|OHKURA, MANA - Oregon State University|
|DAVIS, ELIZABETH - Retired ARS Employee|
Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/14/2022
Publication Date: 5/17/2022
Citation: Weiland, G.E., Ohkura, M., Scagel, C.F., Davis, E.A., Beck, B.R. 2022. Cool temperatures favor growth of Oregon isolates of Calonectria pseudonaviculata and increase severity of boxwood blight on two Buxus cultivars. Plant Disease. https://doi.org/10.1094/PDIS-04-22-0769-RE.
Interpretive Summary: Boxwood blight is a serious plant disease affecting the $141 million US boxwood industry. The disease was first detected in Oregon in 2011. In the eastern US, boxwood blight is most severe during warm (25°C), wet weather. However, little is known about how the disease is affected by environmental conditions in Oregon. Experiments were conducted to determine which temperatures are preferred by the boxwood blight pathogen and to determine how temperature and leaf wetness affect boxwood blight on a susceptible and a resistant boxwood variety. Our study showed that the pathogen grew faster and caused more disease at cooler temperatures (15°C) than at warmer temperatures (25°C). The resistant boxwood variety generally had much less disease than the susceptible variety at 25°C, but both varieties developed severe disease at 15°C. We also showed that disease was worse when the leaves remained wet for a longer period of time (9 versus 24 hours). These results are important because they demonstrate that cool temperatures are more favorable for boxwood blight in Oregon and will help boxwood growers make better decisions about the best time of year to scout for the disease.
Technical 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 20°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 much 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 cooler temperatures than has been reported previously.