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Effects of Soil Solarization and Trichoderma asperellum on Soilborne Inoculum of Phytophthora ramorum and Phytophthora pini in Container Nurseries

    Affiliations
    Authors and Affiliations
    • F. Funahashi , Department of Crop and Soil Science
    • J. L. Parke , Department of Crop and Soil Science and Department of Botany and Plant Pathology, Oregon State University, Corvallis 97331

      Published Online:https://doi.org/10.1094/PDIS-04-15-0453-RE

      Abstract

      Infested container nursery beds are an important source of soilborne Phytophthora spp. for initiating disease through movement with surface water or splashing onto foliage. We investigated the effects of soil solarization, alone or with subsequent amendment with a Trichoderma asperellum biocontrol agent, on the survival of Phytophthora spp. inoculum. In field trials conducted with Phytophthora ramorum in San Rafael, CA and with P. pini in Corvallis, OR, infested rhododendron leaf inoculum was buried at 5, 15, and 30 cm below the soil surface. Solarization for 2 or 4 weeks during summer 2012 eliminated recovery of Phytophthora spp. buried at all depths in California trial 1, at 5 and 15 cm in California trial 2, but only at 5 cm in Oregon. There was no significant reduction of Phytophthora spp. recovery after T. asperellum application. Although the population densities of the introduced T. asperellum at the 5-cm depth were often two- to fourfold higher in solarized compared with nonsolarized plots, they were not significantly different (P = 0.052). Soil solarization appears to be a promising technique for disinfesting the upper layer of soil in container nurseries under certain conditions.

      The quarantine pathogen Phytophthora ramorum Werres, De Cock & Man in’t Veld, causal agent of sudden oak death (SOD), has killed over 1 million trees in coastal California and southwest Oregon in the United States. The pathogen is spread by movement of nursery plants (Goss et al. 2009), and there were a total 561 nursery detections in the United States between 2001 and 2014 (United States Department of Agriculture–Animal and Plant Health Inspection Service [USDA-APHIS]). Once infested by the pathogen, nursery beds are difficult to disinfest and many individual nurseries are recurrently positive.

      P. ramorum can survive for extended periods in soil (Linderman and Davis 2008; Shishkoff 2007; Tooley et al. 2008; Vercauteren et al. 2012). Vercauteren et al. (2012) showed that P. ramorum could be recovered from infested soil for at least 33 months under field conditions. Therefore, effective remediation of nurseries infested by the pathogen will require thorough disinfestation of potting media and soil. Historically, soil fumigants have been applied to disinfest nursery beds; however, recent restrictions on the use of soil fumigants require the development of alternative methods.

      Soil solarization has been used for managing many soilborne plant pathogens (Gamliel and Katan 2012). Solarization employs solar radiation to heat the soil under a transparent plastic film to achieve temperatures that are detrimental to soilborne pathogens. Previous studies have demonstrated that soil solarization is effective against some Phytophthora spp. (Juarez-Palacios et al. 1991; Porras et al. 2007), and a preliminary study investigated the effects of solarization on P. ramorum (Yakabe and MacDonald 2010). Although P. ramorum chlamydospores were killed by exposure to 30 min at 50°C (Linderman and Davis 2008), 2 days at 40°C, and 4 days at 35°C (Tooley et al. 2008), it was not known whether solarization could achieve temperatures high enough to kill P. ramorum.

      Most solarization studies have been conducted in warmer climate areas, such as California and Israel. The Pacific Northwest has a cooler climate but there are some reports of successful solarization of Phytophthora spp. in field soil (Pinkerton et al. 2000, 2002, 2009). Temperatures attained with solarization at different soil depths are highly influenced by soil physical properties such as soil moisture, soil texture, and bulk density (Gamliel and Katan 2012). Little information is available to guide the use of soil solarization in container nursery beds, where soil is usually very compacted and gravelly. Moreover, advances in the development of horticultural plastic films with anticondensation coatings may increase the potential for solar heating; solarization effectiveness with these newer films has not been reported.

