Identification ofPythiumspecies associated with cavity-spot lesions on carrots in eastern Quebec

In this study, 248 isolates of Pythium spp. were recovered from cavity-spot lesions of carrot and identified by DNA sequencing. The isolates came from carrots harvested in different fields in the area of the city of Québec. The majority of the isolates belonged to Pythium sulcatum and P. sylvaticum, which represented 93% of the isolates collected. Pythium macrosporum, P. debaryanum, P. dissotocum, P. attrantheridium, P. rostratum, and P. acanthicum were also found to be associated with cavity-spot lesions. Among these species, only P. acanthicum failed to cause cavity-spot lesions when inoculated on carrots. Based on mycelial growth, 3%, 40%, 80%, and 100% of P. sylvaticum, P. sulcatum, P. macrosporum, and P. dissotocum isolates, respectively, were tolerant to metalaxyl while all isolates of P. attrantheridium, P. debaryanum, P. rostratum, and P. acanthicum were sensitive. In the area of the city of Québec, P. sulcatum and P. sylvaticum were by far the two most important species associated with the disease. This reinforces the concept that Pythium spp. causing cavity spots in carrot are diverse and may vary according to geographical areas.