Objective 1

Two trials were conducted to assess time of planting and cover crops on tree growth. Cover crops were established in October 2018 and maple trees were transplanted into the field in fall 2018 and spring 2019. Baseline levels of soil moisture have been recorded since cover crop was established and the initial height and diameter of the trees has been measured. Cover crop densities were evaluated in March 2019 and will be repeated again in April 2019. 

Cover crop and weedy middles. 

Data from insect populations, tree growth and soil health were collected from April-July 2019. Insect samples were collected using yellow sticky cards held just above the cover crop and pitfall traps arranged at the center of each plot. Samples will be processed to the lowest classification possible. Soil samples were collected and bioassayed for pathogenicity. Preliminary results suggest that cover crop and weedy plots have different insect communities and resistance to soilborne pathogens. Also, spring transplanted trees grew more than fall transplanted trees in the first year. We hypothesize that this is due to earlier bud break of spring transplanted trees due to the deeper dormancy of fall planted trees compared to trees which were held under plastic over winter.  

In fall 2019, a second trial of the same experiment was initiated. Fall trees were transplanted in November 2019 and spring trees were planted in March 2020.

Plant Growth Measurement

In Trial 1, there was a significant difference in trunk diameter growth, with Cover Crop Spring Transplant adding an average of 17.3 ± 0.6 mm (average ± SE) after 1-yr post- transplant and 14.8 ± 0.5 mm (average ± SE) for Cover Crop Fall Transplant. Additionally, the Weed Spring Transplanted trees were larger with an average trunk diameter of 16.3 ± 0.5 mm and 15.9 ± 0.5 mm (average ± SE) for the Weed Fall Transplanted trees. The same trend was observed for plant height with taller trees occurring in the Cover Crop Spring Transplanted trees (average of 95.8 ± 3.4 cm) Cover Crop Transplanted trees (average 72.3 ± 2.4 cm). Trees in the Weed Spring Transplant plots were also taller than trees in the Fall Transplant plots; 83.2 ± 4.3 cm and 68.3 ± 2.2 cm, respectively (Table 1).

Table 1. Final trunk diameter and height for Trial 1, in 2019 after one year season grown in crimson clover or weedy plots.

Treatment means within columns with different letters are statistically different by General Linear Model (GLM) with means separated by LSmeans adjusted Tukey analysis (P ˂ 0.05). Treatments: 1) Cover Crop Spring Transplant, 2) Weed Spring Transplant, 3) Weed Fall Transplant and 4) Cover Crop Fall Transplant. Cover crop used was crimson clover.

The results for Trial 2 at the end of year one season were unlike Trial 1. No difference was found in the trunk diameter growth for Cover Crop or Weed Spring or Fall Transplant. Cover Crop Spring and Fall Transplanted trees averaged 139.6 ± 3.0 cm and 136.7 ± 3.0 cm tall, respectively. However, there was a significant difference between Weed Spring Transplanted trees, with Spring Transplants growing taller than Fall Transplants (138.9 ± 2.5 cm and 131.6 ± 3.0 cm respectively) (Table 2). 

Table 2. Final caliper and height for Trial 2, in 2020 after one year season grown in crimson clover or weedy plots.

ᶻ Treatment means within columns with different letters are statistically different by General Linear Model (GLM) with means separated by LSmeans adjusted Tukey analysis (P ˂ 0.05). Treatments: 1) Cover Crop Spring Transplant, 2) Weed Spring Transplant, 3) Weed Fall Transplant and 4) Cover Crop Fall Transplant. Cover crop used was crimson clover.

Cover Crop Coverage and Density

In trial 1, there was no difference detected in the percentage area covered for Cover Crop Spring Transplant and Cover Crop Fall Transplant (F = 2.20, df = 1, P = 0.1979) treatments, but there was a significant difference with cover crop height (F = 9.02, df = 1, P = 0.030). Crimson clover in the Cover Crop Spring Transplanted plots had an average height of 38.3 ± 3.2 cm compared to 29.8 ± 0.8 cm in the Cover Crop Fall Transplanted plots. (Table 3). 

Table 3. Cover crop height and percentage coverage, Trial 1 in April 2019.

ᶻ Treatment means within columns with different letters are statistically different by General Linear Model (GLM) with means separated by LSmeans adjusted Tukey analysis (P ˂ 0.05). Treatments: 1) Cover Crop Spring Transplant, 2) Cover Crop Fall Transplant. Cover crop used was crimson clover.

There was no difference detected in the average dry weight of Cover Crop Fall Transplant and Cover Crop Spring Transplant (43.6 ± 5.7 g and 34.9 ± 3.5 g, respectively), though the trend was for higher biomass in the Fall Transplant plots (Table 4).

