Effects of Azoxystrobin Application Rate and Treatment Interval on the Control ofRhabdocline pseudotsugaeon Douglas-fir Christmas Trees

Abstract

Rhabdocline needlecast caused by Rhabdocline pseudotsugae is the primary disease limiting Douglas-fir Christmas tree production in the northeastern United States. Azoxystrobin (Quadris) was recently registered for control of needlecasts on conifers, but little is known about its efficacy. In 2002, azoxystrobin was applied to field-grown Lincoln N.F. Douglas-fir Christmas trees at 0.14 or 0.28 g a.i./liter as the first or second spray of a chlorothalonil-based control program. In 2003, four sequential sprays of azoxystrobin at 0.28, 0.55, and 1.10 g a.i./liter were compared to chlorothalonil treatments. Untreated trees at both Pennsylvania test sites in both years were heavily infected, confirming high inoculum levels and environmental conditions favorable for infection. The standard program that consisted of 1.29 g a.i./liter chlorothalonil applications was very effective in controlling Rhabdocline in all experiments. In 2002, application interval had a significant effect on efficacy. Trees sprayed with 0.14 and 0.28 g a.i./liter azoxystrobin 9 days prior to an infection period had a higher disease index rating than those sprayed 2 days prior to the same infection period. Application rate did not appear to have an effect on efficacy. Trees receiving 1.10 g a.i./liter azoxystrobin had the same disease index rating as trees receiving the 0.28 g a.i./liter rate. In all cases, azoxystrobin treatments had significantly higher infection rates than the standard chlorothalonil treatment. The level of natural inoculum present at each site also appeared to play a role in azoxystrobin efficacy. Azoxystrobin is more than twice the cost of chlorothalonil and the data presented does not support any cost incentive for its inclusion in a Rhabdocline control program.Accepted for publication 19 May 2005. Published 17 June 2005.