ABSTRACT
Field experiments were conducted to evaluate the tolerance of strawberry cultivars to terbacil. Treatments (84, 168, and 336 g ha–1) were applied at either dormant stage or 3–4 leaf. Initial crop injury at 1 and 3 weeks after treatment (WAT) ranged between 0% and 43%; all cultivars fully recovered at 6 WAT. Terbacil applied at 168 g ha–1 at dormant stage caused approximately 20% yield reduction in ‘Brunswick’, ‘Darselect’, and ‘Honeoye’, which was not observed with other treatments. A negative impact of the herbicide on yield of these cultivars is an unlikely outcome when sprayed at 3–4 leaf stage.
Introduction
Weed control in matted-row strawberry is one of the most challenging pest control problems for growers who are seeking profitability and longevity in their strawberry production. The low-statured and shallow-rooted nature of strawberry makes it a poor competitor against weeds for light, water, and nutrition (Demchak, Citation2013; Ellis et al., Citation2006). Newly planted strawberries are most sensitive to the presence of weeds in the matted-row system, and only 2 months of weed competition has been shown to cause up to 65% yield loss (Pritts and Kelly, Citation2001).
Compared to mechanical cultivation, mulching, and hand hoeing, herbicides provide a more cost-effective and labor-saving option of weed management and control for the growers. Terbacil is one of the few herbicides registered to control susceptible broadleaf and grass weeds during the strawberry establishment period. It is a soil residual herbicide that can provide control of germinating weeds and germinated weed seedlings for an extended period of time. The duration of weed control is largely dependent on terbacil rate, soil texture, organic matter, and moisture (Rahman, Citation1977). The only commercially available form of terbacil is Sinbar, an 80% wettable granule that is mixed with water. Rainfall or irrigation is required after application for terbacil activation.
Terbacil acts as a photosynthetic inhibitor, and is normally absorbed by crop roots and transported via xylem to its action site in mesophyll chloroplasts (Ashton and Monaco, Citation1991). Terbacil provides the best weed control when applied shortly before or after weed emergence. Strawberry is considered to be moderately tolerant to terbacil (Genez and Monaco, Citation1983a). The tolerance of strawberry has been partially attributed to restricted distribution throughout the plant following root absorption. When treated with 14C-labeled terbacil via roots in solution culture, strawberry plants were found to accumulate the label mainly in the roots, and the label localized in leaf tissue was restricted primarily to the vascular system. These observations were in contrast to cucumber, which is terbacil-sensitive (Genez and Monaco, Citation1983a). Terbacil metabolism also contributes to the tolerance of strawberry. Genez and Monaco (Citation1983b) found that strawberry metabolized terbacil into non-toxic derivatives faster than cucumber. Strawberry growth stage at the time of terbacil application influences sensitivity, with injury much less pronounced when terbacil is applied at the dormant stage rather than at the three-leaf and six-leaf stages (Polter et al., Citation2004).
Even though strawberry moderately tolerates terbacil, individual cultivars may vary significantly in sensitivity. Lindstrom and Swartz (Citation1987) evaluated the sensitivity of renovated and non-renovated plants from 22 strawberry cultivars and observed considerable differences between tolerant and susceptible cultivars. Sensitive cultivars like Guardian showed severe chlorosis and necrosis in both renovated and non-renovated plants, whereas tolerant cultivars, such as Honeoye and Earliglow, exhibited little injury.
A large variation of tolerance to terbacil among strawberry cultivars calls for great caution when applying terbacil. Ohio is one of the leading strawberry-producing states in the north central United States. Available information of injury and yield responses of the most popular cultivars currently planted in Ohio to terbacil will guide strawberry growers when choosing terbacil as a weed control option. Therefore, the objectives of this research were to evaluate injury response of six popular strawberry cultivars currently planted in the state to terbacil and the effect of timing of terbacil treatment on the yield of matted-row strawberry.
