© 2006 Plant Management Network.
Control of Phytophthora Root Rot in Field Plantings of Fraser Fir with Fosetyl-Al and Mefenoxam
D. M. Benson, Professor, Department of Plant Pathology, North Carolina State University, Raleigh 27695; J. R. Sidebottom, Mountain Conifer IPM Specialist, Department of Forestry, North Carolina State University, Raleigh 27695; and J. Moody, County Agent, North Carolina Cooperative Extension, Avery County, Newland 28657
Benson, D. M., Sidebottom, J. R., and Moody, J. 2006. Control of Phytophthora root rot in field plantings of Fraser fir with fosetyl-al and mefenoxam. Online. Plant Health Progress doi:10.1094/PHP-2006-0331-01-RS.
Fungicides were evaluated for control of Phytophthora root rot for five growing seasons in two field plantings of Fraser fir (Abies fraseri) affected by Phytophthora cinnamomi in western North Carolina. At the first site, which had a fairly well-drained soil, treatment programs with Aliette (5 lb/100 gal, three applications per year), Subdue Maxx (3.7 fl oz/1.15 gal/1000 ft2, two applications per year), and Subdue GR (5.75 lb/1000 ft2, two applications per year) maintained low rates of mortality (< 10%) for three growing seasons, whereas tree mortality in the untreated plots reached 13 and 37% by the second and third growing seasons, respectively. At the second site, which was in a flood plain, disease did not develop during the first 2.5 years, even in untreated control plots. However, mortality increased rapidly following several high-rainfall events, but none of the fungicides had any effect on disease development. Apparently, at least under conditions not overly conducive to the disease, the fungicide treatment programs can delay the onset of high mortality rates caused by P. cinnamomi in Fraser fir for up to three growing seasons.
Phytophthora root rot of Fraser fir (Abies fraseri), caused primarily by Phytophthora cinnamomi, is a serious disease problem in North Carolina (1) when the host is planted as a Christmas tree species in poorly-drained, heavy soils (Fig. 1). Phytophthora root rot losses averaged 9% in a survey of Fraser fir plantations in western North Carolina (1). Fraser fir is the dominant species in North Carolina’s Christmas tree industry, which ranks second only to Oregon in total production of trees (9). During the 5-month growing season (mid-April to mid-September), when soils are near saturation following heavy rains, chlamydospores of the pathogen germinate to produce sporangia that in turn form zoospores that are responsible for root tip infections. Inoculum of Phytophthora spp. may already be present in a field site, or may be introduced either on infected Fraser fir transplants or in runoff from adjacent infested areas following heavy rains.
Fraser fir seeds are broadcast in nursery beds and grown for 3 years. Seedlings are then transplanted and grown in line-out beds for an additional 2 years. Disease-free production of transplants requires the use of pre-plant fumigation, cultural practices and post-plant fungicide applications for effective management of Phytophthora root rot (8). Once transplants are planted in field sites, trees normally are grown 7 or more years to reach a marketable size. For field sites, growers have relied on clean planting stock and cultural practices to limit losses to the disease (8) as host resistance in not available (5,7). In the past, preventative fungicide application for Phytophthora root rot control in field sites had not been considered due to steep slopes, difficulty of moving large volumes of water in the site, and cost of fungicide. However, several fungicides have proven effective for control of Phytophthora root rot on Fraser fir in seed beds and in containers, including fosetyl-Al (Aliette, Bayer CropScience, Research Triangle Park, NC) and mefenoxam (Subdue Maxx, Syngenta Professional Products, Greensboro, NC) (2,3,4). The objective of this research was to determine if fosetyl-Al and mefenoxam could suppress the development of Phytophthora root rot of Fraser fir in field sites where the disease was already active at the time of first application.
