Stomatal openings on boxwood leaves yield entry portals for leaf infection by Calonectria pseudonaviculata

The distribution, density, and size of leaf stomata for eight boxwood taxa were investigated using both light and electron microscopy. Stomata, the pores that facilitate gaseous exchange during photosynthesis, were primarily located on the abaxial (lower) epidermis of boxwood leaves with minimal distribution on or along the central vein of the adaxial (upper) surface. Inoculation of boxwood leaves with Calonectria pseudonaviculata at different leaf sites showed that infection occurred only in areas where stomata were present. Scanning and transmission electron microscopic examination of the inoculated leaves also confirmed that the fungal pathogen gains leaf entry through stomatal openings and not by direct penetration of the cuticle. Among the eight boxwood taxa examined, stomatal densities ranged from 116 ± 15 ( B. sinica ‘NaNa’) to 326 ± 21 ( B. harlandii ) stomata per mm 2 ; stomatal size ranged from 500 ± 84 um 2 ( B. harlandii ) to 1052 ± 117 um 2 ( B. sinica ‘NaNa’); and stomatal area per leaf (density × size) ranged from 88.63 mm 2 (‘Justin Brouwers’) to 154.3 mm 2 ( B. harlandii ). No relationship between stomatal density or size and infection rate was observed; relationships between infection rate and stomatal location and stomatal area on the abaxial surface were demonstrated.