PARENT SESSION
Poster Session # 15: Plant Ecology.

Wednesday, August 6 Presentation from 5:00 PM to 6:30 PM. SITCC Exhibit Hall B.

How does elevated CO₂ affect a foliar fungal disease of field-grown Acer rubrum?

Hoye, Katherine¹, McElrone, Andrew¹, Reid, Chantal¹, Jackson, Robert ¹, ¹ Duke University, Durham, NC

ABSTRACT- Little is known about the impact of disease on plants grown at projected elevated CO₂ concentrations. To address these poorly understood interactions, we surveyed Acer rubrum trees for a leaf spot disease caused by the fungal pathogen, Phyllosticta minima, at the Duke FACE experiment, Durham, NC. Disease incidence and severity were lower under elevated CO₂ with 21% fewer trees and 23% fewer leaves per tree exhibiting leaf spot, and reduced mean lesion area on infected leaves (5.12 and 6.35 mm² for elevated and ambient CO₂, respectively). Because the fungus enters the leaf through stomata, we investigated whether a decrease in leaf stomatal density and/or size with elevated CO₂ could explain the reduced fungal infection. In the field, we tested this hypothesis by measuring leaf stomatal density and size of the stomatal aperture on leaf impressions of A.rubrum collected in the Fall 2002 at Duke FACE. Leaf impressions were examined with a scanning electron microscope for determination of fungal infection and stomatal morphology. For leaves grown at elevated CO₂, stomatal size and aperture were increased with elevated CO₂ while stomatal density was reduced. In growth chambers, we grew red maple seedlings at ambient and elevated CO₂ (350 and 560 mol CO₂ mol^-1, respectively) and inoculated young fully expanded leaves with P. minima to follow its mode of entry into the leaf and rate of infection. Scanning electron micrographs of infected leaves were taken at regular intervals following inoculation. The direct impact of elevated CO₂ on the growth and reproduction of P. minima in culture was also monitored. Results suggest that changes in stomatal morphology under elevated CO₂ partly explain the reduced fungal infection on A. rubrum. Changes in leaf surface wax deposition may also contribute to reduced infection by altering fungal hyphae signaling and growth towards the stomata.

Key words: stomata, red maple, plant-pathogen, global change