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Peer Reviewed

© 2005 Plant Management Network.
Accepted for publication 19 May 2005. Published 17 June 2005.

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

Ricky M. Bates, Assistant Professor, Department of Horticulture, Penn State University, University Park, PA, 16802; and David A. Despot, Project Assistant, Department of Horticulture, Penn State University, University Park, PA 16802

Corresponding author: Ricky M. Bates.

Bates, R. M., and Despot, D. A. 2005. Effects of azoxystrobin application rate and treatment interval on the control of Rhabdocline pseudotsugae on Douglas-fir Christmas trees. Online. Plant Health Progress doi:10.1094/PHP-2005-0617-01-RS.


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.


Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) is one of the most popular conifers grown for use as cut Christmas trees in the eastern United States due to its fast growth rate, attractive foliage, good form, and excellent postharvest characteristics (8,10,18). Use of Douglas-fir in commercial and residential landscape plantings is also increasing because of its wide availability and adaptability to a variety of sites (5).

Intermountain forms of Douglas-fir (P. menziesii subsp. glauca) comprise the majority of trees grown in the northeastern and mid-Atlantic United States. Trees derived from these Rocky Mountain seed sources have better cold hardiness and postharvest needle retention traits than coastal forms (P. menziesii subsp. menziesii) grown in the Pacific Northwest (10). However, research has shown that intermountain provenances are also very susceptible to Rhabdocline needlecast disease (Rhabdocline pseudotsugae) (14,16). Rhabdocline sp. are the major pathogens limiting production of Douglas-fir in the Northeast (14,16).

In late winter or early spring, the previous year’s Rhabdocline-infected needles develop spots that are at first yellow and then characteristic brown to brick red (Fig. 1). A common diagnostic feature is a distinct border between healthy and infected areas of a needle. As the current-year shoots are elongating, fruiting bodies erupt through the epidermis of infected one-year-old needles, exposing the orange-bronze apothecia (Fig. 2). Rhabdocline ascospores are expelled from the fruiting bodies and disseminated during rainy periods and require surface moisture on the current year’s developing needles for infection to occur. Infected older needles are cast during summer, leaving severely infected trees with only one year’s complement of needles (Fig. 3). Initially, disease severity and subsequent defoliation is greatest on the lower branches. Repeated severe Rhabdocline infections may weaken and kill young trees and even a single infection may render larger trees unmarketable (2,15).


Fig. 1. Rhabdocline-infected Douglas-fir needles in early spring exhibiting characteristic lesions.


Fig. 2. Rhabdocline pseudotsugae apothecia on the underside of Douglas-fir needle.


Fig. 3. Douglas-fir Christmas trees derived from the Lincoln National Forest. Infected tree (left) has one age class of needles; resistant tree (right) has three age classes of needles.


Strategies recommended to reduce disease levels include the use of resistant seed sources, disease-free stock, removal of inoculum reservoirs, and cultural practices to promote rapid drying of foliage (13,16,20). These practices alone, however, will not control Rhabdocline on susceptible trees grown from intermountain seed sources under favorable infection conditions. As a result, growers must resort to multiple fungicide applications.

There are relatively few fungicides registered for needlecast control on conifers and all are solely preventative (19). This is a serious limitation, as the pathogen that causes Rhabdocline needlecast infects during the period of tissue elongation in the spring. At this time, protectant fungicides must be applied frequently enough to maintain coverage on the rapidly expanding tissue. If long periods of rainfall occur during this critical time of tissue elongation, it becomes extremely difficult to prevent infection. Management of Rhabdocline with chlorothalonil has traditionally been effective and represents the fungicide of choice in most chemical control programs. Other fungicides have been tested but were found to be ineffective in preventing infection (19,22). Pressure is also mounting to use lower-risk, environmentally compatible fungicides to control common diseases in commercial production settings as well as in home and commercial landscapes.

Azoxystrobin, which is marketed under the trade name Quadris (Syngenta, Greensboro, NC), is the first of a new class of reduced-risk fungicides with a very favorable toxicological and environmental profile to be registered for needlecast control on conifers. Azoxystrobin controls plant pathogens from the Phycomycetes, Ascomycetes, Basidiomycetes, and Deuteromycetes fungal groups and exhibits systemic properties on some crops. Effective disease control has been reported in tree fruit (11), herbaceous perennials and bulbs (6), vegetables (1), turfgrass (12), small grain (9), and small fruit (21). There are, however, no reports on the use of azoxystrobin to control Rhabdocline pseudotsugae on Douglas-fir. The objective of this study was to assess the effect of azoxystrobin fungicide application rate and treatment interval on the efficacy of Rhabdocline needlecast control on Douglas-fir.