Materials and methods
Experiments were established at the Ohio Agricultural Research and Development Center, Wooster, OH (40.73° N, –81.90° W, and elevation 358 m) during the 2005 growing season. The soil type was a silty loam (15% sand, 67% silt, and 15% clay) with a pH of 6.3 and organic matter content of 3.0%. Strawberry cultivars included Brunswick, Cabot, Darselect, Evangeline, Honeoye, and Jewel (dormant transplants were donated by Nourse Farms, South Deerfield, MA, USA). The matted-row cultural system was used, as it is the major planting system in the region. Dormant plants were hand planted on 10 May (Trial 1, all six cultivars were transplanted) and machine transplanted using a one-row water-wheel transplanter (Holland Transplanter, Holland, MI, USA) on 27 May (Trial 2, Cabot was not transplanted to the designated plots due to unavailability). The plot consisted of a double row, 1.52 m × 1.52 m in Trial 1 with plant spacing at 15.2 cm, and 3.05 m × 1.52 m in Trial 2 with plant spacing at 30.5 cm. The split plot arrangement of treatments, with cultivar as the main plot and herbicide rate as the subplot, was arranged into a randomized complete block with three replications. For Trial 1, terbacil (Sinbar E. I. du Pont de Nemours and Company, Wilmington, DE, USA) was applied on the same day when strawberry was still dormant, without any new leaf emergence. Herbicide treatments for Trial 2 were applied on 20 June 2005 when the crop was at the 3 to 4 leaf stage. Treatments included three terbacil rates: 84, 168, and 336 g ha–1. Applications were made using a CO2-pressurized sprayer, calibrated to deliver 234 L ha−1 at 241 kPa through 8002VS flat-fan spray nozzles (TeeJet Technologies, Wheaton, IL, USA). Air temperature at the time of application was 17.8 °C and 19.8 °C for Trials 1 and 2, respectively, and wind speed at the time of application for both experiments was below 6 km h–1. Untreated weed-free controls were included for comparison. During the experimental period, weeds that tolerated herbicide application were controlled by cultivation and hand-weeding to eliminate the impact of weed competition on strawberry growth.
Crop injury symptoms, including chlorosis and necrosis, were assessed visually using the 0 to 100 scale, in which 0% indicated no crop injury, and 100% indicated death of the crop. Data were collected 1, 3, and 6 weeks after treatment (WAT) on 17 May, 31 May, and 21 June 2005 for Trial 1, and for Trial 2 on 27 June, 11 July, and 1 Aug. 2005. The crop was harvested eight times between 5 June and 30 June 2006 from the central 1.52 m in each plot. Total yield per plot was determined by summing the yield of each harvest.
Data from all experiments were subjected to ANOVA using PROC GLM (α = 0.05) in SAS 9.2 (SAS Institute Inc., Cary, NC, USA). Means for cultivar and rate were separated with the use of LSD test. Untreated weed-free control data were included in the ANOVA for yield but not for crop injury.
