Dogwood Anthracnose Caused by Discula destructiva on Cornus spp. in Canada
The most important fungal disease of dogwoods in North America is anthracnose caused by Discula destructiva. This disease affects Cornus florida (flowering dogwood), C. nuttallii (Pacific dogwood), and C. kousa (kousa dogwood). It has not been well studied in Ontario nor anywhere in Canada. In this study, over 2,500 fungal isolates were obtained from symptomatic samples of C. alba, C. alternifolia, C. amomum, C. kousa, C. florida, C. nuttallii, C. racemosa and C. sericea. To help distinguish between foliar symptoms of different etiologies, a “Dogwood Disease Symptom Guide” was produced. Isolates were divided into 13 fungal morphotypes, of which D. destructiva accounted for 39% of all isolates. Pathogenicity testing of Discula destructiva on C. florida satisfied Koch’s postulates, and this fungus was confirmed as the causal agent of dogwood anthracnose in southwestern Ontario (C. florida) and southwestern British Columbia (C. nuttallii). Wounds and leaf trichomes may provide a point of entry and help the pathogen survive endophytically without producing symptoms on “non-host” plants such as oak, maple and pear. The pathogen was found to survive for over 12 weeks at -20 °C, and the optimal growth temperature was found to be between 20-25 °C, but temperatures as high as 30 °C inhibited the growth, and the fungus died after one week incubation at 40 °C. The finding of only one mating type within D. destructiva populations (122 isolates) explains the lack of sexual reproduction of this fungus in North America, and along with the SSR results, reconfirms the low genetic variability within its populations.