Table 4. Dry weight of middle biomass from experimental field plots (mean ± SEM) in trial 1 in 2019.

ᶻ Treatment means within columns with different letters are statistically different by General Linear Model (GLM) with means separated by LSmeans adjusted Tukey analysis (P ˂ 0.05).

In trial 2, there was no difference detected in the percentage area covered for Cover Crop Spring Transplant and Cover Crop Fall Transplant (F = 0.49, df = 1, P = 0.51), as well as for cover crop height (F = 0.35, df = 1, P = 0.578) (Table 5).

Table 5. Average (± SE) height and percentage coverage of crimson clover crop after one year for Trial 2 (2020).

ᶻ Treatment means within columns with different letters are statistically different by General Linear Model (GLM) with means separated by LSmeans adjusted Tukey analysis (P ˂ 0.05). Treatments: 1) Cover Crop Spring Transplant, 2) Cover Crop Fall Transplant. Cover crop used was crimson clover.

Soil Moisture and Temperature Measurements

In trial 1, there was no significant difference detected for soil moisture and soil temperature between Cover Crop and Weed plots. The volumetric water content in the Cover Crop plots averaged 21.7 ± 1.2 % and 20.3 ± 1.2 % for Weeds. For the soil temperature, Cover Crop and Weed plots averaged 18.6 ± 1.1 °C and 18.6 ± 1.1 °C, respectively. Data were collected and analyzed for 7 mo, and February 2019 was the month with the lowest soil temperature for both treatments (average 6.5 ± 0.1 C°). The highest average soil temperature recorded was in June 2019 at 31.2 ± 0.2 C°. January 2019 was the month with the highest average volumetric water content (VWC) of 32.8 ± 2.1 %, and the lowest average (VWC) was May at 7.9 ± 0.9 %.

In trial 2, there was a similar trend with no significant difference detected for soil moisture and soil temperature between Cover crop and Weed plots. The volumetric water content in the Cover Crop and Weed plots averaged 22.94 ± 0.61% and 22.63 ± 1.03%, respectively. For the soil temperature, Cover Crop and Weed plots averaged 13.7± 1.1 °C and 13.7 ± 1.12 °C respectively. The month with the lowest average soil temperature was February at 3.9 ± 0.07 C°. The month with the highest average soil temperature was May at 27.23 ± 0.14 C°. February was the month with the highest average VWC at 24.7 ± 0.8%, and the lowest average VWC was January at 19.23 ± 0.7%.

Pest and Beneficial Insect Assessments

Key families of beneficial insects found in the plots included Staphylinidae, Carabidae Chalcidoidae, Platygastridae, Ichneumonidae, Anthocoridae, and Braconidae. Key families of pest species include Curculionidae, Thripidae, Aphididae, Cercopidae, Cicadellidae, and Cydnidae. There were no significant differences in family diversity indices between Cover Crop and Weeds plots in the pitfall traps, with a diversity index value for the Cover Crop and Weed plots of 5.1 ± 0.7 D and 4.4 ± 1.1 D, respectively (Table 6). Treatments were compared for equitability (evenness) with no statistical difference for average equitability of Cover Crop or Weed plots at 0.3 ± 0.02 ED and 0.3 ± 0.07 ED, respectively. A similar tendency was detected for the family richness with 15.0 ± 2.1 S and 14.0 ± 0.9 S for Cover Crop and Weed plots, respectively. 

Table 6. Results of the diversity indices for the arthropod samples from the pitfall traps collected from May 2019 to July 2019 at the NRC Trial 1 plot.

ᶻTreatment means within columns with different letters are statistically different by General Linear Model with means separated by General Linear Model (GLM) with means separated by LSmeans adjusted Tukey analysis (P˂ 0.05). Cover crop used was crimson clover.   

After analyzing the sticky traps collected during the summer in 2019, there was likewise no significant differences detected for diversity index (Table 7), equitability, and family richness between the Cover Crop and Weed plots.

Table 7. Results of the diversity indices for the arthropod samples from the sticky traps collected from May 2019 to July 2019 at the NRC Trial 1 plot.

ᶻ Treatment means within columns with different letters are statistically different by General Linear Model with means separated by General Linear Model (GLM) with means separated by LSmeans adjusted Tukey analysis (P˂ 0.05). Cover crop used was crimson clover.   

