Prevalence of Race 2 Strains of Verticillium dahliae Causing Verticillium Wilt of Mint in Oregon

Series/Report Number: COARC2016

Abstract: Verticillium dahliae is a plant pathogenic fungus that causes Verticillium wilt, which is the most detrimental disease of commercially grown peppermint (Mentha piperita) in Oregon. Symptoms in peppermint include asymmetrical growth, reddening, chlorosis, necrosis, stunted growth, wilt, and premature plant death. Two pathogenic races of V. dahliae have been identified. Race 1 isolates contain the Ave1gene. Several plants that are resistant to race 1 have been identified, including lettuce and tomato. These resistant plants contain a gene which encodes for plant immune-receptors that can recognize the avirulence gene in race 1 strains of V. dahliae (de Jonge et al. 2012). Race 2 isolates of V. dahliae do not possess the avirulence gene and therefore do not trigger an immune response in plants containing the resistance gene. Several studies have reported an increased prevalence of race 2 relative to race 1, likely owing to its success in colonizing a greater variety of plants in crop rotations (Short et al. 2014). Despite being the top mint producing area in the country, there has been relatively little work done to investigate the races of V. dahliae infecting mint in the Pacific Northwest. One study tested 16 V. dahliae isolates collected from Washington mint and all 16 isolates were determined to be race 2. However, it is not known which race(s) of V. dahliae cause Verticillium wilt in commercial peppermint production fields of Oregon. We hypothesize that race 2 V. dahliae strains are responsible for Verticillium wilt in peppermint grown in Oregon. The objectives of this study were to: (i) determine which race of V. dahliae is infecting Oregon peppermint; and (ii) determine which race is predominately responsible for causing Verticillium wilt in important crop plants other than peppermint.

Chapter Number: 40