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ARS Home » Northeast Area » Washington, D.C. » National Arboretum » Floral and Nursery Plants Research » Research » Publications at this Location » Publication #343869

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

item GUO, YONGHONG(HENRY) - Rutgers University
item KILCREASE, JAMES - Orise Fellow
item Hammond, John
item Pooler, Margaret

Submitted to: Journal of Plant Pathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/26/2019
Publication Date: 11/7/2019
Citation: Guo, Y., Kilcrease, J., Hammond, J., Pooler, M.R. 2019. Stomatal openings on boxwood leaves yield entry portals for leaf infection by Calonectria pseudonaviculata. Journal of Plant Pathology.

Interpretive Summary: Boxwood (Buxus spp.) are slow-growing evergreen shrubs and small trees commonly grown as hedges and for topiary. Over 13 million boxwood plants are sold in the U.S. each year, with an annual market value of over $100 million. Boxwood blight, caused by the fungus Calonectria pseudonaviculata, is a destructive leaf-drop and stem-lesion disease, and is of significant concern throughout Europe, the U.S., and Canada. ARS scientists in Beltsville, MD examined boxwood leaves to determine the method of entry of the boxwood blight fungus into the plant. They discovered that the fungus enters the boxwood leaves only through the pores (stomata) in boxwood leaves. Analysis of the distribution of these pores among different boxwood varieties may lead to a strategy to identify resistant varieties of boxwood.

Technical Abstract: 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 (upper) epidermis of boxwood leaves with minimal distribution on or along the central vein of the adaxial (lower) 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 mm2. Stomatal size ranged from 500 ± 84 um2 (B. harlandii) to 1052 ± 117 um2 (B. sinica 'NaNa'). No linear relationship between stomatal density or size and host resistance was observed; however a relationship between stomatal location and infection rate was demonstrated.