Cool Temperatures Favor Growth of Oregon Isolates of Calonectria pseudonaviculata and Increase Severity of Boxwood Blight on Two Buxus Cultivars

Abstract

Controlled environment experiments were conducted to evaluate the effects of temperature on Calonectria pseudonaviculata mycelial growth and effects of temperature and infection period on boxwood blight severity. In experiment 1, 15 Oregon isolates (representing five genotypes) were grown on potato dextrose agar (PDA) and malt extract agar (MEA) at six temperatures from 5 to 30°C. Growth (culture diameter) was measured after 2 weeks. Optimal growth occurred at 25°C on PDA and 20°C on MEA. Isolates of genotype G1 also grew faster than genotype G2, but only on MEA at 25°C. In experiment 2, Buxus cultivars Green Velvet (GV, more susceptible) and Winter Gem (WG, more resistant) were inoculated and incubated in moist chambers for 9 or 24 h at 22°C (infection period), then moved into growth chambers at 15 or 25°C. After 4 weeks, chamber temperatures were switched and plants were incubated for 4 more weeks. Disease severity was evaluated weekly. During the first 4 weeks, disease was generally more severe on GV than WG, on plants with a 24 h versus a 9 h infection period, and on plants incubated at 15°C versus 25°C. However, disease was just as severe on WG as GV when the 24 h infection period was followed by incubation at 15°C. After the temperatures were switched, disease increased only on WG that were cooled from 25 to 15°C. Results show that Oregon isolates of C. pseudonaviculata are capable of growing faster and causing more severe disease at temperatures cooler than those reported previously.

Combating an Invasive Boxwood Pathogen – Calonectria pseudonaviculata – in the United States by Shifting Production to Less-Susceptible Cultivars

“…Prevailing weather conditions at different location across the U.S such as cool temperatures have varying effects on Cps infection on various cultivars (Weiland et al 2022). Growers reportedly have taken advantage of this to continue sales of HS cultivars in some states, shipping to regions that are not favorable for boxwood blight in the U.S. (Barker et al 2022).…”