Soilborne Pathogen Resistance

Soils taken from cover cropped and weedy plots in objective 1 were assessed for resistance to soilborne pathogens. Red maple rooted cuttings were grown in inoculated cover cropped soil in 2019 (71.4 g) and non-inoculated cover cropped soil in 2020 (53.2 g) had a significantly higher total plant fresh weight than inoculated and non-inoculated control plants (Table 1). Similarly, plants grown in inoculated cover cropped soil had a significantly greater root fresh weight relative to non-inoculated non-cover cropped soil (50.9 g vs 38.7 g) in 2019, while plants in both inoculated (30.3 g) and non-inoculated cover cropped (33.2 g) soil had a significantly greater root fresh weight relative to inoculated non-cover cropped soil (24.5 g) in 2020. There were no significant differences in total plant and root fresh weight among cover cropped and non-cover cropped treatments.

Table 1. Means (±SE) of height increase (cm), total plant fresh weight (g) and root fresh weight (g) of maple plants grown under four cover crop treatments in two transplant seasons when Phytopythium vexans was inoculated in 2019 and 2020. Bold numbers in P-values denote significant treatment effects at P < 0.05.

^zSeason represents timing of cover crop disturbance caused by red maple plantation (fall transplant or spring transplant); ^yCover crop: soils for bioassay were collected from cover crop plots; non-cover crop: soils for bioassay were collected from non-cover crop plots; inoculated: plants were inoculated with Phytopythium vexans; non-inoculated: plants were not inoculated with pathogen. When the interaction (cover crop*season) is not significant (P > 0.05), treatment means for main effects in a column followed by the same letter are not significantly different. When the interaction is significant (P ≤ 0.05), treatment means for interactive effects (cover crop*season) followed by the same letter are not significant, otherwise the treatment means are presented without letter for informational purpose.

Disease severity in the roots of maple plants grown under inoculated non-cover cropped soils in 2019 and 2020 were 52.3% and 48.0%, which were significantly reduced to 23.5% and 18.5% under inoculated cover cropped soil, respectively (Figure 3a). Among the non-inoculated plants, significantly lower disease severity was observed under cover cropped in 2019 (8.0%), and numerically lower in 2020 (4.8%) as compared to non-cover cropped soils (14.8% in 2019 and 9.8% in 2020). Similarly, the significantly higher pathogen recovery was observed in plants grown under inoculated non-cover cropped soils in 2019 (63.0%) and in 2020 (48.0%), and was significantly lower, at 29.0% and 18.5%, under inoculated cover cropped soil in 2019 and 2020 (Figure 1). No significant differences were detected in pathogen recovery between non-inoculated cover cropped and non-inoculated non-cover cropped treatments. Disease severity and pathogen recovery were significantly lower in soils disturbed by fall transplant than spring transplant in 2019 (Table 3).

Table 2. P-values of two-way analysis of variance (ANOVA) test for the main and interactive effects of transplant season and cover crop on disease severity (%) and pathogen recovery (%) of maple plants inoculated with Phytopythium vexans in the year 2019 and 2020. Bold numbers denote significant treatment effects at P < 0.05.

^zSeason represents timing of cover crop disturbance caused by red maple plantation (fall transplant or spring transplant); ^yCover crop: soils for bioassay were collected from cover crop or without cover crop plots.

Table 3. Means (±SE) of disease severity (%) and pathogen recovery (%) of maple plants grown under two transplant seasons when Phytopythium vexans was inoculated in 2019 and 2020.

^zSeason represents timing of cover crop disturbance caused by red maple plantation (fall transplant or spring transplant). Treatments means followed by the same letter in the column are not significantly different at P < 0.05.

Figure 1. Mean (±SE) of disease severity (%) and pathogen recovery (%) of maple plants grown under four cover crop treatments when Phytopythium vexans was inoculated; CC (cover crop): soils for bioassay were collected from cover crop plots; non-CC (non-cover crop): soils for bioassay were collected from non-cover crop plots. inoculated: plants were inoculated with Phytopythium vexans; non-inoculated: plants were not inoculated with pathogen (control). Treatments followed by the same letter are not significantly different at P < 0.05.

Objective 1 Discussion

A recommendation is needed for nursery growers regarding the optimal timing of red maple transplant into the cover crop (fall or spring) in order to maximize the beneficial impact of the cover crop for plant health.  Cover crop is likely to be disturbed less while planting red maple soon after seeding the cover crop in the fall. Cover crops and soils structure will be disturbed more with spring transplant of the main crop. The disturbance timing is likely to impact soil microbial activity and pathogen pressure. We hypothesized that cover crops would reduce root rot disease severity caused by soilborne pathogens. As predicted, we found that the use of crimson clover as a cover crop significantly reduced the disease severity and pathogen recovery in maple plants from soils in both years. This finding is consistent with the significant reduction in disease severity and pathogen recovery demonstrated recently in two previous cover crop studies in maple plants. What was unexpected was the trend for greater growth of trees in spring transplanted plots. This was consistent across 2 trials and contradicts some other previous work. Two important details to note are 1) the trees used in this study were container trees, not bare root and 2) the spring transplanted trees were overwintered in a plastic covered hoop house. It was noted that the spring transplanted trees broke dormancy faster than the fall transplanted trees. This 'head start' is the likeliest cause of the observed increase in growth. Based on these results, we recommend use of a winter cover crop as it was better at disease suppression than soils with naturally occurring weeds.  And while either planting time appeared acceptable for cover crop stand recovery, spring planting of containerized plants may aid in increasing first year growth.  

