Weed hosts and winter survival of Stemphylium vesicarium on onion in Ontario, Canada

Stemphylium leaf blight (SLB), caused by the fungal pathogen Stemphylium vesicarium, is an important disease of onion (Allium cepa). This pathogen also causes disease on a wide range of other plant species. SLB on onion is managed primarily through fungicide application. The source of primary inoculum and whether S. vesicarium can overwinter in the region were not known. Inoculation and subsequent re-isolation of the pathogen from common weed species present near onion fields demonstrated that many of these weedy species are hosts of S. vesicarium. Redroot pigweed (Amaranthus retroflexus), yellow nutsedge (Cyperus esculentus), yellow marshcress (Rorippa palustris), perennial sow thistle (Sonchus arvensis) and purslane (Portulaca oleracea) were identified as new hosts of S. vesicarium and bull thistle (Cirsium vulgare) was confirmed as a host. Sporulation on dead weed residue in spring may be a source of primary inoculum for infection of adjacent onion fields. Also, a study of pathogen survival in infected leaf pieces of onion buried in soil or left at the soil surface demonstrated that S. vesicarium can overwinter in infected onion leaf residue and produce pseudothecia and ascospores in early spring. These conclusions support existing management recommendations for the burial or removal of onion debris and managing weeds in and around onion fields.