Effect of Temperature, Leaf Wetness Period, and Cultivar Susceptibility on Boxwood Blight Disease Development and Sporulation.

Boxwood blight causes great losses to the boxwood nursery industry and landscapes in 30 states in the United States. Understanding the epidemiological factors governing disease development will be important for disease forecasting and design of best management practices. We evaluated the effect of leaf wetness period (lwp) and temperature on lesion development and sporulation on three boxwood cultivars under controlled conditions to develop predictive models for disease development. We conducted detached leaf assays at 18 to 27°C and various lwp with the cultivars 'Suffruticosa' (highly susceptible), . × var. 'Green Velvet' (moderately susceptible), and var. 'Winter Gem' (less susceptible). Detached leaves were inoculated with 200 conidia in 50 µl of suspension and disease incidence was recorded at 3 to 13 days postinoculation (dpi). Cultivar, lwp, temperature, and most interactions significantly influenced disease development. A minimum of 5 h of leaf wetness was required for any disease. Lesion development increased most rapidly between 12 and 15 h and continued to increase to about 21 h of leaf wetness. Temperatures between 21 and 25°C were optimal for lesion development. There was about a 7-day lag between appearance of lesions and maximal incidence of sporulation. The two less-susceptible cultivars had fewer lesions than Suffruticosa under the same infection conditions; in addition, leaf lesions of Winter Gem exhibited delayed sporulation and sporulation from a smaller proportion of symptomatic leaves. Response surfaces were developed for each cultivar to predict the disease incidence using the lwp and dpi. Our findings will help refine disease forecast models to improve management of boxwood blight.