Evaluation of Infection

Evaluation of foliar infection levels were performed in early March during each year of the study. The effect of fungicide treatments on disease development was assessed using a five-class disease severity rating system (17), where 1 = no obvious infection of the foliage; 2 = light-moderate infection of foliage in the bottom ¼ of crown and no infection in the higher portions of the crown; 3 = moderate-severe infection of the bottom ¼ and light-moderate infection of the second ¼ of the crown, no infection in higher portions of the crown; 4 = severe infection bottom half and light-moderate infection of the third ¼ of the crown; and 5=severe infection of the bottom ¾ of the crown. A disease incidence rating was determined by arbitrarily collecting a 3-to-4-inch (7.6-to-10.2-cm) stem section from the north, south, east, and west quadrant of each tree, approximately 3 ft above ground level, and arbitrarily selecting 10 needles from each stem section. Needles with 1 or more lesions were considered diseased, and ratings from the four quadrants averaged. The disease severity rating was multiplied by the mean disease incidence rating to derive an overall disease index. Thus the disease indices could range from 0 to 50. Trees having a disease index rating above 10 were considered commercially unacceptable. Data were subjected to analysis of variance (ANOVA) using the GLM procedure of SAS (SAS, Version 8.2, SAS Institute Inc., Cary, NC). Treatment means were compared with Fisher's protected least significant difference (LSD; P = 0.05).

Needle color on treated trees was evaluated 2 weeks after the final fungicide application and rated on a 1 to 5 scale where 1 = healthy dark green needles, 2 = healthy green or light green needles, 3 = dull green needles with light to moderate chlorosis, 4 = moderate to extensive yellowing or browning, and 5 = yellow needles displaying necrotic tips or spots. No treatments in any of the experiments had an adverse effect on needle color and no phytotoxicity was noted (data not shown).

2002 Field Trials

During spring of 2002, two replicated field trials were conducted in commercial Douglas-fir Christmas tree farms in Schuylkill and Clinton Counties, PA. At the Schuylkill site, all work was conducted in a planting of approximately 12 acres of sheared trees, near Schuylkill Haven, PA. Trees were 4 to 6 ft tall and planted on 6-ft centers. At the Clinton site, the planting consisted of approximately 15 acres of sheared 7-to-9-ft trees planted on 7-ft centers, near Beech Creek, PA. All trees at both sites were derived from the same seed source from the Lincoln National Forest. During spring 2001, needlecast caused by Rhabdocline pseudotsugae was found evenly distributed throughout both farms. Interviews with the farm managers indicated that the sites suffered severe disease levels during the previous two seasons. In order to minimize variation, treatment trees were selected based on presence of uniform, heavy, natural infection and well-formed buds.

At each site, treatments were arranged as a randomized complete block design comprised of 8 blocks. Treatments were applied to individual trees in each block. Experiments consisted of 6 fungicide treatments and a nontreated control (Table 1). All fungicides were applied to the point of drip as a directed spray with a 4.7 horsepower Solo Port backpack mistblower (Model 450; Solo, Inc., Newport News, VA) at approximately 1100 m3/h. In all experiments, chlorothalonil (Bravo Weather Stik; Syngenta, Greensboro, NC) was applied four times at 1.29 g a.i./liter between April 30 and June 7, and represented local industry standard. The initial fungicide application was made at the onset of budbreak. Treatments 1 through 5 were designed to evaluate the effectiveness of azoxystrobin as a part of a chlorothalonil-based control program. Treatments 1, 2, and 3 replaced the first chlorothalonil application of the standard program with 0.14 or 0.28 g a.i./liter azoxystrobin and incorporated a 2 or 3 week interval prior to the second chlorothalonil application (Table 1). Treatments 4 and 5 were identical to the standard chlorothalonil treatment, except the second chlorothalonil application on 8 May, 2002 was replaced with azoxystrobin at 0.14 and 0.28 g a.i./liter.

Table 1. Azoxystrobin (Quadris) and chlorothalonil (Bravo Weather Stik) treatment rates, combinations, and intervals for 2002 and 2003 spray trials in Clinton and Schuylkill Counties, Pennsylvania.