Results and discussion
Visual assessments of strawberry injury response to terbacil in Trial 1 showed that there was a large variation among cultivars, as indicated by chlorosis and necrosis (). At 1 WAT, a wide range of chlorosis symptoms was observed when terbacil was applied at 168 g ha–1, which is the recommended rate for post-transplanting application. Chlorosis was greatest in ‘Evangeline’ at 18%, followed by ‘Honeoye’ and ‘Jewel’ at 12%. Chlorosis symptoms were not detected in ‘Brunswick’, ‘Cabot’, and ‘Darselect’. More necrosis was observed than chlorosis at the time of the first assessment. At the recommended rate, response was greatest in ‘Jewel’ at 33%, followed by ‘Evangeline’ at 25% and ‘Honeoye’ at 22%. Necrosis did not always correspond to the severity of chlorosis. ‘Jewel’ showed the highest necrotic injury, even though it only had moderate chlorotic symptoms. However, both necrosis and chlorosis were remarkably greater in ‘Evangeline’, ‘Honeoye’, and ‘Jewel’ than in ‘Brunswick’, ‘Cabot’, and ‘Darselect’. Cultivars generally showed more response to 168 and 336 g ha–1 terbacil. The overall effect (chlorosis and necrosis) of 336 g ha–1 terbacil on six cultivars was significantly greater than the effect at 84 g ha–1 1WAT (data not reported). In spite of the general trend, each cultivar varied in response as rate increased. Necrosis of ‘Honeoye’ dramatically increased from 5% at the lowest rate to 27% at the highest rate, while necrosis of ‘Brunswick’, ‘Cabot’, and ‘Darselect’ remained consistently low (). Regardless of initial chlorosis and necrosis, symptoms rapidly decreased as new leaves emerged, except for ‘Cabot’, which exhibited deferred chlorosis symptom at 3WAT. At 6WAT, symptoms of herbicide injury were almost non-detectable. Yield varied among strawberry cultivars, with the highest yield observed in Honeoye and the lowest in Cabot (). Significant yield reductions of 20%, 23%, and 18%, respectively, were detected in ‘Brunswick’, ‘Darselect’, and ‘Honeoye’ when terbacil was applied at 168 g ha–1, compared to the untreated control. However, reduced yields were not observed in any other treatments with either 84 g ha–1 or 336 g ha–1 terbacil ().
Table 1. The injury response of six new-planted strawberry cultivars to different rates of terbacil applied at the dormant stage.
Figure 1. Yield of six strawberry cultivars treated with different rates of terbacil (0, 84, 168, and 336 g ha–1) applied at the dormant stage. Significant effects on yield within each cultivar are indicated by different letters, according to the LSD (0.05) test. Letters are not denoted if there is no significant difference of injury among rates in each cultivar.
Visual assessments of strawberry injury response to terbacil in Trial 2 indicated that when herbicide treatments were applied at the 3–4 leaf stage, crop injury symptoms persisted to 3WAT (), longer than that when herbicides were applied at the dormant stage. At 1WAT, the highest chlorotic injury was observed in ‘Honeoye’ at 33% when terbacil was applied at 168 g ha–1. ‘Brunswick’, ‘Darselect’, and ‘Evangeline’ showed approximately 20% chlorosis. Chlorosis was not observed in ‘Jewel’ with 168 g ha–1 terbacil. Chlorotic injury increased with the rate of terbacil in general. Terbacil at 336 g ha–1 doubled the chlorotic injury in ‘Brunswick’, ‘Darselect’, and ‘Evangeline’ compared to the injury caused by 84 g ha–1 terbacil. Necrosis was noted among most cultivars at 1WAT. ‘Brunswick’, ‘Honeoye’, and ‘Jewel’ displayed approximately 30% necrotic injury at 168 g ha–1 terbacil. A trend of increased necrosis with increased herbicide rate was observed. In contrast to Trial 1, in which ‘Brunswick’ showed only very slight injury symptoms, ‘Brunswick’ in Trial 2 was much more sensitive to terbacil treatment, showing 40% chlorosis and 40% necrosis symptoms at the highest terbacil rate. Symptoms of injury declined across all of the cultivars over time, but were still obvious at 3WAT. One exception was ‘Jewel’, in which chlorosis increased from 1WAT to 3WAT. All cultivars had almost fully recovered from terbacil injury by 6WAT. Regardless of the initial injury, yield response the following year was not influenced by terbacil (). Slight but not significant yield increase was observed in ‘Brunswick’, ‘Darselect’, and ‘Jewel’ at 336 g ha–1 terbacil.
Table 2. The injury response of five strawberry cultivars to different rates of terbacil applied at the 3–4 leaf stage.
Figure 2. Yield of five strawberry cultivars treated with different rates of terbacil (0, 84, 168, and 336 g ha–1) applied at the 3–4 leaf stage. Significant effects on yield within each cultivar are indicated by different letters, according to the LSD (0.05) test. Letters are not denoted if there is no significant difference of injury among rates in each cultivar.