      Trichoderma spp. have been shown to be effective biocontrol agents of soilborne fungi (Benítez et al. 2004; Harman 2006) and plant-parasitic nematodes (Affokpon et al. 2011). They also have been shown to inhibit the growth of Phytophthora spp. and reduce the diseases caused by them (Harman 2000; Kelley and Rodriguez-Kabana 1976; Porras et al. 2007; Smith et al. 1990; Washington et al. 1999). Porras et al. (2007) reported that the combination of solarization and Trichoderma spp. was effective in reducing P. cactorum (Lebert & Cohn) J. Schröt soil populations. Trichoderma asperellum isolate 04-22 was reported to be an effective biocontrol agent against soilborne P. ramorum chlamydospores (Widmer 2014). Because solarization has previously been shown to improve the establishment of microorganisms introduced subsequently (Jayaraj and Radhakrishnan 2008), we sought to determine whether solarization could improve the establishment of T. asperellum and provide biocontrol of P. ramorum.

      The objectives of this research were to evaluate the potential use of soil solarization for eradicating P. ramorum in the surface soil of nursery beds in California and Oregon, and to evaluate its effect on establishment of the T. asperellum biocontrol agent. Because P. ramorum is a quarantine pathogen, field trials with P. ramorum were allowed only in the National Ornamentals Research Site at Dominican University of California (NORS-DUC) facility. In the Oregon field trial, we substituted P. pini Leonian as a surrogate for P. ramorum. P. pini (previously P. citricola Sawada) is also a destructive plant pathogen in container nurseries but has been reported to tolerate higher temperatures than does P. ramorum (Jung and Burgess 2009; Werres et al. 2001).

      Materials and Methods

      Field experiments.

      Three field trials with a similar experimental design were conducted in two sites: at NORS-DUC, San Rafael, CA, and the Oregon State University Botany Farm and Field Lab, Corvallis. Two trials were conducted in California with P. ramorum: trial 1 began 22 July 2012 and trial 2 began 17 August 2012. One trial was carried out with P. pini in Oregon, beginning 24 August 2012.

      The soil at the California site was a sandy “fill” (sand 55.2%, silt 25.2%, clay 19.6%; gravel fraction 27.0%, bulk density 1.85 g/cm3, pH 6.4, 4.7% organic matter). Soil in Oregon was a Camas gravelly sandy loam (sand 57.4%, silt 27.1%, clay 15.5%; gravel fraction; 12.2%, bulk density 1.93 g/cm3, pH 6.5, 2.6% organic matter).

      The experimental design for each trial was a split plot, with solarization as the main plot and biocontrol amendment as the subplot. Each trial consisted of 12 main plots, each 2.5 by 2.5 m, and treatments were arranged in a randomized complete block design with six replications per solarization treatment. Plots in the solarized treatment were each covered with a transparent plastic sheet and plots in the nonsolarized treatment were left uncovered. Subplots for subsequent amendment with a Trichoderma biocontrol treatment were established by dividing each main plot into two subplots. After the initial solarization treatment period, a T. asperellum biocontrol agent was applied to one of the subplots in each main plot (TA) and the other subplot was not amended (non-TA).

      Phytophthora spp. inoculum.

      P. ramorum isolate Pr-1418886 was used in California and P. pini isolate Pc98-517 was used in Oregon to produce infested leaf disk inoculum. P. ramorum zoospores were produced according to established methods (Parke and Lewis 2007) but with dilute (one-third strength) V8 broth agar substituted for V8-CMA. P. pini zoospores were produced according to Ochiai et al. (2011). Rhododendron ‘Catawbiense Boursault’ leaves were collected from plants maintained in a greenhouse at Oregon State University. Leaves were dipped into a zoospore suspension (5 × 104 zoospores/ml) and incubated in a moist chamber at 20°C (P. ramorum) or 24°C (P. pini). After 2 weeks, 6-mm-diameter disks were removed from leaf lesions using a hole punch. The presence of chlamydospores (P. ramorum) or oospores (P. pini) within leaf tissue was confirmed by microscopic observation of cleared leaves prior to use as inoculum. Mesh bags (4 by 4 cm) were constructed from nylon phytoplankton netting (100-μm opening; Aquatic Ecosystems, Apopka, FL) and filled with sieved (<2 mm) soil from each site and 10 infested leaf disks. Eight columns (8 cm in diameter by 40 cm in depth) were prepared for each main plot (Fig. 1). Columns were made from plastic window screen material to allow for drainage and aeration. The columns were filled with field soil, and mesh bags with leaf disk inoculum and sieved soil were inserted at depths corresponding to 5, 15, and 30 cm below the soil surface. Columns were placed in cylindrical holes (12 cm in diameter by 40 cm deep) arranged in a circular pattern 40 cm from the center of each main plot. Soil water content reflectometers (CS655; Campbell Scientific. Logan, UT) were placed at depths of 5, 15, and 30 cm in the center of three main plots in each solarization treatment. Soil temperature, volumetric water content, and bulk electrical conductivity data were recorded every 30 min with a CR1000 datalogger (Campbell Scientific).

