Tolerance of Young Blackberries to a Selection of Preemergence Herbicides and Rates

Abstract

Limited preemergence herbicides are registered for new blackberry (Rubus subgenus Rubus) plantings. This greenhouse experiment was designed to investigate the effects of a broad selection of preemergence herbicides at multiple rates on blackberry transplants. Screening was initiated Aug 2021 and repeated Mar 2022 in Fayetteville, AR, USA, in a greenhouse at the Milo J. Shult Agricultural Research and Extension Center. ‘Ouachita’ blackberry plugs were transplanted into utility pots that contained field soil and growth media treated with preemergence herbicides. After transplanting, plant height was measured from the substrate to the highest apical meristem of 25 representative plants. Initial blackberry plant heights were 13.5 and 9.2 cm in 2021 and 2022, respectively. Twenty-five treatments were evaluated, consisting of 12 preemergence herbicides at 1× and 2× field rates, and one untreated control. Herbicide treatments included diuron, flumioxazin, halosulfuron, indaziflam, mesotrione, napropamide, oryzalin, pendimethalin, rimsulfuron, S-metolachlor, simazine, and sulfentrazone applied to substrate in containers at their respective 1× or 2× field rates. Data were collected on plant height, blackberry injury ratings, internode length, leaf chlorophyll content, and destructive harvest, including leaf count, leaf dry biomass, and aboveground dry biomass. Specific leaf areas and leaf area-to-dry matter ratios were calculated. When observed, plant injury tended to increase from 7 days after treatment (DAT) until 42 DAT. Greater injury levels were observed in response to treatment with mesotrione at the 1× (78%) and 2× rates (90%), halosulfuron at the 1× rate (58%), halosulfuron at the 2× rate (68%), and diuron at the 2× rate (73%). Injury from diuron was rate dependent, with the 1× rate causing relatively low injury (19%). At both the 1× and 2× rates, flumioxazin, indaziflam, napropamide, S-metolachlor, and pendimethalin treatments exhibited similar responses to the untreated control.

Abstract

Limited preemergence herbicides are registered for new blackberry (Rubus subgenus Rubus) plantings. This greenhouse experiment was designed to investigate the effects of a broad selection of preemergence herbicides at multiple rates on blackberry transplants. Screening was initiated Aug 2021 and repeated Mar 2022 in Fayetteville, AR, USA, in a greenhouse at the Milo J. Shult Agricultural Research and Extension Center. ‘Ouachita’ blackberry plugs were transplanted into utility pots that contained field soil and growth media treated with preemergence herbicides. After transplanting, plant height was measured from the substrate to the highest apical meristem of 25 representative plants. Initial blackberry plant heights were 13.5 and 9.2 cm in 2021 and 2022, respectively. Twenty-five treatments were evaluated, consisting of 12 preemergence herbicides at 1× and 2× field rates, and one untreated control. Herbicide treatments included diuron, flumioxazin, halosulfuron, indaziflam, mesotrione, napropamide, oryzalin, pendimethalin, rimsulfuron, S-metolachlor, simazine, and sulfentrazone applied to substrate in containers at their respective 1× or 2× field rates. Data were collected on plant height, blackberry injury ratings, internode length, leaf chlorophyll content, and destructive harvest, including leaf count, leaf dry biomass, and aboveground dry biomass. Specific leaf areas and leaf area-to-dry matter ratios were calculated. When observed, plant injury tended to increase from 7 days after treatment (DAT) until 42 DAT. Greater injury levels were observed in response to treatment with mesotrione at the 1× (78%) and 2× rates (90%), halosulfuron at the 1× rate (58%), halosulfuron at the 2× rate (68%), and diuron at the 2× rate (73%). Injury from diuron was rate dependent, with the 1× rate causing relatively low injury (19%). At both the 1× and 2× rates, flumioxazin, indaziflam, napropamide, S-metolachlor, and pendimethalin treatments exhibited similar responses to the untreated control.