Chlorosis is a condition where leaf cells produce insufficient chlorophyll. It progresses into necrosis if the symptoms aggravate. In our trials, we observed more necrosis than chlorosis in most treatments at the time of the first assessment, for the symptoms progressed so fast that we missed the opportunity to observe them. At the time of the second assessment, plants had quickly recovered from initial injury. This may have resulted from self-healing of herbicide injury, but can also be partially attributed to the growth of new healthy leaves.
The Sinbar label cautions that application after new foliage development from strawberry transplants may result in severe injury. In our trials, strawberry was more tolerant to terbacil applied before leaf growth began, than terbacil applied at the 3–4 growth stage when new leaves were present. This result is also in agreement with an experiment (Polter et al., Citation2004) in which terbacil injury to greenhouse-grown strawberry was greatest in plants treated at the three-leaf stage compared to plants treated before leaf emergence or at the six-leaf stage. The different sensitivity to terbacil at two growth stages may be partially attributed to the disparity of herbicide absorption site. Though terbacil is a soil residual herbicide that is normally absorbed by root (Ashton and Monaco, Citation1991), it is also capable of penetrating foliar tissue (Barrentine and Warren, Citation1970). As the tolerance of strawberry to root-absorbed terbacil is partially resulted from restricted distribution within root and vascular system (Genez and Monaco, Citation1983a), foliar-absorbed terbacil is more likely to circumvent the restriction and move directly to mesophyll chloroplasts, where it exerts effect so as to cause more injury. This is supported by another experiment conducted by Polter et al. (Citation2004) where crop injury resulted from foliar-applied terbacil on strawberry was significantly greater than that caused by soil-applied terbacil. However, our result is in contrast with a field study in which injury was more obvious when terbacil was applied prior to new growth (Polter et al., Citation2005). The inconsistency may partially be attributed to the time span between transplanting and terbacil application. In our study and the greenhouse study of Polter et al. (Citation2004), herbicide was applied immediately after transplanting on the same day, while in the field study plants were treated 4 days after transplanting when new leaves had likely started to emerge.
The different sensitivity to terbacil in the two experiments might also be a result of precipitation difference during the first few days following herbicide application. Strawberry in Trial 1 received 4.2 cm rainfall during the first 5 days following treatment, which is much higher than the 0.2 cm rainfall received by the plants in Trial 2. Rainfall, like the effect of irrigation (Polter et al., Citation2005), removed terbacil from the foliage and reduced its phytotoxic damage to strawberry. Intense rainfall might have also leached part of terbacil in soil out of root zone in Trial 1 during the first few days following treatment, which did not happen in Trial 2. Terbacil is a soil residual herbicide whose behavior is largely determined by a complex of environmental factors. The study conducted by Rahman (Citation1977), which aimed to characterize persistence of terbacil in soil under different climatic conditions, tested 10 soil types over New Zealand, and found that organic matter, clay content, and rainfall level were able to explain 57% of the variance in the residual activity of terbacil. Rainfall also affects terbacil movement and distribution in soil (Milanova and Grigorov Citation1996; Mora et al., Citation1997). Compared to diuron, dichlobenil, and simazine, terbacil had the highest mobility. Further, 40 cm of irrigation was found to leach more than half of the applied terbacil from a 30-cm sandy loam soil (Hogue et al., Citation1981).