Objective 2

Cover crops were established in October 2018 in established tree fields. Baseline levels of soil moisture have been recorded since cover crops were established. Cover crop densities were evaluated in March 2019 and will be repeated again in April 2019. 

Field seeded in October. Flower City, McMinnville, TN.   

Cover Crop Establishment. 

Triticale established better with a Drill system, as expected by its recommended planting depth. Crimson clover, however, established similarly if applied by Broadcast or Drill (Table 1&2). Soil moisture content and temperature was similar across all treatments including weedy controls (Table 3&4).  

Table 1.  Average dry weight from samples collected at Pleasant Cove in cover crop and weed treatments.

Treatments: 1) Triticale Drill, 2) Triticale Broadcast, 3) Crimson Clover Drill, 4) Crimson Clover Broadcast, 5) Weeds as control.

Table 2.  Average dry weight from samples collected at Flower City in cover crop and weed treatments.

ᶻTreatment means that do not share the same letter are significantly different (α=0.05). Treatments; 1) Triticale Drill, 2) Triticale Broadcast, 3) Crimson Clover Drill, 4) Crimson Clover Broadcast, 5) Weeds as control.

Table 3.  Average soil moisture and soil temperature at Pleasant Cove in 2019.

Treatments; 1) Triticale Drill, 2) Triticale Broadcast, 3) Crimson Clover Drill, 4) Crimson Clover Broadcast, 5) Weeds as control.

Table 4.  Average soil moisture and soil temperature at Flower City in 2019. 

Treatments; 1) Triticale Drill, 2) Triticale Broadcast, 3) Crimson Clover Drill, 4) Crimson Clover Broadcast, 5) Weeds as control.

Pest and Beneficial Insect Assessments in Pleasant Cove

Key families of beneficial insects included, Anthocoridae, Carabidae, Charipidae, Cryptophagidae, Diapriidae, Ichneumonidae, and Platygastridae. Key families of pest species include Aphididae, Curculionidae, Cicadellidae, Drosophilidae, Mordellidae, and Thripidae. There was no significant difference in the diversity index between Broadcasted and Drilled Crimson Clover (3.7 ± 0.7 D and 6.0 ± 0.8 D, respectively). A similar trend was found when comparing Broadcasted and Drilled Triticale (6.8 ± 2.0 D and 5.7 ± 1.4 D). Arthropod equitability (evenness) was similar for Broadcasted and Drilled Crimson Clover (0.2 ± 0.05 ED and 0.3 ± 0.06 ED). No difference was detected between Broadcasted and Drilled Triticale (0.3 ± 0.07 ED and 0.3 ± 0.05 ED). A similar trend was repeated when comparing the family richness for Broadcast Crimson Clover (18.0 ± 1.3 S) and Drilled Crimson Clover (21.5 ± 2.3 S). Both triticale plots performed similarly with 23.5 ± 2.3 S for Broadcast Triticale and 19.7 ± 1.1 S for Drilled Triticale (Table 5).

The sticky traps collection had, no significance difference detected for any treatment when comparing the diversity index, evenness, and richness (Table 6).

Pest and Beneficial Insect Assessments in Flower City

Key families of beneficial insects included, Anthocoridae, Charipidae, Carabidae, Cryptophagidae, Diapriidae, Ichneumonidae, and Platygastridae. Key families of pest species include Aphididae, Curculionidae, Cicadellidae, Drosophilidae, Mordellidae, and Thripidae. There was no difference in the diversity index for Broadcast and Drilled Crimson Clover (5.9 ± 0.6 D and 6.3 ± 1.2 D). Triticale plots performed similarly with no difference for diversity index for Broadcast Triticale (6.9 ± 1.2 D) or drilled Triticale (3.7 ± 0.7 D).