A national survey of blackberry (Rubus subgenus Rubus) growers identified weed control as a key limitation for production, particularly among southern stakeholders (Worthington 2021). Unfortunately, limited registered herbicides exist for blackberry production, and there is relatively little interest on the part of chemical companies in securing new labels for this specialty crop. Pesticide registrants often see negligible value in registering pesticides for use in specialty crops because of the low return on investment and liability risk (Gast 2008). This disinterest is largely a result of the low acreage and limited market opportunity these crops offer compared with agronomic crops. Herbicide discovery for specialty crops is often the by-product of investigation of chemical use for agronomic crops (Gast 2008). Chemical company consolidations have also resulted in reduced investigation into new agrichemical development (Gast 2008). Registration of additional herbicide chemistries could reduce the risk of herbicide resistance among weed populations by allowing growers to rotate herbicide active ingredients (Mitchem and Czarnota 2023; Norsworthy et al. 2012).

There are relatively few broadcast, postemergence herbicide options for blackberry production; therefore, preemergence herbicides are critical for weed control. Annual applications of registered preemergence herbicides are a standard practice to manage weeds in blackberry production (Mitchem and Czarnota 2023). Standard recommendations to maximize yields and profitability are to maintain a minimum area free of weeds centered on the blackberry crowns, called a weed-free strip width measuring 0.9 m for new plantings when blackberry plants are still small, and 1.2 m for established plantings as canes develop and need more space for production (Basinger et al. 2017; Fernandez et al. 2023; Meyers et al. 2014, 2015). Newly transplanted blackberry plants are less competitive than established blackberry plants, making them more sensitive to weed interference; thus, it is critical to assess which preemergence herbicides are safe for use on young blackberry plants.

A further limitation on preemergence herbicides used in blackberries is the age of the crop and fruit-bearing status. Some herbicides are labeled for use only in established plantings, whereas others are not similarly restricted. To be considered established, a planting must be in the ground at least 365 d. Some herbicide labels are further restricted to plantings that have been established for 2 to 3 years (Gowan Company LLC 2017; Valent U.S.A. LLC 2021). Given the limited herbicide options for blackberry plantings, the objective of our study was to assess the response of newly planted blackberries to a broad selection of preemergence herbicides at two rates in a greenhouse setting.

Materials and methods

A greenhouse trial was initiated 31 Aug 2021 and repeated 3 Mar 2022 at the University of Arkansas System Division of Agriculture Milo J. Shult Agricultural Research and Extension Center in Fayetteville, AR, USA (lat. 36.09962°N, long. 94.17194°W). The trial was arranged as a randomized complete block design, with 12 preemergence herbicides applied at both 1× and 2× the recommended field rates (Table 1). All treatment combinations were replicated five times in each trial run, and an untreated control receiving no herbicide was included in each replication.

Of the selected herbicides for investigation, flumioxazin, mesotrione, napropamide, oryzalin, and sulfentrazone are currently labeled for use in newly planted blackberry plantings (FMC Corporation 2020; Syngenta Crop Protection LLC 2018; United Phosphorus, Inc. 2012, 2014; Valent U.S.A. LLC 2021). Diuron is not labeled for use in blackberries, with the exception of a California-specific registration (Alligare LLC 2019). The formulation of pendimethalin used in this trial, Prowl^® H2O (3.8 lb/gal pendimethalin; BASF Corp., Research Triangle Park, NC, USA), is not labeled for use in blackberries (BASF Corporation 2022); however, another product with the same concentration of pendimethalin, Satellite HydroCap^® (3.8 lb/gal pendimethalin; United Phosphorus, Inc., King of Prussia, PA, USA) is labeled for surface application before transplanting blackberries (UPL NA, Inc. 2022). S-metolachlor is labeled for use in blackberries under a section 24(c) special local need label in Oregon and North Carolina (Syngenta Crop Protection LLC 2021, 2023a). Furthermore, three of the herbicides (indaziflam, halosulfuron, and rimsulfuron) are only registered for use in blackberries established for 1 year or more (Bayer Cropscience 2022; Corteva Agriscience 2021; Gowan Company LLC 2017). Simazine is labeled for use in blackberries but includes two restrictions: do not apply when fruit are present, and reduce the application rate by half if the plants are 6 months of age or younger (Syngenta Crop Protection LLC 2023b). Of these labeled products, napropamide, simazine, and oryzalin are recommended for use in blackberries for all growth stages (Burgos et al. 2014, Mitchem and Czarnota 2023). Flumioxazin, indaziflam, mesotrione, and rimsulfuron are labeled and recommended for use in established plantings (Mitchem and Czarnota 2023).