Terbacil at 168 g ha–1 caused a significant yield reduction in ‘Brunswick’, ‘Darselect’, and ‘Honeoye’ in Trial 1. This was not observed at 336 g ha–1 terbacil. The contradictory results that significant yield reduction was only observed at 168 g ha–1 but not at 336 g ha–1 may be attributed to weed competition. Despite the effort to keep plots free of weeds, it is possible that the highest rate failed to reduce yield because it provided a more consistent control of weed than did the lower rate, compared with hand-weeding. Except for ‘Honeoye’, ‘Brunswick’ and ‘Darselect’ both displayed limited chlorosis and necrosis during the application year with a terbacil rate at 168 g ha–1, indicating that yield reduction is not necessarily correlated with initial herbicide injury. In Trial 2 where terbacil was applied at the 3–4 leaf stage, yield reduction was not observed even though the initial injury was more severe than that of Trial 1. Brunswick, Darselect, and Jewel exhibited nearly 40% necrotic symptoms 1WAT at 336 g ha–1; however, all of these cultivars yielded slightly more than the control at the highest rate.
Conclusion
Our results demonstrated that the response of strawberry cultivars to terbacil was dependent on time and rate of herbicide application. Cultivars initially varied in response to terbacil, but they all fully recovered from terbacil injury within the growing season. Results indicated that a negative impact of terbacil on yield of these tested cultivars is an unlikely outcome for strawberry at the 3–4 leaf stage. However, cautions must be taken in ‘Brunswick’, ‘Darselect’, and ‘Honeoye’ before they develop new leaves after transplanting.
Acknowledgments
The authors wish to thank Nourse Farms, South Deerfield, MA for their donation of strawberry plants. The authors also thank former research associate Tim Koch.
Funding
Funding was provided by the OARDC IPM competitive grants program.
Additional information
Funding
Literature cited
- Ashton, F.M., and T.J. Monaco. 1991. Weed science: Principles and practices, 3rd ed. Wiley, New York.
- Barrentine, J.L., and G.F. Warren. 1970. Isoparaffinic oil as a carrier for chlorpropham and terbacil. Weed Sci. 18:365–372.
- Demchak, K. 2013. The mid-Atlantic berry guide for commercial growers, 2013–2014. Pennsylvania State Univ., College Agr. Sci., Coop. Ext. Publ. AGRS-97.
- Ellis, M.A., R.C. Funt, S. Wright, K. Demchak, E. Wahle, D. Doohan, C. Welty, R.N. Williams, and M. Brown. 2006. Midwest strawberry production guide. Ohio State Univ. Bul. 926.
- Genez, A.L., and T.J. Monaco. 1983a. Uptake and translocation of terbacil in strawberry (Fragaria×ananassa) and goldenrod (Solidago fistulosa). Weed Sci. 31:56–62.
- Genez, A.L., and T.J. Monaco. 1983b. Metabolism of Terbacil in strawberry (Fragaria×ananassa) and goldenrod (Solidago fistulosa). Weed Sci. 31:221–225.
- Hogue, E.J., A. Gaunce, and S. Khan. 1981. Leaching of four orchard herbicides in soil columns. Can. J. Soil Sci. 61:401–407.
- Lindstrom, J.T., and H.J. Swartz. 1987. Strawberry genotype responses to terbacil. Adv. Strawberry Prod. 6:44–46.
- Milanova, S., and P. Grigorov. 1996. Movement and persistence of imazaquin, oxyfluorfen, flurochloridone and terbacil in soil. Weed Res. 36:31–36.
- Mora, A., M.C. Hermosin, and J. Cornejo. 1997. Mobility of terbacil as influenced by soil characteristics. Intl. J. Environ. Anal. Chem. 66:149–161.
- Polter, S.B., D. Doohan, and J.C. Scheerens. 2004. Tolerance of greenhouse-grown strawberries to terbacil as influenced by cultivar, plant growth stage, application rate, application site and simulated postapplication irrigation. HortTechnology 14:223–229.
- Polter, S.B., D. Doohan, and J.C. Scheerens. 2005. The effect of irrigation on terbacil tolerance in field-grown strawberry. HortTechnology 15:560–564.
- Pritts, M.P., and M.J. Kelly. 2001. Early season weed competition reduces yield of newly planted matted row strawberries. HortScience 36:729–731.
- Rahman, A. 1977. Persistence of terbacil and trifluralin under different soil and climatic conditions. Weed Res. 17:145–152.