No difference was detected for arthropod equitability with 0.3 ± 0.07 ED for Broadcasted Crimson Clover and 0.2 ± 0.05 ED for Drilled Crimson Clover. Triticale plots were similar between, Broadcast Triticale (0.3 ± 0.04 ED) and Drilled (0.2 ± 0.05 ED). Again, no difference was found when comparing for arthropod richness with 24.2 ± 5.5 S Broadcast, and 25.0 ± 0.7 S for Drilled Crimson Clover. With the triticale plots, no difference was detected with 22.7 ± 1.4 S for Broadcast and 23.5 ± 1.7 S for Drilled Triticale (Table 7). Likewise, there was no significant difference after analyzing the sticky traps for any treatment when comparing the diversity index, evenness, and richness (Table 8).

Table 5. Mean arthropod diversity index, equitability, and family richness at the Pleasant Cove plots from the pitfall traps collected from May 2019 to July 2019.

Treatments; 1) Triticale Drill, 2) Triticale Broadcast, 3) Crimson Clover Drill, 4) Crimson Clover Broadcast, 5) Weeds as control.

Table 6. Mean arthropod diversity index, equitability, and family richness at the Pleasant Cove plots from the sticky traps collected from May 2019 to July 2019.

ᶻ Treatments means within columns followed by different letters were significantly different (P < 0.05). With treatments; 1) Triticale Drill, 2) Triticale Broadcast, 3) Crimson Clover Drill, 4) Crimson Clover Broadcast, 5) Weeds as control.

A summer cover crop trial was established in spring 2019. While the cover crop did germinate, all plants died due to extended drought and did not recover. Winter cover crops senesced over the early summer and provided some mulch/weed suppression. While we may investigate summer cover crops in the future, the high likelihood of drought at this time of year suggests that such crops would not be ideal for nursery fields lacking irrigation.  An additional summer cover crop trial was initiated in 2020 which also failed to establish. Based on two consecutive years of crop failure, we would not recommend nursery growers rely on summer cover cropping and instead suggest focusing on winter cover crops which may provide summer weed suppression as winter biomass senesces.   

Table 7. Mean arthropod diversity index, equitability, and family richness at the Flower City plots from the pitfall traps collected from May 2019 to July 2019.

ᶻ Treatments means within columns followed by different letters were significantly different (P < 0.05). With treatments; 1) Triticale Drill, 2) Triticale Broadcast, 3) Crimson Clover Drill, 4) Crimson Clover Broadcast, 5) Weeds as control.

Table 8. Mean arthropod diversity index, equitability, and family richness at the Flower City plots from the sticky traps collected from May 2019 to July 2019.

ᶻ Treatments means within columns followed by different letters were significantly different (P < 0.05). With treatments; 1) Triticale Drill, 2) Triticale Broadcast, 3) Crimson Clover Drill, 4) Crimson Clover Broadcast, 5) Weeds as control.

Soil Microbial Biomass Content

Microbial biomass assays were conducted to determine whether small changes in soil C and N could be detected over a single growing season. There was no difference in microbial carbon at Pleasant Cove between Triticale Drilled treatment (1.4 ± 0.04 MBC), Weeds (0.97 ± 0.25 MBC) and Crimson Clover Drill (1.1 ± 0.09 MBC) (Table 9). A similar trend was detected when comparing the average microbial nitrogen content with no difference for Triticale Drill, Crimson Clover Drill, or Weeds at 0.04 ± 0.01 MBN, 0.03 ± 0.01 MBN, 0.04 ± 0.01 MBN, respectively.

The results obtained at the second location; Flower City indicated that no statistical difference was detected for any of the treatments evaluated either for the microbial biomass carbon or nitrogen content (Table 10).

Table 9. Microbial biomass carbon and nitrogen content at Pleasant Cove in 2019.

ᶻ Treatments means within columns with different letters are statistically different by General Linear Model with means separated by General Linear Model (GLM) with means separated by LSmeans adjusted Tukey analysis (P˂ 0.05). With treatments; 1) Triticale Drill, 2) Crimson Clover Drill, and Weeds as control.

Table 10. Microbial biomass carbon and nitrogen content at Pleasant Cove in 2019.

ᶻ Treatments means within columns with different letters are statistically different by General Linear Model with means separated by General Linear Model (GLM) with means separated by LSmeans adjusted Tukey analysis (P˂ 0.05). With treatments; 1) Triticale Drill, 2) Crimson Clover Drill, and Weeds as control.

Soilborne Pathogen Suppression

Soils from established field plots with and without winter cover crops (crimson clover (Trifolium incarnatum L.) or triticale (× Triticosecale W.)) were sampled following the senescence of the cover crops. Separate bioassays were performed using red maple cuttings on inoculated (with Phytopythium vexans, Phytophthora nicotianae or Rhizoctonia solani) and noninoculated field soils. The results indicated that winter cover crop usage was helpful for inducing soil disease suppressiveness. There was lower disease severity (Fig. 1) and pathogen recovery when the cover crops were used compare to the non‐cover cropped soil. However, there were no differences in maple plant fresh weight and root weight between the treatments. The rhizosphere pseudomonad microbial population was also greater when the cover crops were used. Similarly, the C:N ratio of the soil was improved with the cover crop usage. Thus, in addition to improving soil structure and reducing erosion, cover crops can provide improved management of soilborne diseases.