Each experimental unit was a 1-gal plastic container (top width, 6.25; bottom width, 5.75 inches; height, 6.25 inches) filled with a 1:1 v/v ratio of Roxana silt loam field soil from the University of Arkansas Vegetable Research Station, Kibler, AR, USA (lat. 35.37907°N, long. 94.23318°W) and general-use potting soil (PRO-MIX BX Mycorrhizae; Pro-Mix, Quakertown, PA, USA). The field soil was sourced from an untreated site at the Vegetable Research Station that had not been sprayed with herbicides for more than 20 years. The resulting soil and media mixture had a pH of 6.2, an electrical conductivity of 281 μmho⋅cm^–1, and 4.2% organic matter.

Before treatment, filled containers were watered thoroughly, then allowed to settle and drain to field capacity. Herbicide treatments were applied to prepared containers using a compressed air-powered spray chamber calibrated to deliver 20 gal/acre at 1 mph and was fitted with two tapered-edge flat fan 1100067 nozzle tips (TeeJet^® Technologies, Glendale Heights, IL, USA) placed 50 cm apart. ‘Ouachita’ blackberry plugs (Agri-Starts, Apopka, FL, USA) from 72-cell trays were transplanted 6 cm deep into the 1-gal utility pots within 24 h after herbicide application. At the time of transplanting, blackberries were 13 and 9 cm in height for the 2021 and 2022 runs, respectively. Care was taken to displace as little substrate as possible to allow for a more accurate representation of root uptake of the preemergence herbicides. This procedure simulates root uptake of herbicides of blackberry plugs transplanted into herbicide-treated fields, rather than simulating root uptake of preemergence herbicides by shielded transplants using impermeable grow tubes or low-density polyethlyene (LDPE)-coated milk/juice cartons. Herbicide application to the substrate is particularly important for herbicides with a known potential for phytotoxicity with foliar exposure on young plants (e.g., simazine, sulfentrazone, diuron, flumioxazin).

Potted plants were then placed in the greenhouse on tables and randomized within each replication. Plants were watered twice weekly to field capacity, limiting excessive drainage. Plants were fertilized once each week with 4 fl oz of prepared solution containing 3.0 g⋅L^–1 of a 24N–3.5P–13.3K soluble fertilizer (Sta-Green Plant Food; Parker Fertilizer Co., Inc., Dallas, TX, USA). Visual injury was rated at 7, 14, 21, 28, 35, and 42 d after treatment (DAT). Injury ratings were recorded on a 0% to 100% scale, with 0% indicating no injury and 100% indicating dead plants. Total canopy reduction (i.e., stunting, reduced leaf size) were included in injury ratings in addition to necrotic or chlorotic leaf surfaces. Chlorophyll content, internode length, and plant height were recorded at 14 and 42 DAT. Chlorophyll content was measured on a representative leaflet of the youngest fully expanded leaf of each plant using a soil plant analysis development (SPAD) chlorophyll meter (SPAD-502Plus; Konica Minolta, Ramsey, NE, USA). The SPAD meter (SPAD-502Plus) measures the absorbances of a leaf in the red and near-infrared regions to calculate a numerical SPAD value, proportional to the amount of chlorophyll present in the leaf (Konica Minolta 2009). Internode length was recorded between the first and second nodes proximal from the apical meristem using an electronic digital caliper (CID Bio-Science Inc., Camas, WA, USA). Plant height was measured from the substrate to the highest apical meristem. Destructive harvest was conducted at 42 DAT, and data were collected on leaf area and leaf count. Leaf area was determined using a leaf area scanner (LI-3100C Area Meter; LI-COR^® Biosciences, Lincoln, NE, USA). At the termination of the study, leaves and stems were harvested separately, oven-dried in a laboratory oven (Blue M Electric Company, New Columbia, PA, USA) at 63 °C for 4 d, and weighed using a laboratory balance (BP61S; Sartorius, Goettingen, Germany). Total aboveground biomass (dry weight) was recorded as the sum of leaf biomass and stem biomass from each container. Specific leaf area (SLA) was calculated as the ratio of leaf area to leaf biomass, and the leaf area-to-dry matter ratio (LADMR) was calculated as the leaf area-to-total aboveground biomass ratio.

