Application of phosphonate to prevent sudden oak death in south-western Oregon tanoak (Notholithocarpus densiflorus) forests

Abstract

We conducted four experiments to evaluate the effectiveness of phosphonate application to tanoak (Notholithocarpus densiflorus (Hook. & Arn.) Manos, Cannon & S.H.Oh) forests in south-western Oregon: (1) aerial application to forest stands; (2) trunk injection; (3) foliar spray of potted seedlings; and (4) foliar spray of stump sprouts. We compared aerial spray treatments: (1) no treatment (unsprayed); (2) low-dose (17.35 kg a.i. ha^-1); and (3) high dose (34.5 kg a.i. ha^-1), applied by helicopter in a carrier volume of 188 L ha^-1 to 4-ha treatment plots. Treatments were applied in November 2007, in May 2008, and in December 2008 and May 2009 (double treatment). At the same time as the aerial application we injected phosphonate into the trunk of nearby mature tanoak trees at the standard label rates of 0.43 g a.i. cm-dbh-1. We used three different biological assays to measure uptake of phosphonate: (1) canopy twig dip in zoospore suspension; (2) in situ bole inoculation with Phytophthora gonapodyides (Petersen) Buisman; and (3) laboratory inoculation of log bolts with Phytophthora ramorum S. Werres, A.W.A.M. de Cock & W.A. Man in 't Veld and P. gonapodyides. We also simulated an aerial spray of potted seedlings, comparing an untreated control, a low dose (2.9 kg a.i. ha^-1 applied in 935 L spray solution ha^-1#, and a high dose #17.35 kg a.i. ha^-1applied in 187 L spray solution ha^-1).

Aerial spray with phosphonate consistently resulted in smaller bole lesions on trees challenge inoculated with Phytophthora gonapodyides in situ and in logs inoculated with P. ramorum. This effect persisted for 18 months post treatment. Results from detached canopy twig assays were variable and showed only small treatment effects. Trunk injection consistently reduced bole lesion size in trees and logs, but gave inconsistent results in the canopy twig assay, possibly due to the twig assay methodology. In the spring and autumn trunk injection treatment, canopy twig lesion length was reduced by 32 percent compared to untreated controls, indicating that trunk-injected phosphonate was mobilised to the outer twigs of the tree crown. Trunk injection with phosphonate resulted in a greater reduction in bole lesion area than aerial spray. Spray application of phosphonate to tanoak seedlings did not protect them from infection when exposed to artificial or natural inoculum of P. ramorum. Foliar application of phosphonate to stump sprouts reduced lesion length by 44% of control in a shoot-dip assay three months post-treatment.

Citation

Kanaskie, Alan; Hansen, Everett; Sutton, Wendy; Reeser, Paul; Choquette, Carolyn. 2011. Application of phosphonate to prevent sudden oak death in south-western Oregon tanoak (Notholithocarpus densiflorus) forests. New Zealand Journal of Forestry Science. 41S: S177-S187. [published on-line: 25/11/2011]