, the outermost layer of the vasculature. Infection of V. dahliae in spinach seed was systemic and transmissible to developing seedlings. Additional analyses indicated that fungicide and steam seed treatments reduced detectable levels of the pathogen but did not eliminate the pathogen from the seed. This information will assist in the development of seed treatments that will reduce the seedborne inoculum transmission to crop production fields.