All data were subjected to analysis of variance (ANOVA) as a randomized complete block design using the GLIMMIX procedure in SAS v. 9.4 (SAS Institute Inc., Cary, NC, USA). Main effects of herbicide, rate, and herbicide-by-rate interaction were treated as fixed effects, whereas block (nested in trial) and trial were treated as random effects. Data were checked for heteroskedasticity by reviewing residual plots in SAS, and means were separated using Tukey’s honestly significant difference (HSD) multiple comparisons adjustment (α = 0.05). Untreated containers served as a reference for injury ratings and were excluded from ANOVA when analyzing the effect of herbicide and rate on blackberry injury. For plant height, internode length, leaf chlorophyll content, and biomass, untreated containers were excluded from the initial means separation analyses because the untreated control applied to both factors (rate and herbicide) and could not be accommodated appropriately in the model. Instead, data from untreated containers were included in a subsequent ANOVA using Dunnett’s procedure to determine whether each treatment combination was significantly different (α= 0.05) from the untreated control.

Results

Injury

Depending on herbicide active ingredients, plant injury symptoms manifested as chlorosis, bleaching, necrosis, leaf deformation, or general stunting (Fig. 1). Because injury ratings were assessed visually relative to an untreated control, stunting in response to herbicides and rates may not have been initially apparent, but later ratings revealed greater injury levels because of the difference in growth relative to the untreated control. Thus, blackberry injury symptoms, when present, tended to worsen with time. At 7, 14, and 21 DAT, no significant interaction of herbicide × rate was observed (P = 0.86, 0.71, and 0.25 for 7, 14, and 21 DAT, respectively), so the main effect of herbicide was pooled across both the 1× and 2× rates (Table 2). Initial blackberry injury was relatively minor across all treatments, with 11 of the 12 herbicide treatments causing ≤ 3% injury at 7 DAT (Table 2). The greatest injury (7%) at 7 DAT was observed in mesotrione-treated blackberries, which exhibited minor bleaching symptoms on both old and newly formed leaves. For many treatments, injury increased over time with the mesotrione, halosulfuron, and diuron (2× rate) treatments, where initial injury at 7 and 14 DAT was mild (between 0% and 16%), but became severe (58%–90%) by 42 DAT (Table 2, Fig. 1). At 28, 35, and 42 DAT, the interaction of herbicide × rate was observed to be significant (P = 0.0011, P < 0.0001, and P < 0.0001 for 28, 35, and 42 DAT, respectively), and herbicides are presented separately by rate (Table 2). Although the least square means of the 1× rates were numerically less than the 2× rates for each herbicide, only the 1× and 2× rates of diuron fell into separate statistical groupings according to Tukey’s HSD (Table 2). Therefore, herbicide active ingredient was a stronger determinant of blackberry injury rather than herbicide rate. Blackberries treated with flumioxazin, indaziflam, napropamide, S-metolachlor, and pendimethalin exhibited injury levels ≤ 6% throughout the trial, even at 2× rates.

