Aerial application of Agri-Fos® to prevent Sudden Oak Death in Oregon tanoak forests

We have been testing the practicality and efficacy of aerial application of Agri-Fos for control of sudden oak death (SOD) in the Oregon tanoak (Lithocarpus densiflorus) forest. Helicopter application to forest stands has been compared with bole injection and groundbased spray application to seedlings and stump sprouts. We bio-assayed for Agri-Fos uptake using twig and seedling inoculation with zoospores and bole wound inoculation. Experiments are continuing. Preliminary results indicate small but significant reductions in growth of Phytophthora ramorum in Agri-Fos sprayed trees, comparable to the results obtained from bole injection. Introduction Phosphonate (Agri-Fos) has been shown to be effective in limiting development of Phytophthora ramorum when sprayed directly onto or injected into individual tree boles. There have been no tests, however, of its application and uptake under forest conditions, or of its efficacy as a disease protectant in landscape scale application on tanoak (Lithocarpus densiflorus). Proper governmental registration and permission for use of phosphonate in the forest requires demonstration of efficacy. We are conducting trials in Oregon to demonstrate the feasibility of aerial application from helicopter, phytotoxicity at various spray concentrations, and the uptake and translocation of the chemical within treated trees. We are also testing the efficacy of Agri-Fos as a protectant fungicide by spraying tanoak seedlings and then exposing them to natural inoculum of P. ramorum in the field. Results presented here are preliminary and do not represent the complete data set. Final results and analysis will follow completion of the final assays in November 2009. Methods Three methods of Agri-Fos application are being compared: helicopter application to mature tanoak forest trees; backpack pressurized spray to simulate the helicopter application to tanoak seedlings and sprouts; and bole injection of mature tanoak trees. The aerial spray application rates were: no treatment; 3 gal Agri-Fos 400/20 gal water/acre; and 6 gal Agri-Fos 400/20 gal water/acre. The seedling application rates were the same as the aerial rates except the 6 gal per acre rate was replaced by 0.1 gal 1 A version of this paper was presented at the Fourth Sudden Oak Death Science Symposium, June 1518, 2009, Santa Cruz, California. 2 Oregon Dept. of Forestry, 2600 State Street, Salem, OR 97301. 3 Dept. Botany and Plant Pathology, Oregon State University, Corvallis, OR 97331. Corresponding author: akanaskie@odf.state.or.us. GENERAL TECHNICAL REPORT PSW-GTR-229 226 Agri-Fos 400/20 gal water/acre. Pentra-Bark was added to all spray mixes. Tanoak trees were injected with Agri-Fos 400 at the label rate of 10 ml injection fluid (1 part Agri-Fos:2 parts water) per 15 cm trunk circumference. Five different biological assays are being used to measure Agri-Fos uptake: 1. Twig assay: Ten treated trees per treatment plot were felled and 20, 25 cm long shoot tips (twigs) from the outer canopy were collected from each and transported to the laboratory. Cut ends of shoot tips from forest trees or sprout clumps are immersed in a 1 cm deep zoospore suspension then incubated 3 weeks. Resultant lesion lengths are measured. 2. In situ bole inoculation with P. gonapodyides: Intact tanoak trees are wound inoculated in the field. Lesion area is measured after 5 weeks. P. gonapodyides was used in field inoculations instead of P. ramorum due to quarantine regulations. 3. “Log” bolt inoculation with P. ramorum: Trees are felled and 1 m bolts cut and returned to the laboratory. Logs are wound inoculated in the laboratory, and lesion area measured after 5 weeks. 4. Artificial inoculation seedling assay: Treated seedlings are challenge inoculated with a zoospore and sporangium suspension of P. ramorum, and incubated in a growth chamber until symptoms develop. 5. Treated seedlings are exposed to natural inoculum beneath infected tanoak trees in Curry County, Oregon. Treatment areas for helicopter spray treatments were selected in extensive stands of tanoak north of the SOD infestation in Curry County, Oregon. Treatments were applied by contract helicopter November 2007, May 2008, December 2008, and May 2009 (table 1). At each application time, we sprayed three blocks, each consisting of three 10 acre treatment plots. Table 1—Aerial application of Agri-Fos to tanoak forests: tests, treatments, and assays Test Spray Dates Treatments Area Treated Assay Assay Date 1 Nov 07 0, 3, and 6 gal /Acre 3, 10 Acre Blocks/Treatment Branch dip In situ P. g May 08 2 May 08 0, 3, and 6 gal /Acre 3, 10 Acre Blocks/Treatment Log-P.ram Branch dip In situ P. g Jan 09 Nov 09 3 Nov 08 and May 09 0, 3, and 6 gal /Acre at each time 3, 10 Acre Blocks/Treatment Branch dip In situ P. g June 09 Nov 09 Results Aerial Application The twig assay (fig. 1), the in situ assay with P. gonapodyides (fig. 2), and the log assay with P. ramorum (fig. 3), all indicated an Agri-Fos effect in sprayed trees 6 months or 18 months after treatment. Lesion length differences with the twig assay were usually small, but significant. The in situ and log assays gave larger and more consistent differences in lesion area between treatments. There was no consistent difference between the two fungicide doses. Proceedings of the Sudden Oak Death Fourth Science Symposium 227 Bravo (original data)