Reaction of Sclerotinia stem rot resistant Brassica napus plant introduction genotypes to inoculations with multiple Sclerotinia sclerotiorum isolates

Sclerotinia sclerotiorum affects many crops of economic importance to our nation, including canola (Brassica napus) where it causes Sclerotinia stem rot (SSR). Effective genetic resistance to SSR is still not available in commercial cultivars. A common practice used to identify sources of resistance is to screen relatively large number of genotypes with a single, aggressive strain of the pathogen. While this process is quick, it also may allow weak and strong sources of resistance to be selected as a group. Adding a second round of screenings with multiple isolates may help purge the weaker sources of resistance. This study aimed to identify B. napus plant introduction genotypes with resistance to multiple S. sclerotiorum isolates. Plants from 16 genotypes considered resistant to SSR were inoculated with five highly aggressive S. sclerotiorum isolates originally collected from multiple states and hosts. The isolates also were characterized by their growth and ability to produce sclerotia in vitro at various temperatures. The plants were challenged at the flowering stage using the agar plug stem inoculation technique in replicated trials conducted twice in a greenhouse. The resulting lesions were measured after seven days. All isolates grew well at all temperatures tested and caused severe symptoms on susceptible control plants. Six of the 16 genotypes evaluated were resistant or moderately resistant to all isolates. These results confirmed the value of a second round of selections using multiple isolates to identify materials with broader resistance and helped establish a priority order for mapping population development.