Plant height

Plant height was similar among treatments at 14 DAT, but differences in height were observed at 42 DAT (Table 3). The interaction of herbicide × rate was not significant (P = 0.88 and 0.67 for 14 and 42 DAT, respectively), so the main-effect herbicide pooled across both the 1× and 2× rates is presented. Relative to the plant height of the untreated control (46 cm) at 42 DAT, plants treated with mesotrione and halosulfuron were much shorter, measuring 12.4 and 13.6 cm, respectively; however, means separation with Tukey’s HSD showed that these treatments were not significantly shorter than plants treated with diuron, rimsulfuron, and sulfentrazone, which measured 24.9, 22.3, and 25.7 cm, respectively. A Dunnett’s test determined that plants treated with mesotrione, halosulfuron, diuron, rimsulfuron, and sulfentrazone were significantly shorter than the untreated control at 42 DAT (Table 3). Reduced height likely resulted from combinations of leaf bleaching, chlorosis, and necrosis, which could reduce overall plant growth. All other treatments, including oryzalin, simazine, flumioxazin, indaziflam, napropamide, S-metolachlor, and pendimethalin, did not stunt the blackberry plants relative to the untreated control (Table 3).

Internode length

The herbicide × rate interaction effect on internode length was not significant (P = 0.92 and 0.39 for 14 and 42 DAT, respectively); therefore, the herbicide means presented are averaged across rates. At 14 DAT, the internode length did not differ between herbicide treatments, and none of the treatments differed according to Tukey’s multiple comparisons adjustment nor from the untreated control (5.5 cm) according to Dunnett’s procedure (Table 3). At 42 DAT, relative to the untreated control (39.2 cm), shortened internode lengths were most evident in mesotrione-, halosulfuron-, and sulfentrazone-treated plants, which measured 1.7, 6.0, and 17.0 cm, respectively. At 42 DAT, internode lengths of blackberries treated with mesotrione, diuron, rimsulfuron, halosulfuron, and sulfentrazone were reduced and significantly different from the untreated control. The same herbicides that reduced internode length also reduced plant heights (Table 3), indicating that height reduction was a result, at least in part, of shortening of internodes (“stacking”) rather than fewer nodes per plant, although nodes per plant were not counted.

Leaf chlorophyll content (SPAD)

At 14 DAT, the herbicide × rate interaction effect on leaf chlorophyll content as measured by SPAD was not significant (P = 0.27); thus, the herbicide means presented are averaged across both the 1× and 2× rates. At 14 DAT, mesotrione- and halosulfuron-treated blackberries exhibited reduced leaf chlorophyll content as measured by SPAD readings relative to the untreated control (42.0). SPAD readings were the lowest in mesotrione-treated plants (26.6) and were also significantly reduced in halosulfuron-treated plants (35.4) at 14 DAT (Table 3). This is expected because mesotrione is an inhibitor of hydroxyphenyl pyruvate dioxygenase (HPPD), a key enzyme that facilitates chlorophyll synthesis. At 42 DAT, the herbicide × rate effect was significant (P = 0.03) for SPAD readings; therefore, means are presented separately for rate and herbicide. Blackberries treated with mesotrione at the 1× and 2× rates, and with diuron at the 2× rate had reduced SPAD readings (21.9, 26.2, and 26.5, respectively), and each was significantly different from the untreated control (45.3) SPAD reading (Table 3). At 42 DAT, 21 of the 24 treatment combinations (herbicide and rate) did not differ from the untreated control regarding leaf chlorophyll content as measured by SPAD.

Leaf biomass and total aboveground biomass

The interaction effect of herbicide × rate on leaf biomass was not significant (P = 0.72); thus, the main-effect herbicide is presented (Table 4). Leaf biomass was reduced in plants treated with mesotrione (0.4 g), diuron (1.3 g), rimsulfuron (1.3 g), and halosulfuron (0.5 g), each producing significantly less leaf biomass compared with the untreated control (3.9 g) (Table 4). The remaining eight herbicide treatments did not reduce leaf biomass relative to the untreated control. For total aboveground biomass, the interaction effect of herbicide × rate was significant (P = 0.04); therefore, means were analyzed separately by rate for each herbicide (Table 4). At the 1× rate, total aboveground biomass was reduced in blackberry plants treated with mesotrione (1.5 g), halosulfuron (1.8 g), oryzalin (6.4 g), and rimsulfuron (3.1 g) relative to the untreated control (Table 4). At the 2× rate, total aboveground biomass was significantly reduced in blackberry plants treated with 7 of the 12 selected herbicides (Table 4). At the 2× rate, plants treated with diuron, simazine, and sulfentrazone exhibited reductions in total aboveground biomass relative to the untreated control; however, biomass was not significantly different from the untreated controls at the 1× rate of these herbicides (Table 4).