      Fig. 1.

      Fig. 1. Arrangement of eight soil-filled columns in each plot. Mesh bags containing leaf disk inoculum were placed in the columns at depths corresponding to 5, 15, and 30 cm below the soil surface. Soil water content reflectometers (not shown) were placed at the center of each plot at depths of 5, 15, and 30 cm for recording soil temperature, volumetric water content, and bulk electrical conductivity.

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      Field sites were irrigated to saturation and allowed to drain overnight, and initial samples were collected the next day. Plots in the solarization treatment were each covered by a 6-mil (0.15-mm-thick) anticondensation polyethylene sheet (Thermax; AT Films, Edmonton, Alberta, Canada). Edges of the plastic sheets were “sealed” and held in place by a 15-cm-wide layer of gravel along the margin. The nonsolarized plots were left uncovered. Solarization was conducted for 6 weeks in the California trials and 4 weeks in the Oregon trial. The mesh bags from the three depths in each main plot were retrieved after 4 and 6 weeks in California trial 1; after 2, 4, and 6 weeks in California trial 2; and after 2 and 4 weeks in Oregon. During the solarization period, samples from the solarized treatment were collected by making small cuts on the plastic sheets, retrieving samples, and sealing the cuts with clear tape. There was a minimal effect on soil temperature and moisture. After solarization treatments were concluded, the plastic sheets were removed, and subplots receiving the Trichoderma biocontrol agent were amended with T. asperellum isolate 04-22 inoculum at 84 g/m2 (1.72 × 109 CFU/g) provided by Dr. Timothy Widmer, United States Department of Agriculture–Agricultural Research Service (USDA-ARS, Ft. Detrick, MD). Inoculum consisted of a dried conidial powder produced by a proprietary process that was added to the plots to achieve a final concentration of 1 × 107 CFU/cm3 of soil (Widmer 2014). Subplots were irrigated by hand to wash Trichoderma inoculum into the soil. Additional mesh bags from the three depths in each subplot were retrieved 2 and 8 weeks after the biocontrol treatment was applied in California trial 1, and after 4 and 6 weeks in both the California trial 2 and the Oregon field trial.

      P. ramorum recovery.

      All samples were sent to Oregon State University and kept cool (4°C) until processing. Leaf disks were removed \from the mesh bags, rinsed in water to remove soil, and plated on Phytophthora-selective medium (PARPH) (Jeffers and Martin 1986) with modified amounts of antibiotics (ampicillin at 200 mg/liter, rifamycin at 10 mg/liter, pentachloronitrobenzene at 66.7 mg/liter, hymexazol at 25 mg/liter, and Delvocid at 20 mg/liter; DSM, Delft, The Netherlands). Plates were examined after 20 days for outgrowth of colonies of P. ramorum or P. pini, respectively. Recovery was quantified as percentage of leaf disks (out of 10) with outgrowth into the medium.

      Quantifying T. asperellum soil populations.

      Soil samples were collected from mesh bags buried at the 5-cm depth for quantification of the T. asperellum population. Soil samples (1 g) were suspended in 25 ml of sterile water agar (0.2%) and placed on a shaker for 20 min. A dilution series was prepared (1:500, 1:5000, and 1:50000) and 1 ml of each suspension was spread onto a Trichoderma-selective medium (Askew and Laing 1993). Colonies were counted after 1 week of incubation at 20°C. T. asperellum colonies were distinguished from other Trichoderma spp. by their characteristic colony morphology and verified by sequencing the internal transcribed spacer (ITS) of representative colonies using primers ITS1F and ITS4 (White et al. 1990). Dry weight of soil samples was determined by oven drying soil subsamples and reweighing.

      Statistical analysis.

      The nonparametric Mann-Whitney U test was used to compare average mean daily soil temperature of solarized and nonsolarized treatments at each soil depth. The test was used because three replications would not be enough to ascertain a normal distribution. All inoculum recovery data were logit transformed. Analysis of variance (ANOVA) was performed to test the fixed effects of solarization and TA application on the weighted average of logit transformed inoculum recovery. ANOVA was also applied to determine whether the T. asperellum population density was significantly different between TA and non-TA treatments, and to determine the effect of solarization treatment on the subsequent establishment of a T. asperellum population in TA plots in each trial. The reduced model was derived by stepwise Akaike Information Criterion comparisons before applying ANOVA. All statistical tests were conducted using SPSS Statistics 19.0 software (SPSS Inc., Chicago) at a P < 0.05 level of significance.

