Pacific Southwest

Title: Effectiveness of fungicides in protecting conifers and rhododendrons from Phytophthora ramorum

Author: Chastagner, Gary A.; DeBauw, Annie; Riley, Kathy; Dart, Norm

Date: 2008

Source: In: Frankel, Susan J.; Kliejunas, John T.; Palmieri, Katharine M., tech. coords. 2008. Proceedings of the sudden oak death third science symposium. Gen. Tech. Rep. PSW-GTR-214. Albany, CA: U.S. Department of Agriculture, Forest Service, Pacific Southwest Research Station. pp. 325-333

Station ID: GTR-PSW-214

Description: The effectiveness of 19 fungicides in protecting noble fir, grand fir, and Rhododendron x ‘Nova ZemblaÂ’ foliage from P. ramorum was tested. The tops of conifer seedlings with newly emerging shoots and mature rhododendron leaves were collected from treated plants 7 days after drench applications or 1 day after foliar applications. Detached seedling tops and leaves were then placed on moistened filter paper in Petri dishes and inoculated. Zoospores from three European genotype isolates of the A1 mating type (EU1 lineage) and three NA1 lineage isolates of the A2 mating type were mixed to create two inoculum sources representing the A1 and A2 mating types for the conifer tests. Detached rhododendron leaves were inoculated with suspensions of zoospores from a EU1 A1 mating type rhododendron isolate and a North American genotype NA1 A2 mating type rhododendron isolate. The rhododendron leaves were inoculated by pipetting three 10 ul drops of zoospore suspension onto the lower leaf surface on each side of the leaf midrib. The leaf tissue was injured beneath 3 drops on one side of the leaf midrib using an insect pin. The tissues beneath the drops on the other side of the leaf were left unwounded. Checks included inoculated and non-inoculated conifer tops and rhododendron leaves that had been treated with plain water. Following inoculation, the Petri dishes were incubated for 7 days at 19°C in plastic tubs. Fungicide efficacy on the conifers was quantified by calculating the percent of visibly diseased shoots on each seedling top. Fungicide efficacy on the rhododendron leaves was quantified by counting the number of inoculation sites that developed symptoms and the area of each resulting lesion using ASSESS. No disease developed on any of the non-inoculated conifer or rhododendron checks. On the conifers, ANOVA analysis indicated that genotype and treatment (P<0.0001) were the only variables that had a significant affect on infection. Overall, the seedling tops inoculated with the A1 mating type had significantly fewer infected shoots than the seedlings inoculated with the A2 mating type. Applications of Dithane, Gavel, Stature, Ranman, Maneb, Polyram, Fenstar, Daconil Ultrex, V-10161, and Insignia were the most effective fungicides in reducing infection of the conifer shoots by both the A1 and A2 mating types of P. ramorum. On rhododendron, ANOVA analysis indicated that wounding and treatment (P = 0.001) were the only variables that had significant effects on the number of infected inoculation sites and lesion size. Overall, fewer fungicides were effective in reducing disease development on the wounded leaf surfaces. Maneb, Gavel, Subdue MAXX, Ranman (3 oz), Stature DM and Dithane were the most effective treatments in controlling disease development on both the wounded and non-wounded rhododendron leaves.

Keywords: Phytophthora ramorum, disease management, fungicides, control, conifers, rhododendrons

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Citation

Chastagner, Gary A.; DeBauw, Annie; Riley, Kathy; Dart, Norm  2008.  Effectiveness of fungicides in protecting conifers and rhododendrons from Phytophthora ramorum.   In: Frankel, Susan J.; Kliejunas, John T.; Palmieri, Katharine M., tech. coords. 2008. Proceedings of the sudden oak death third science symposium. Gen. Tech. Rep. PSW-GTR-214. Albany, CA: U.S. Department of Agriculture, Forest Service, Pacific Southwest Research Station. pp. 325-333.