Treatment Date in 2002
5/1 5/8 5/15 5/21 6/7
Controlx --- --- --- --- ---
Standard Cy C --- C C
1 A(1X)z --- C --- C
2 A(1X) --- --- C C
3 A(.5X) --- C --- C
4 C A(1X) --- C C
5 C A(.5X) --- C C
Treatment Date in 2003
4/30 5/7   5/19 6/3
Control --- --- --- --- ---
Standard C C --- C C
1 A(1X) A(1X) --- A(1X) A(1X)
2 A(2X) A(2X) --- A(2X) A(2X)
3 A(4X) A(4X) --- A(4X) A(4X)

 x Control trees were untreated.

 y C = Chlorothalonil (Bravo Weather Stik), 1.29 g a.i./liter (standard commercial regime).

 z A (.5X, 1X, 2X, 4X) = Azoxystrobin (Quadris), 0.14 g a.i./liter, 0.28 g a.i./liter, 0.55 g a.i./liter, 1.10 g a.i./liter, respectively.

Bio-88, a non-ionic surfactant (Kalo Inc., Overland Park, KS) was used with all azoxystrobin spray applications at a rate of 0.5 ml/liter. At least three rows of buffer trees separated all treatment trees to avoid overlapping treatments or interference from the other fungicide treatments. Temperature, relative humidity, and rainfall were recorded at each site throughout each growing season with a HOBO Weather Station (Onset Computer Corp., Pocasset, MA). Relative humidity and temperature varied during the study at the time of fungicide treatment, but applications were made only on rainless days and under conditions of low wind. After every application, the fungicide sprays were observed to dry on the tree foliage.

Application interval had a significant effect on the efficacy of azoxystrobin sprays against Rhabdocline. For treatments 1, 2, and 3, the first spray was applied 1 May and contained 0.14 or 0.28 g a.i./liter azoxystrobin and the second chlorothalonil spray was not applied until 15 May or 21 May. Treatments 1, 2, and 3 were only slightly more effective than the untreated controls. Disease indices at the Schuylkill site were between 25 and 31 and the Clinton site ratings were between 30 and 34, compared to indices of 39 and 45, respectively, for unsprayed trees (Fig. 4). For treatments 4 and 5, the first chlorothalonil spray was applied on 1 May and the second spray containing azoxystrobin was applied on 8 May. Conditions favoring infection including cool temperatures and heavy rainfall were recorded on 10 May, 13 and 14 May, and 19 May (data not shown). Treatments 4 and 5 provided significantly better control than treatments 1, 2, or 3 (Fig. 4). If the first infection event occurred on 10 May, treatments 1, 2, and 3 were sprayed 9 days prior to infection, whereas treatments 4 and 5 where sprayed only 2 days before infection. These results indicate that 0.14 and 0.28 g a.i./liter azoxystrobin offers poor protection against Rhabdocline infection when the interval between sprays is extended beyond the standard 7 days, particularly when disease pressure is high. Furthermore, recommended rates of azoxystrobin do not offer acceptable control even when applied within 2 days of an infection event compared to the standard chlorothalonil program. Treatments 4 and 5, which replaced the second chlorothalonil spray with 0.14 g a.i./liter and 0.28 g a.i./liter of azoxystrobin, had significantly higher disease indices than the standard chlorothalonil treatment at both sites. There was no significant difference between the 0.28 and 0.14 g a.i./liter rates of azoxystrobin at either site in 2002.


Fig. 4. Efficacy of azoxystrobin (Quadris) and chlorothalonil (Bravo Weather Stik) spray treatments on Rhabdocline needlecast infection of Douglas-fir at two Pennsylvania sites during 2002. Disease Index = disease severity (1 to 5 scale; 1 = no infection) x disease incidence (needles with one or more lesions). Control = untreated trees; Standard = Chlorothalonil 1.29 g a.i./liter, four applications; 1, 2, 3, 4, 5 = Azoxystrobin, 0.14 g a.i./liter, 0.28 g a.i./liter, one application as first or second spray. Letters indicate significant differences between treatments. Mean separation was tested according to Fisher’s protected Least Significance test (LSD; P = 0.05).