Figure 1. Disease severity of rooted maple cuttings grown in triticale crimson clover and weedy control soils inoculated with Phytopythium vexans (top), Phytophthora nicotianae (middle) or Rhizoctonia solani (bottom panel).  

Objective 2 Discussion

There are important aspects to consider when planning to incorporate cover crops. In agriculture, the decision is determined by the seed size, seeding rate, date of emergence and planting method. Establishment of cover crop by selecting the right planting method is crucial to assure good germination. In this study, a comparison on the planting method for two cover crops was made including Drill or Broadcast. The results from this study suggest that the establishment of crimson clover as cover crop can be successfully achieved either by using the Drill or the Broadcast method. However, after analyzing the data collected on both sites the Broadcast Crimson Clover showed better results, even though there was no statistical difference detected. The broadcasted evaluated area had higher numbers of plants. But, broadcasting cover crops on the soil surface can minimize seed contact with the soil and can leave the seed expose to desiccation under dry conditions; thus, potentially reducing the cover crop establishment rate. Triticale established better with the Drill treatment method with significant differences at one site (Pleasant Cove), but not the other (Flower City). The Flower City site did have a higher mean triticale plant establishment. It is recommended that winter triticale should be no more than 2.45 cm deep, since deep planting increases the time to emergence and some winter triticale varieties take longer to develop their maximum cold tolerance . Dry matter from both nursery locations indicated that Drill treatment achieved a good amount of biomass for both cover crops species. While the Broadcast method can be less expensive than the use of Drill, it is considered a riskier method for crop establishment, which can also result in inconsistent cover crop yields.

The usage of cover crops can reduce the soil temperature and increase the soil moisture. Soil moisture and soil temperature from both locations were not statistically different, regardless of the planting method or cover crop species. For this experiment, cover crops were terminated in July 2019, but rolling the cover crop and using it as a mulch can reduce soil temperature and soil water evaporation, which can be beneficial for moisture conservation .

An important finding was that Ichnneuminidae and the Ceraphronidae parasitoid families were found only in the Drill Crimson Clover treatment and not in the Weeds or Triticale. Unfortunately, there was not enough specimens to infer that crimson clover will provide a suitable host for parasitoids, but these insects benefit from the nectar provided by flowering plants.

The impact in soil microbial biomass content is determined by the species utilized as a cover crop. In the past, legumes were well known in agriculture for their capability of N fixation and also helping to enrich surface soil organic C. The effect of legumes in altering N levels is more negligible when the legumes decompose and incorporate to soil. The results obtained from our experiment showed no detectable difference in the microbial content between Drill Crimson Clover and Triticale, with similar results for Weeds. Pathogen inoculated soil studies confirmed that both triticale and crimson clover soils suppressed soilborne pathogens which may infect red maple cuttings and that associated beneficial microbial populations were higher in the cover crop soils. 

Overall we recommend the use of winter cover crops in established nurseries. Growers interested in using crimson clover can achieve good establishment with broadcast or drill methods. Triticale and other grains may require the use of a narrow row drill to achieve sufficient stand quality, unless they are willing to seed at a higher rate. 

Objective 3

(a) Cover crop was established in October and red maple trees ('Sun Valley') were transplanted in December. Initial height and diameter of trees as well as soil moisture and temperature readings have been recorded. Germination of cover crop was evaluated in March and will be repeated in April. Treatments will be applied in early to mid April.

Field plot establishment. Moore Farm, Irving College, TN.

Tree growth was measured in fall 2019 and flatheaded borer damage assessed. Growth results suggest that killing cover crops in early May did not reduce competition between the cover crops and trees as these trees grew shorter and were similar in size to trees where the cover crop was allowed to naturally senesce (Figure 1). Interestingly, while the mulch mat product was able to prevent cover crop from growing up around the base of the tree, tree growth was similar to trees with cover crops. Trees with early killed cover crop had more borer attacks than trees in the live cover crop treatment (Figure 2). The early kill treatment had slightly more attacks than trees in the herbicide treatment, suggesting the early kill treatment itself may be attractive to borers in some way. Also, trees protected from weeds using mulch mats had similar amounts of damage cover crop trees, suggesting that something else about the herbicide treatment other than lack of foliage at the base of trees may draw beetles to that treatment. Since attack counts were more evenly distributed across the treatments in this study, statistical power was low, but herbicide was a clear factor in borer presence (herbicide: 27 vs no herbicide: 12).  

