Susceptibility to Stemphylium vesicarium of asparagus, onion, pear, and rye in Canada

Diseases caused by Stemphylium vesicarium are an increasing problem in the major asparagus and onion production regions of eastern and central Canada. Replicated and repeated controlled environment and field trials were conducted in Ontario to assess the disease reaction of asparagus and onion cultivars to determine if isolates from one crop can cross-infect other hosts, to assess the variation of disease symptoms among cultivars of onion and of asparagus, and to assess the ability of the pathogen to survive in a non-host species. All of the varieties of asparagus and onion assessed were susceptible to infection by S. vesicarium. There were small differences in susceptibility among varieties and in aggressiveness among isolates. These differences were not strongly associated with host crop. Onion cultivars that developed higher numbers of lesions often had lower levels of leaf dieback and vice versa. This indicated that resistance to infection was independent of susceptibility to development of dieback symptoms. In asparagus, infection on spears was not consistently correlated with severity on ferns. All isolates from asparagus and onion produced typical symptoms on wounded, but not unwounded, pear fruit. Finally, the pathogen was shown to colonize and sporulate on necrotic leaves of autumn rye, which is often used as a cover crop in asparagus fields. Stemphylium vesicarium can cross-infect asparagus and onion.