2003 Field Trials

In 2002 experiments were conducted at the same sites using the aforementioned procedures; however, trees selected for treatment and control trees in 2003 were not used in the 2002 trials. Experiments consisted of 5 treatments: 4 fungicide and a non-sprayed control (Table 1). Azoxystrobin was used alone and applied at 0.28 (maximum labeled rate), 0.55, and 1.10 g a.i./liter on 30 April, 7 May, 19 May, and 3 June to evaluate the impact of high azoxystrobin rates on Rhabdocline control. The standard four applications of chlorothalonil treatment as applied in the 2002 trial was also included as a comparison.

Application rate had a significant influence on the efficacy of azoxystrobin against Rhabdocline. Treatment 1 replaced the standard chlorothalonil treatment with four sequential applications of azoxystrobin at the maximum labeled rate of 0.28 g a.i./liter and moderately reduced infection at both sites compared to untreated controls (Fig. 5). Untreated control trees at the Clinton site had disease index ratings of 36.3 while the Schuylkill site control tree disease index rating was 26.1. Treatments 2 and 3 increased the azoxystrobin rate to 0.55 or 1.10 g a.i./liter respectively, and did not significantly improve control at either the Clinton or Schuylkill site. Applications of azoxystrobin applied alone resulted in disease indices of 11 to 14 at the Clinton site and 18 to 23 at the Schuylkill site. Azoxystrobin performed better under the lighter disease pressure of the Schuylkill site in 2003, compared to 2002, and the Clinton site both years.


Fig. 5. Efficacy of azoxystrobin (Quadris) and chlorothalonil (Bravo Weather Stik) spray treatments on Rhabdocline needlecast infection of Douglas-fir at two Pennsylvania sites during 2003. Disease Index = disease severity (1 to 5 scale; 1 = no infection) x disease incidence (needles with one or more lesions). Control = untreated trees; Standard = chlorothalonil 1.7 kg a.i/ha, four applications; 1, 2, 3 = azoxystrobin, 0.28 g a.i./liter, 0.55 g a.i./liter, 1.10 g a.i./liter, four applications. Letters indicate significant differences between treatments. Mean separation was tested according to Fisher’s protected Least Significance test (LSD; P = 0.05).


Spray Intervals

Reports vary with regard to the number of fungicide sprays necessary to achieve control of Rhabdocline. In a 1983 trial, Chastagner (3) reported that only one application of an effective fungicide was necessary to control Rhabdocline, but in some years a second application may be required. Other recommendations call for as many as 3 to 4 sprays per season (15). The Bravo Weather Stik (chlorothalonil) label recommends applying at budbreak and repeating at 3-to-4-week intervals until needles are fully elongated. These application directions assume relatively uniform budbreak. In practice, budbreak timing is often variable due to mixed Douglas-fir provenance or site variation. On average, contact fungicides such as chlorothalonil give about 7 to 10 days of Rhabdocline protection in the northeast U.S. (19).

In areas with intense Rhabdocline pressure, growers have found that timely re-application of fungicides is just as important to success as initial application. The keys to timeliness begin with understanding the fungicide’s residual activity period, and then closely monitoring weather conditions as the period expires. If conditions promoting infection are present at that time, another application may be warranted.

One of the objectives of this study was to investigate if azoxystrobin could effectively lengthen intervals between sprays while maintaining acceptable disease control. Results of this research indicate that azoxystrobin, when used alone, was ineffective even under the tight intervals of a four-spray program. When used in combination with chlorothalonil, the azoxystrobin was not a suitable replacement for chlorothalonil and did not provide a means of lengthening the intervals between applications.

Economic Considerations

In Pennsylvania, a calendar-based spray program consisting of 3 or 4 applications of chlorothalonil is widely followed. This standard chemical control program is effective in controlling Rhabdocline, albeit expensive. Chemical costs are $12.50 and equipment and application costs add approximately $9.54, for a total cost of $22.04 per application, per acre (Table 2). Douglas-fir growers following this regimen spend $66.00 to $88.00 per acre protecting their crop during the infection period and would like to reduce their fungicide applications to save money and time.

Table 2. Fungicide application cost estimates for Rhabdocline needlecast control on 1 acre of late-rotation Douglas-fir Christmas trees.

