Effects of Inoculum Dose, Temperature, Cultivar, and Interrupted Leaf Wetness Period on Infection of Boxwood byCalonectria pseudonaviculata

Abstract

Boxwood blight is an emerging disease of great concern for the ornamental horticulture industry, historic garden managers, landscapers, and homeowners. Controlled-environment experiments were conducted to determine the effects of conidial concentration, temperature, interrupted leaf wetness period, cultivar, and leaf age on infection of boxwood leaves by Calonectria pseudonaviculata. Boxwood blight incidence (BBI) increased with increasing concentration up to 2.0 × 104 spores/ml. BBI also increased as temperature increased from 18 to 25°C, then declined gradually to zero at 29°C. Similar infection effects of inoculum concentration were observed in an experiment with four boxwood cultivars (‘Justin Brouwers’, ‘John Baldwin’, ‘Green Mound’, and ‘Nana’) of various degrees of susceptibility. The hypothesis that younger leaves are more susceptible than older leaves was supported for Justin Brouwers and Nana but not for Green Mound; and younger leaves of John Baldwin were less susceptible than older leaves. When inoculated plants (‘Suffruticosa’) were exposed to dry interruptions of 3 h or longer between 5 or 8 h of initial wetness and 12 h of additional wetness, these plants had significantly lower BBI compared with those exposed to continuous wetness for 20 h, and similar or at most slightly more infection than plants exposed to only the 5- or 8-h initial wetness. Continuous wetness durations beyond 20 h did not increase infection in these experiments. These results advanced our understanding of the environmental requirements of the infection process in boxwood blight development and they are essential for refining disease forecasting models.

Boxwood blight: an ongoing threat to ornamental and native boxwood

“…Inoculum density is a key factor for disease development, with higher numbers of conidia leading to greater disease incidence across a tested range of 1250 to 40000 conidia/mL on 2-year-old plants (Avenot et al 2017 ). Less susceptible cultivars such as Buxus x ‘Green Mound’, B. sinica var.…”

“…Less susceptible cultivars such as Buxus x ‘Green Mound’, B. sinica var. insularis ‘Nana’, and B. microphylla ‘John Baldwin’ show no symptom development at inoculum doses below 5000 spores/mL, but it is not known whether latent infection occurred under these conditions (Avenot et al 2017 ). Germination of conidia is increased when they are exposed to 24 h of darkness following a 14-h photoperiod, compared to germination rates when conidia are exposed to 12 or 24 h of light following the same 14-h photoperiod (Marine et al 2017 ).…”

Effect of initial darkness duration on the pathogenicity of Calonectria pseudonaviculata on boxwood

“…All experiments were conducted in M‐12 growth chambers (Environmental Growth Chambers) set at a 25/21 °C day/night temperature with a 14 h day length of photosynthetically active radiation (PAR; 400–700 nm) of 500 μmol m −2 s −1 light intensity and either 65% (suboptimal) or 95% (optimal) relative humidity. Environmental conditions were selected based on the average day length in Virginia during the boxwood growing season, and the optimal growth temperature range for C. pseudonaviculata (Avenot et al ., ). Initial dark periods evaluated were: (i) 0 h dark (i.e.…”

Breeding and selection of Buxus for resistance to Calonectria pseudonaviculata

“…Within B . sempervirens , various degrees of susceptibility are also observed among different cultivars (Avenot, King, Edwards, Baudoin, & Hong, ). B .…”