In June 2001, two field sites in western North Carolina were selected, one in Avery Co. on Beech Mountain and another in Watauga Co. near Boone. When the experiments were initiated, the incidences of mortality due to Phytophthora root rot were 0.3% in the Avery Co. site and 2.8% at the Watauga Co. site. The Avery Co. site had been in Fraser fir for many years, and the current planting was 4 years old. The soil was a Chestnut coarse loam, a moderately deep, well-drained soil at pH 5.3. The site had sustained significant losses to Phytophthora root rot in previous fir plantings. It had a 14% slope with a southwest aspect. The Watauga Co. site was on a Cullowhee loam, a poorly-drained soil type associated with flood plains. The slope was 3% with a soil pH of 4.2. This bottom land was in pasture before Fraser fir was planted, and had a long history of cabbage production prior to the pasture. Though the previous crops were non-hosts of P. cinnamomi, adjoining upland areas had Fraser fir plantings that drained toward the experimental site.
Layout and Fungicide Applications
Each treatment was applied to a plot of six trees by six trees planted on 5-ft centers giving an area of 900 ft2 per treatment per replication. There were three replications of each treatment plus an untreated control arranged in a randomized complete block design. Fungicides evaluated were fosetyl-Al (Aliette 80W at 5 lb/100 gal), and mefenoxam (Subdue Maxx 2E at 3.7 fl oz/1.15 gal/1000 ft2; and Subdue 1GR at 5.75 lb/1000 ft2). Aliette was applied as a foliar spray to wet the foliage of each tree in the plot with a hand-pumped backpack sprayer equipped with a cone nozzle. Subdue Maxx was applied as a directed spray to the soil surface with a hand-pumped backpack sprayer equipped with a flood nozzle (Fig. 2). Subdue 1 GR was broadcast uniformly over the plot with a hand-cranked pouch seeder (Fig. 2). Subsequent rainfall was relied on for movement of the Subdue fungicides into the soil profile.
Initial fungicide applications were made on 5 June 2001 at the Avery site and 6 June 2001 at the Watauga site. Soil temperature at 4 inches at time of initial application was 71°F at the Avery site and 69°F at the Watauga site. Subsequent applications of Aliette at the Avery and Watauga sites were on 14 August and 25 September 2001; 6 May, 19 June, and 18 September 2002; 13 May, 24 July, and 2 October 2003; 4 May, 28 July, and 26 October 2004; and 9 May 2005 for a total of 13 applications over five growing seasons. The Subdue fungicides were applied only in May and September-October each year for a total of nine applications. Soil temperature (4 inches) at the Avery site for the May application dates were 71, 59, 52, 48 and 55°F in 2001, 2002, 2003, 2004, and 2005, respectively. Soil temperatures (4 inches) at the Watauga site for the May applications were 69, 55, 57, and 50°F in 2001, 2002, 2003, and 2004, respectively.
Symptoms of Phytophthora root rot on Fraser fir, including chlorosis, stunted shoot growth, flagging, and mortality (Fig. 1), were rated each spring and fall at the time of fungicide applications. Mortality was used as the main indicator of Phytophthora infection over growing seasons, because chlorosis and stunted growth also can result from nutrient deficiencies and insect infestations. No symptoms or signs of other root pathogens affecting fir were observed in the field sites during the study period. Isolations were made from representative plants at each location to confirm the presence of P. cinnamomi. The mortality data were subjected to analysis of variance using PROC ANOVA (PC-SAS, SAS Institute Inc., Cary, NC) with mean separation according to the Waller-Duncan k-ratio t test, k = 100.
Disease Progress and Fungicide Protection
At the Avery Co. site, no additional mortality of Fraser fir occurred in any of the treatments over the growing season of 2001 from that present at the beginning of the experiment. However, between October 2001 and May 2002, mortality increased to over 10% in the untreated control plots but did not increase for trees treated with Aliette or either formulation of mefenoxam (Fig. 3A). During this period 21.1 in of rain fell on the site (Fig. 3B). By May of 2003, mortality of Fraser fir due to Phytophthora root rot had reached 37% in the untreated control while mortality in fungicide-treated plots remained near 5%. At each disease assessment date between October 2001 and October 2003 mortality of Fraser fir in the three fungicide treatments was significantly less than the untreated control (P < 0.05). Between October 2002 and May 2003, an estimated 32.1 inches of precipitation fell on the site (Fig. 3B). Mortality of Fraser fir reached 48% in untreated plots between October 2003 and October 2004 (Fig. 3A). During this same period, mortality increased to 22 and 17% in the Subdue 1GR and Aliette-treated plots, respectively, while mortality reached 7% in Fraser fir treated with Subdue Maxx (Fig. 3A). Between October 2003 and May 2004, total precipitation at the site was 26.8 inches. However, almost 20 inches of precipitation fell on the site in September 2004 as a result of three tropical depressions that moved over western North Carolina. From May 2004 through September 2005 mortality increased slightly in all treatments but no significant differences between treatments were found after May 2004.