      Results

      Soil temperatures.

      Soil temperatures are summarized for the first 4 weeks in each trial during the solarization period (Table 1). Across all three depths, mean soil temperatures were 8.1 to 9.4, 7.0 to 7.8, and 5.9 to 6.1°C higher in solarized plots than in nonsolarized plots in California trial 1, California trial 2, and the Oregon trial, respectively. Mean soil temperatures in solarized plots in trial 2 in California and the trial in Oregon were 1.9 to 2.6 and 4.8 to 6.1°C lower than those in trial 1 in California across all three depths. The average maximum daily soil temperature at the 5-cm depth in solarized plots achieved 51, 48, and 41°C in trials 1 and 2 in California and the trial in Oregon, respectively. The amplitude of diurnal fluctuation in soil temperature in solarized plots was 18.5 to 24.1°C at the 5-cm depth in contrast to 2.2 to 3.3°C at the 30-cm depth. Also, the amplitude of daily fluctuation was 3.5 to 6.0, 1.6 to 2.9, and 0.6 to 0.9°C greater at depths of 5, 15, and 30 cm, respectively, than in nonsolarized plots. More than 100 h of temperatures greater than 35°C were achieved with 4 weeks of solarization at all three depths in trial 1 in California, at the 5- and 15-cm depths in trial 2 in California, and at the 5-cm depth in the trial in Oregon in solarized plots. Even in nonsolarized plots, 138 and 87 h of temperatures greater than 35°C were achieved with 4 weeks at the 5-cm depth in trial 1 and trial 2 in California, respectively.

      Table 1. Soil temperature summary during the first 4 weeks in solarized and nonsolarized plots in trial 1 (CA 1; 22 July to 19 August 2012) and trial 2 (CA 2; 17 August to 14 September 2012) in San Rafael, CA, and in Corvallis, OR (OR; 24 August to 21 September 2012)

      Recovery of Phytophthora spp. after solarization and TA applications.

      P. ramorum was not recovered from leaf disk inoculum retrieved from depths of 5, 15, and 30 cm in solarized plots in trial 1 or at 5 and 15 cm in trial 2 in California at the first sampling during solarization in each trial (4 weeks in trial 1 and 2 weeks in trial 2; (Fig. 2; Table 2). P. ramorum was recovered from the 30-cm depth in trial 2 in California even after 6 weeks of solarization. P. ramorum was recovered from all depths in nonsolarized plots, including 5 cm in California trial 1, where more than 100 h of temperatures greater than 35°C was achieved (Table 1). P. pini was not recovered from leaf inoculum retrieved from the 5-cm depth in solarized plots in the trial in Oregon (Fig. 2), except for one sample retrieved after 4 weeks of solarization, in which 2 of 10 leaf disks yielded P. pini. The inoculum was always recovered from 15- and 30-cm depths in solarized plots and also from all three depths in nonsolarized plots. TA application did not result in any significant difference in Phytophthora spp. recovery for any depth in any trials (P > 0.05, ANOVA test; Fig. 2; Table 2).

      Fig. 2.

      Fig. 2. Frequency of recovery (%) of Phytophthora from infested leaf disks buried at soil depths of 5 (○ or ●), 15 ( or ■), and 30 (∆ or ▲) cm during the field trials. Open symbols represent nonsolarized treatments and solid symbols represent solarized treatments. Open arrows indicate the beginning of solarization and solid arrows indicate the time of Trichoderma asperellum (TA) application (dashed line = TA and solid line = non-TA) in three field trials (CA 1, CA 2, and OR). Trials CA 1 and CA 2 were conducted with Phytophthora ramorum in San Rafael, CA beginning 22 July 2012 or 17 August 2012, respectively. The OR trial was conducted with P. pini in Corvallis, OR beginning 24 August 2012.