Figure 1. Final tree height after 1-yr. Values with different letters are statistically different by General Linear Model (GLM) with means separated by LSmeans adjusted Tukey analysis (P ˂ 0.05) (F = 10.12, df = 3, P = 0.0001). Treatments: 1) Cover Crop, 2) Early Kill Cover Crop, 3) Herbicide and 4) Mulch Mat. Cover crop used were crimson clover and triticale.

Figure 2. Total number of trees damaged by flatheaded appletree borer in treatments after 2-yr.  Tree damage was analyzed by using a General Linear Model fitted to a binomial (Proc Genmod) (Chi-Square = 7.96, df = 3, P = 0.0468).  Treatments: 1) Cover Crop, 2) Early Kill Cover Crop, 3) Herbicide and 4) Mulch Mat. Cover crop used were crimson clover and triticale.

(b) Height, diameter and canopy size was recorded for trees in Fall 2018. Tree rows were maintained bare with pre-emergent herbicides. The final year of growth/borer attacks (year 4) was assessed for this previous study plot. We found that by the final year of production, trees in the cover crop plots remained smaller than herbicided trees (Figure 3), however, the trees had grown more in the previous year than the herbicided trees, suggesting they would recover lost growth in time. The reduction in tree growth was most acute in the first year of establishment, after which trees in cover crop plots grew at similar rates to those with bare rows. Treatments continued to accumulate a few borer attacks each year even after the establishment phase (Figure 4).

Figure 3. Tree height and caliper growth over time after 4-yr production. Treatments: 1) Herbicide + Insecticide (BareIns), 2) Herbicide (Bare), 3) Cover Crop + Insecticide (CoverIns) and 4) Cover Crop + non-insecticide (Cover). Cover crops used were winter wheat, crimson clover, and annual rye grass.

Figure 4. Total number of trees damaged by flatheaded borers in treatments after 4 yr. Tree damage was analyzed by using a General Linear Model fitted to a negative binomial distribution (Proc Genmod) (Chi-Square = 34.40, df = 3, P = 0.0001).  Treatments: 1) Herbicide + Insecticide, 2) Herbicide + Non-Insecticide, 3) Cover Crop + Insecticide and 4) Cover Crop + non-insecticide. Cover crops used were winter wheat, crimson clover, and annual rye grass.

Objective 3 Discussion

Our attempts to balance tree growth with cover crop protection from borers showed that early-kill of cover crop at the end of spring did not provide the same protection from borers as the living cover crop. In fact, the application of herbicide at this time may have increased borer attraction to the early kill trees. Borers may be attracted to trees emitting some stress compounds, and while care was taken to shield tree trunks from direct contact with herbicides, chemical drift could have resulted in exposure. Additionally, borers may have been attracted to general plant odors from stressed or dying foliage, which then drew them to the vicinity of trees in this treatment. Not only did the early kill treatment fail to protect trees from borers, but it also provided no benefit to tree growth. From these studies we conclude that cover crops should be allowed to senesce naturally in fields. We will be conducting future work looking at irrigation in the first year following transplant in order to mitigate growth loss during this period.  Additionally, we found that testing of the cover crop system works better in blocked trials with a group of trees grown with or without cover crops rather than individual treatments replicated across a field. Future tests will evaluate the cover crop method on a larger scale. 

Objective 4

In summer, Phytophthora experimental plots were inoculated with P. nicotianae infested rice grains. Four rice grains were placed 5 cm below the surface soil every 30 cm. Rhizoctonia experimental plots were drench-inoculated with slurry of R. solani (7-day old cultures on PDA were homogenized) at a rate of 100 ml/ 0.1 m². Non-treated, non-inoculated and inoculated plots served as controls. Selected cover crop seeds (Table 1) were seeded into the plots with 1.76 oz of coarse sand for even distribution. At the flowering stage cover crops plant were chopped and incorporated properly into 15 cm deep using ploughs.

Table 1. Selected cover crops seed and organic input rates for the TSUNRC field experiment.

Then the dedicated plots were covered for two and four weeks with 3.0-mil transparent polythene (clear poly polyethylene sheeting, Wrap Bros, Chicago, IL) with no opening at the edges. After two and four weeks polythene covers were removed from specific field plots and mixed it properly. Solarization alone- plots were covered with polythene on 3 July for six weeks and irrigated properly to increase moisture for trapping more temperature (Fig. 1). Non-treated plots served as control. Average temperature of solarized beds, cover crop incorporated solarized beds and non-treated beds were 31.03℃, 29.48℃ and 27.06℃, respectively.