At the Watauga Co. site, mortality in Fraser fir did not increase in any treatment between the initiation of the experiment in June 2001 and May 2003 (Fig 4A). Rainfall between October 2001 and May 2002 was 20.5 inches and 38.1 inches between October 2002 and May 2003 with almost 7 inches of that in April 2003 (Fig. 4B). However, mortality increased from 8 to 15% and from 2 to 19% between May and October 2003 in Fraser fir plots treated with Subdue Maxx and Subdue GR, respectively (Fig. 4A). Rainfall from April to October 2003 was over 34 inches. A high incidence of mortality was concentrated in a downhill portion of the site, which received a disproportionate share of water runoff in the April to October 2003 period. The wet area with high mortality fell predominately across some plots of the two Subdue treatments. Mortality caused by Phytophthora root rot reached 25% in the Subdue Maxx and Subdue GR plots by May 2004 (Fig. 4A). There was a slight rise in mortality of Fraser fir in the untreated control and no change in mortality in Aliette-treated plots during this same period. No differences (P = 0.46) were observed between treatments. No change in mortality occurred in any plots during the period June 2004 and June 2005, regardless of treatment. Interestingly, 75 inches of rain was recorded during this period, including over 23 inches in September 2004. At the final assessment on 29 September 2005, however, a drastic increase in dead and dying trees occurred across all plots (Fig 4A). Mortality in the untreated control went from 11 to 44% with a similar increase for trees in fungicide-treated plots. For instance, mortality in Subdue Maxx plots rose from 25 to 66%. Rainfall in September 2005 at the site was only 0.67 inches (Fig. 4B). Overall, no significant differences in mortality among Fraser fir in the various fungicide treatments compared to the untreated controls was found at any assessment.
Patterns of Mortality
Two different patterns of mortality developed, depending on the site. At the Avery Co. site, disease increased constantly in the untreated plots over the 5 years, reaching 50% mortality by year 5. The three fungicides were effective in delaying this increase in mortality for 3 years, after which a sharp increase in mortality occurred in the Aliette and Subdue GR plots, although mortality in the Subdue Maxx plots remained low. The high rainfall in September 2004 did not result in an increase in mortality of Fraser fir across treatments throughout the next year. At the final assessment date in late September 2005, no increase in mortality was observed even though trees had been subjected to drought stress for about 4 weeks as only 0.89 inch of rainfall was recorded. Apparently, the chestnut coarse loam drained well enough during the September 2004 rainy period such that a large percentage of trees were not infected but held enough water during September 2005 to avoid the effects of drought stress and subsequent mortality of any asymptomatic infected trees.
Conversely, at the Watauga site, no mortality occurred in any treatment including the untreated control until 2.5 years after initial fungicide application. The sharp increase in mortality that was observed after 2.5 years in the Subdue Maxx and Subdue GR treated plots was not tied to an unusually high rainfall month, but rather to a wet summer in July and August and then only because of the drainage pattern in the field site. These two patterns suggest a different distribution of initial inoculum of P. cinnamomi at each field site. The steady increase in mortality at the Avery Co. site in the untreated control plots suggests inoculum was distributed generally across the entire field site. However in the Watauga Co. site, inoculum was introduced as water drained across the field site from adjoining Fraser fir plantings. The exceptionally large rainfall from the tropical storms in September 2004 may have resulted in an increase in the number of infected trees in the bottom-land site that were not symptomatic by the June 2005 assessment. However, at the end of September 2005, when only 0.67 inch of rainfall was recorded in the previous 4 weeks, the remaining healthy roots of these asymptomatic trees could no longer support foliage water requirements so these trees died rapidly. Indeed, subsequent root samples from dead and dying trees again confirmed the presence of P. cinnamomi.