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      Table 2. P values of solarization and Trichoderma asperellum application (TA) effects from analysis of variance (ANOVA) on recovery (%) of Phytophthora ramorum and P. pini from infested leaf disks buried at soil depths of 5, 15, and 30 cm in trials CA 1 and CA 2 in San Rafael, CA, and trial OR in Corvallis, OR

      The enhanced recovery of P. ramorum from soil following 30 days of storage at 4°C was reported previously (Tooley and Carras 2011). Therefore, we sought to determine whether P. ramorum could be recovered after the winter. We sampled in March 2013 for trial 1 in California, 7 months after solarization, and did not see any significant difference in pathogen recovery when compared with recovery the previous October. We also tested the effect of storage on recovery by storing samples under moist conditions at 4°C and did not observe any significant difference in pathogen recovery. We conclude that the lack of recovery indicated that the pathogen was dead and not simply dormant.

      T. asperellum population densities.

      Application of TA to plots resulted in a population density of T. asperellum at the 5-cm depth that was greater than in non-TA plots (Tables 3 and 4; P < 0.001, ANOVA). Inconsistent T. asperellum population densities with relatively large standard errors (Table 3) were observed among replicates. There was no significant T. asperellum establishment observed in non-TA plots (P = 0.811, ANOVA). The effect of solarization on T. asperellum population density was not significant (Table 4; P = 0.052, ANOVA), although population densities were often two- to fourfold greater in solarized plots compared with nonsolarized plots (Table 3). There were a few T. asperellum colonies that grew from non-TA-amended soil samples. Because there was no detectable indigenous T. asperellum in the soil prior to TA amendment based on plating the field soil, we concluded there was some lateral movement of T. asperellum.

      Table 3. Trichoderma asperellum population density (CFU × 104 g−1 soil) at the 5-cm soil depth after Trichoderma asperellum application (TA) following solarization treatments in trials CA 1 and CA 2 in San Rafael, CA and trial OR in Corvallis, OR

      Table 4. Analysis of variance (ANOVA) of Trichoderma asperellum population density (CFU × 104/g soil) in relation to T. asperellum application (TA), trial, and solarization treatmenta

      Discussion

      This study demonstrates that solarization is a potential method for eradicating P. ramorum in the surface soil in both California (depths of 0 to 15 or 0 to 30 cm) and Oregon (depths of 0 to 5 cm). Temperatures achieved in solarization trial 1 in San Rafael, CA were 4 to 5°C lower than in previous studies in Davis, CA (Juarez-Palacios et al. 1991; Pullman et al. 1981). The plot size in our study (6.3 m2) was smaller than the 12 or 36 m2 used in previous studies (Juarez-Palacios et al. 1991 and Pullman et al. 1981, respectively). Larger plots for solarization likely would have achieved higher soil temperatures by minimizing the border effects (Grinstein et al. 1995). Therefore, the smaller plots in our trials may have underestimated the efficacy of larger-scale solarization in commercial nurseries. Davis, CA also has a 4 to 5°C higher mean air temperature than San Rafael, CA in summer (meteorological data from Remote Automatic Weather Stations; http://raws.fam.nwcg.gov/). Trial 2 in California had a lower temperature because it was carried out later in the season than trial 1. Soil temperature in the Oregon trial was within the range of soil temperature measured in a previous study (Pinkerton et al. 2000). Because 3 to 4°C higher soil temperature was observed in mid-August (Pinkerton et al. 2000) as compared with the period of our field trial, a higher soil temperature in Oregon is likely to have been achieved in our study if the solarization had been conducted in July or earlier in August.

      P. pini was not recovered from at least the upper 5-cm depth in Oregon in solarized plots (Fig. 1). Because P. pini is more heat tolerant than P. ramorum (Jung and Burgess 2009; Werres et al. 2001), it is not likely that P. ramorum would have survived where P. pini was not recovered. Because P. ramorum cannot be tested outside of quarantine facilities, P. pini can be a good indicator of solarization efficacy. The low effectiveness of solarization in deeper horizons in Oregon could result from either the lower temperature or the higher thermal tolerance of P. pini compared with P. ramorum. Direct comparison between P. ramorum and P. pini in the same location would help interpret the usefulness of P. pini as a surrogate for P. ramorum in field studies.

      Cumulative hours of solarization in the surface layer of soils appeared to exceed the threshold required to kill the pathogens. The reported length required to kill P. ramorum is 96 h at 35°C or 48 h at 40°C (Tooley et al. 2008). Therefore, it may be possible to shorten the period of solarization, making it more feasible for growers to apply solarization to nurseries. Solarized strawberry or raspberry fields in Washington were also reported to have had more than 96 cumulative hours above 35°C in the surface 0 to 10 cm with 8 weeks of solarization (Pinkerton et al. 2002, 2009). These data indicate the possible use of solarization to eradicate P. ramorum in surface soil in Washington nurseries as well.