Ground application cost
   chlorothalonil (Bravo Weather Stik) @ 32 oz/acre $12.50
   azoxystrobin (Quadris) @ 12 oz/acre $29.10
   maintenance/repair   $0.43
   insurance/depreciation/interest   $0.85
   labor   $6.00
   insurance/benefits   $1.20
   fuel   $1.06
Total application cost using chlorothalonil $22.04
Total application cost using azoxystrobin $38.64
Aerial (helicopter) application cost
   chlorothalonil (Bravo Weather Stik) @ 32 oz/acre $12.50
   azoxystrobin (Quadris) @ 12 oz/acre $29.10
Custom application ratez $11.45
Total application cost using chlorothalonil $23.95
Total application cost using azoxystrobin $40.55

 w Price quote (Helena Chemical Corp., Mifflinville, PA) March 21, 2005.

 x Costs calculated for 30 horsepower tractor and 50 gal sprayer.

 y Hired labor @ $9.00/h plus 20% fringe costs.

 z Mean of two custom service quotes for 100 acre application, March 21, 2005.

Prior to this study, there was no data on the efficacy of azoxystrobin for control of Rhabdocline. Azoxystrobin’s protective, curative, and eradicant activity had been documented for other crops (1,12) but it was not known how the inclusion of azoxystrobin in a chlorothalonil-based control program would impact disease control costs for Douglas-fir. An informal 2003 survey of Pennsylvania Douglas-fir growers indicated that use of azoxystrobin for the control of Rhabdocline was increasing.

The data presented here does not support any cost incentive to include azoxystrobin as part of a Rhabdocline control program. At the recommended rate, azoxystrobin costs $38.64 per acre compared to $22.04 for chlorothalonil (Table 2). However, it is also unlikely that four applications of chlorothalonil are economically justified and the cost savings of reducing the overall number of sprays is potentially substantial. There does appear to be an opportunity for growers to reduce fungicide use through a better understanding of disease risk factors. Use of local monitored and forecast weather information and improved scouting for ascospore inoculum levels would help growers determine the need for additional applications beyond a second spray. In a good year when the danger of Rhabdocline is low, growers may be able to reduce fungicide applications, saving over $22.00 per acre for every application eliminated.

Aerial fungicide application has proven successful in controlling Rhabdocline. Because they can cover large acreages very quickly, and are not limited by wet soil conditions, helicopters have certain advantages in timeliness over ground sprayers. Aerial applications also compare favorably in cost with ground application (Table 2) and may represent an opportunity for growers with medium or large operations. Custom spray costs in Table 2 represent average costs for a 100-acre application, and this cost would likely increase significantly for smaller isolated acreages. An additional disadvantage to aerial application includes difficulty in scheduling due to high demand for services during a relatively narrow application window.

Growers need to balance potential savings in production costs from fewer fungicide applications with possible revenue losses from a higher percentage of diseased trees. The degree of Rhabdocline infection depends on how much inoculum is present in the plantation, as well as the number of infection periods in a season. There will always be variability in disease levels between farms regardless of how many fungicides are applied.

Conclusions and Management Implications

Needlecast diseases continue to limit Douglas-fir production in the northeastern United States. A recent survey of Christmas tree farms in New York found Rhabdocline pseudotsugae at nearly 90% of the collection sites and discovered Swiss needlecast (Phaeocryptopus gauemanii) to be more prevalent than previously thought (7). In this study, chlorothalonil was effective in controlling Rhabdocline pseudotsugae. In both years four applications of chlorothalonil at 1.29 g a.i./liter between 30 April and 7 June were highly effective against Rhabdocline at both sites and resulted in disease indices of less than 4 (Figs. 4 and 5). While light infection on the lower quarter of the crown of some trees was observed, all trees receiving the standard chlorothalonil treatment were marketable. However in recent years anecdotal evidence from growers suggests that chlorothalonil is not as effective as it has been in the past (7). The reduced efficacy of chlorothalonil should be verified and alternatives to chlorothalonil-based control programs identified if necessary. Results of this study indicate that azoxystrobin at the label rate and up to four times the label rate does not adequately control Rhabdocline either when used alone or in combination with chlorothalonil. These results are in agreement with Chastagner (4) who found azoxystrobin to be ineffective in controlling Swiss needle cast on stands of Douglas-fir timber at sites with high disease pressure. Results from this study also suggest that the level of natural inoculum contributed to azoxystrobin performance.

Additional studies are needed to develop alternatives to chlorothalonil that are effective in controlling needlecast diseases and economically sustainable. Use of seed sources more resistant to Rhabdocline than the Lincoln N.F. and selection of resistant trees may represent a less expensive and sustainable answer to losses due to this disease.


We wish to thank Davey Kramer and Jim Moore for providing us with the experimental locations and the Pennsylvania Christmas Tree Growers Association for their financial support.

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