Phytophthora Root Rot Management with Fungicides
Aliette and Subdue fungicides have been used routinely in the past to protect Fraser fir seedlings and transplants in nursery beds, but fungicides are not generally used on field planted stock due to cost and terrain considerations (8). Both Aliette and Subdue fungicides delayed increase in mortality due to Phytophthora root rot in Fraser fir at the Avery Co. site. Application of metalaxyl (Subdue 2E) to Douglas fir seedlings infected with Phytophthora spp. in nursery beds in Oregon resulted in suppression of further disease development, fewer new infections, and a greater number of healthy roots on seedlings (6). A three-growing-season delay in onset of mortality that we observed when fungicides were used compared to a steady increase in mortality in untreated plots indicates that field-applied fungicides could be an effective management tool for Phytophthora root rot of Fraser fir under certain conditions. It was not until the fourth growing season, when mortality approached 45% in the untreated plots, that mortality began to increase in fungicide-treated plots, and even then mortality was only 10% in plots treated with Subdue Maxx. Since Fraser firs are typically grown 7 to 8 years in the field prior to harvest as Christmas trees, it would not be practical to treat entire fields for the whole production cycle. However, portions of fields where Phytophthora root rot began to occur later in the production cycle could be treated with fungicides to delay further losses and get the trees to a harvestable size.
We applied Aliette three times per growing season and Subdue fungicides twice per season. On an acre basis (approximately 1740 trees) this treatment schedule would require about 6.3 lb of Aliette per season given that trees were 3 to 4 years in the field at first application. At a cost of about $22/lb this equates to about 8 cents per tree per year for material only. Subdue Maxx at two applications per year would require 2.5 gal of product, while 500 lb of Subdue GR would be needed per acre. At about $590/gal, Subdue Maxx would cost about 85 cents per tree per year and Subdue GR would cost about $1.01 per tree per year. Given an average wholesale price of about $20 per tree, fungicide material costs only would be about 0.4, 4.3, and 5% of tree value per year for Aliette, Subdue Maxx, and Subdue GR, respectively.
Mortality of Fraser fir due to Phytophthora root rot was delayed up to three growing seasons with three foliar spray applications of Aliette per year or two soil applications of Subdue Maxx or Subdue GR at the Avery Co. site with acceptable soil water drainage. When mortality had reached over 45% in untreated plots, only 10% mortality was observed for trees in plots treated with Subdue Maxx. At a second field site, which apparently suffered from high rainfall events, poor soil water drainage, and irregular distribution of inoculum of P. cinnamomi, none of the fungicide treatment programs (i.e., with Aliette, Subdue Maxx , or Subdue GR) resulted in significant disease control, compared to that in untreated plots. The results suggest that appropriate foliar spray programs with Aliette or soil treatment programs with mefenoxam can, at least under conditions not overly favorable to the disease, delay high rates of mortality caused by P. cinnamomi in Fraser fir.
Acknowledgments and Disclaimers
This research was supported by a grant from the North Carolina Christmas Tree Association and by the North Carolina Agricultural Research Service, North Carolina State University, Raleigh. We thank the North Carolina Christmas tree growers who provided their farms for our research and Kala Parker for assistance with the statistical analysis.
The mention of a trade name, proprietary product, or vendor does not constitute an endorsement, guarantee, or warranty by the North Carolina Agricultural Research Service and does not imply its approval or the exclusion of these or other products that may be suitable.
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7. Hinesley, L. E., Parker, K. C., and Benson, D. M. 2000. Evaluation of seedlings of Fraser, momi, and Siberian fir for resistance to Phytophthora cinnamomi. Hortscience 35:87-88.