      A study in three retail nurseries in Washington in which the soil had previously tested positive for P. ramorum showed that P. ramorum was not limited to the organic layer of nursery soil but was detected in the top 0-to-10-cm soil layer (Dart et al. 2007). Solarization may not be able to eradicate pathogens at deeper horizons in cooler locations. Because the field surface in container nurseries tends to be covered by crushed rock to facilitate drainage, further tests are required to determine the effect of the rock layer on the elevated soil temperature by solarization. P. ramorum was still recovered from the 5-cm depth in nonsolarized plots in trial 1 in California even though the cumulative hours exposed to high soil temperature should have been adequate to kill the pathogen. During the 4-week trial, the inoculum at the 5-cm depth was exposed to 138 h above 35°C and 48 h above 40°C. In contrast, P. ramorum was not recovered at the 30-cm depth in solarized plots, where the inoculum only experienced 130 h above 35°C and 0 h above 40°C. This indicates that the pathogen response to heat is most likely not cumulative and may involve other factors. In nonsolarized plots, surface soil was drier than in solarized plots due to evaporation (data not shown). Dry conditions have been reported to increase the tolerance of pathogens to heat, as observed with Fusarium oxysporum f. sp. radicis-lycopersici (Shlevin et al. 2004), Alternaria porri f. sp. solani (Rotem 1968), and Sclerotium oryzae (Usmani and Ghaffar 1986). Diurnal fluctuations are large at the surface horizons, resulting from greater maximum and lower minimum temperatures, and leading to the intermittent exposure of the pathogen to heat. Continuous exposure to heat had a greater effect than intermittent exposure on S. sclerotiorum in soil (Porter 1991). Consideration of soil moisture and intermittent heat may more precisely predict pathogen responses to solarization.

      Our results did not show a strong effect of solarization on establishment of the T. asperellum biocontrol agent even though there was a moderate increase in T. asperellum population in solarized plots compared with nonsolarized plots. Solarization was expected to enhance establishment of the introduced biocontrol agent by reducing competition by other soil microorganisms (Jayaraj and Radhakrishnan 2008). Our results contrast with the reduced P. ramorum populations in TA-amended plots associated with the increased T. asperellum population over time observed by Widmer et al. (2011). One reason may be that the T. asperellum population density in the field soils was not high enough to be effective. Preliminary studies showed that T. asperellum concentration of 1 × 107 CFU/cm3 soil is necessary to effectively eliminate P. ramorum (T. L. Widmer, unpublished data). Infiltration of biocontrol inoculum into the soil by watering may not be effective in soils with a high bulk density, as with our soil (Van Elsas et al. 1991). Another reason may be the type of inoculum used. In this study, we used infected leaf inoculum instead of “naked” chlamydospores, as used by Widmer et al. (2011). T. asperellum is not known to colonize leaf material; therefore, inoculum of the pathogen inside leaves may have been protected from parasitism. A single surviving Phytophthora chlamydospore is expected to result in colony outgrowth from the leaf disk in our evaluation of survival. Although T. asperellum might have killed a portion of the pathogen population in the leaf disks, our experiment was not designed to detect the decline of pathogen population within each leaf disk. Nevertheless, a potential benefit of using T. asperellum would be to prevent reinfestation of the previously disinfested soil by creating soil suppressiveness; this aspect would require future study.

      This study showed that soil solarization has an excellent potential for disinfesting surface soil in container nurseries. P. pini could be used as a surrogate for P. ramorum to test the efficacy of solarization outside of quarantine facilities. Soil solarization also has the potential to control other pathogens that might be present in the soil. On the other hand, T. asperellum did not appear to reduce recovery of the pathogens in our system. Further studies are necessary to determine the effect of different water potentials and diurnal temperature fluctuation on the response of pathogens to heat and to test how a layer of crushed rock on the soil surface affects solarization efficacy. Additional research is also needed to test solarization on other soilborne Phytophthora spp. common in container nurseries (Parke et al. 2014).

      Acknowledgments

      This work was supported by Farm Bill funds provided by the USDA-APHIS. We thank T. Widmer at the USDA-ARS Foreign Disease and Weed Science Research Unit, Ft. Detrick, MD, for providing inoculum of the biocontrol agent and providing helpful comments on the manuscript; and the staff at NORS-DUC, S. Cluskey at the Oregon State University Botany Farm and Field Lab, and E. Larson for their skilled and dedicated assistance.

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