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Comparative Pathogenicity, Biocontrol Efficacy, and Multilocus Sequence Typing of Verticillium nonalfalfae from the Invasive Ailanthus altissima and Other Hosts

First, third, and fifth authors: Department of Plant Pathology and Environmental Microbiology, The Pennsylvania State University, University Park 16802; and second and fourth authors, Department of Plant Pathology, University of California, Davis, 1636 E Alisal Street, Salinas 93905.

Verticillium wilt, caused by Verticillium nonalfalfae, is currently killing tens of thousands of highly invasive Ailanthus altissima trees within the forests in Pennsylvania, Ohio, and Virginia and is being considered as a biological control agent of Ailanthus. However, little is known about the pathogenicity and virulence of V. nonalfalfae isolates from other hosts on Ailanthus, or the genetic diversity among V. nonalfalfae from confirmed Ailanthus wilt epicenters and from locations and hosts not associated with Ailanthus wilt. Here, we compared the pathogenicity and virulence of several V. nonalfalfae and V. alfalfae isolates, evaluated the efficacy of the virulent V. nonalfalfae isolate VnAa140 as a biocontrol agent of Ailanthus in Pennsylvania, and performed multilocus sequence typing of V. nonalfalfae and V. alfalfae. Inoculations of seven V. nonalfalfae and V. alfalfae isolates from six plant hosts on healthy Ailanthus seedlings revealed that V. nonalfalfae isolates from hosts other than Ailanthus were not pathogenic on Ailanthus. In the field, 100 canopy Ailanthus trees were inoculated across 12 stands with VnAa140 from 2006 to 2009. By 2011, natural spread of the fungus had resulted in the mortality of >14,000 additional canopy Ailanthus trees, 10,000 to 15,000 Ailanthus sprouts, and nearly complete eradication of Ailanthus from several smaller inoculated stands, with the exception of a few scattered vegetative sprouts that persisted in the understory for several years before succumbing. All V. nonalfalfae isolates associated with the lethal wilt of Ailanthus, along with 18 additional isolates from 10 hosts, shared the same multilocus sequence type (MLST), MLST 1, whereas three V. nonalfalfae isolates from kiwifruit shared a second sequence type, MLST 2. All V. alfalfae isolates included in the study shared the same MLST and included the first example of V. alfalfae infecting a non-lucerne host. Our results indicate that V. nonalfalfae is host adapted and highly efficacious against Ailanthus and, thus, is a strong candidate for use as a biocontrol agent.

Additional keywords: tree-of-heaven, Verticillium albo-atrum.