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

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 × 10 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.