Phytophthora root rot disease severity in boxwood was high experiment with non-treated, inoculated control plants showing 58.94% disease severity. All the tested cover crops- amara mustard, astro arugula mighty mustard, radish and turnips in combination with solarization (2-weeks or 4-weeks), solarization alone (6-weeks) and organic inputs (compost and mustard meal) significantly reduced root rot severity on boxwood roots compared to the non-treated control plants (Table 2). There were no significant differences among tested cover crops in combination with solarization (2-weeks or 4-weeks) compared to the other treatments in reducing Phytophthora root rot disease. There were no significant differences among treatments in plant fresh weight, plant height, and plant width (Table 2).

Table 2. Effect of solarization and organic inputs for the control of Phytophthora root rot disease of boxwood in field conditions

^xDisease severity was based on percentage of roots affected. ^yValues are the means of four replicates; treatments followed by the same letter within a column are not significantly different at P<0.05.

Rhizoctonia root rot disease severity in viburnum was high with non-treated, inoculated control plants showing 68.94% disease severity. All the tested cover crops except astro arugula and radish in combination with solarization (2-weeks or 4-weeks), solarization alone (6-weeks) and compost organic input significantly reduced Rhizoctonia root rot severity on viburnum roots compared to the non-treated, inoculated control plants (Table 3). There were no significant differences among all the tested cover crops in combination with solarization (2-weeks or 4-weeks) and other treatments except astro arugula in combination with 2 weeks solarization in disease severity. There were no significant differences in plant fresh weight, root weight, plant height and plant width among treatments (Table 3).

Table 3. Effect of solarization and organic inputs for the control of Rhizoctonia root rot disease of viburnum plants in field conditions

^yDisease severity was based on percentage of roots affected. ^xValues are the means of four replicates; treatments followed by the same letter within a column are not significantly different at P<0.05.

Root pieces taken from boxwood and viburnum plants were placed on Rhizoctonia and Phytophthora (PARPH-V8) selective media. According to the analysis, highest Phytophthora root rot plant pathogen recovery was observed in compost treated plants and non-treated, inoculated control plants (more than 30%); and there were no differences among treatments in Rhizoctonia root rot pathogen recovery (Table 4).

Table 4. Pathogen recovery percentages from the roots at the end of the field experiment

^xPathogen recovery was based on percentage of roots cultured.  ^yValues are the means of three replicates; treatments followed by the same letter within a column are not significantly different at P<0.05.

In this study we have evaluated five selected cover crops (Amara mustard, astro arugula, turnips, mighty mustard ® pacific gold, and radish) in combination with solarization (Liyanapathiranage 2017), solarization alone and two organic amendments (compost and mustard meal) against soilborne diseases such as Phytophthora and Rhizoctonia root rot diseases. The results of these studies showed that solarization with or without cover crops can significantly reduce root rot diseases.

An integrated management approach should be used to control soilborne pathogens (Rhizoctonia and Phytophthora). Results of both on-farm and field experiments indicate that cover crops (Amara mustard, astro arugula, turnips, mighty mustard ® pacific gold, and radish) can be used in combination with solarization to control soilborne pathogens in nursery productions. Nursery producers could benefit from using solarization alone or in a combination with cover crop to control soilborne diseases.

Each Phytophthora experimental plot consisted of 4 boxwood rooted cuttings spaced 60 cm apart with 2 m between rows. Each Rhizoctonia experimental plot consisted of 5 viburnum rooted cuttings spaced 60 cm apart with 2 m between rows. Boxwood rooted cuttings and viburnum rooted cuttings were planted. Plants were fertilized with 10 g of 18-6-8 Nutricote controlled-release fertilizer. Plants were watered as needed using drip irrigation system. The herbicide (Finale 31.3 ml/L) was applied as spot treatment into the test field.

Objective 4 Discussion

Soil solarization and biofumigant mulches can reduce pathogen inoculum in soils, however, results can vary with soil type, location and target pathogen. To achieve success utilizing these methods for soilborne disease management it is important that the proper crop be used, soil temperatures be raised for a sufficient amount of time to kill the pathogen. In addition to using the appropriate biofumigant crop, the crop stand must be sufficiently healthy and vigorous to provide the added chemical benefit to the solarization process. 

Although amara mustard green and astro arugula are not currently used as commercial biofumigation cover crops, according to our study they show promising results in controlling soilborne pathogens of woody ornamental plants. Further experiments will be conducted to test the ability of these Brassica cover crops to control other major pathogens in woody ornamental nursery production and assess there safety for different woody plants. This research is ongoing.