Leaf number and leaf area

Reductions in leaf number indicate a developmental delay whereas reductions in leaf area indicate reduced photosynthetic area of the plant, which could be a result of fewer leaves or smaller leaves. The interaction of herbicide × rate was not significant for leaf number (P = 0.13) and leaf area (P = 0.21); thus, only the main effect of the herbicide is presented in Table 5. At 42 DAT, the untreated plants had 26 leaves and a 1173-cm² leaf area. Mesotrione and halosulfuron treatments reduced leaf number to 10 per plant and reduced leaf area to 59 and 125 cm², respectively. Plants treated with mesotrione and halosulfuron had the lowest leaf number whereas mesotrione-, halosulfuron-, and rimsulfuron-treated plants had the lowest leaf area (Table 5). Mesotrione and halosulfuron were the only herbicides that reduced leaf number significantly relative to the untreated control. Interestingly, plants treated with pendimethalin had significantly more leaves (n = 37) and, consequently, a greater leaf area (1603 cm²) relative to the untreated control (n = 26 and 1173 cm²). Mesotrione- (59 cm²), diuron- (518 cm²), rimsulfuron- (352 cm²), halosulfuron- (125 cm²), and oryzalin-treated (737 cm²) plants exhibited a significantly lower leaf area compared with the untreated control (1173 cm²) (Table 5).

SLA and leaf-to-dry matter ratio

For SLA, the interaction of herbicide × rate was significant (P = 0.03); thus, herbicide means are presented by rate (Table 5). The mesotrione treatment caused the lowest SLA among all herbicides at the 1× rate and was significantly different from the untreated control, with a 59% reduction in SLA. The decrease in SLA indicates that leaves from mesotrione-treated plants require more biomass per unit of leaf area, meaning plants require more leaf biomass to produce an equivalent leaf surface area of those treatments with greater SLA values. However, means separation by Tukey’s HSD showed little separation between SLA means across all herbicides and rates (Table 5).

The LADMR is the ratio of leaf area (measured in square centimeters) to total aboveground biomass (measured in grams), so a reduced LADMR indicates that a plant canopy area is diminished, with fewer or smaller leaves, whereas an increased LADMR indicates more leaves or increased surface area per leaf relative to the total aboveground biomass of the plant. A greater LADMR is indicative of increased resource allocation to leaves rather than stems. For the LADMR, the interaction of herbicide × rate was significant (P < 0.001); thus, herbicide means are presented by rate. The LADMR of untreated blackberries was 136 (Table 5). At the 1× rates, mesotrione- and halosulfuron-treated plants exhibited the lowest LADMRs, which were significantly less than the untreated control (Table 5). Plants treated with mesotrione, diuron, and halosulfuron at the 2× rate had the lowest LADMRs, which were significantly less than the untreated control. In general, the LADMR did not differ between the 1× and 2× rates of the same herbicide. However, the LADMR of plants treated with the 2× rate of diuron was reduced by 64% compared with the untreated control, whereas the 1× rate did not differ from the untreated control (Table 5). The LADMRs of 19 of 24 treatment combinations (herbicide × rate) were similar to the untreated check.

Discussion

These findings are a helpful demonstration of blackberry growth and injury responses for a selection of herbicides at 1× or 2× rates when grown in a controlled environment. Application of the herbicides directly to media before transplanting allowed for characterization of herbicidal activity by root uptake. Each of these herbicides is reported to be absorbed by roots when applied to soil (González-Delgado and Shukla 2020; Shaner 2014). It is important to contextualize these findings from a greenhouse trial with container-grown blackberries as distinct from a field trial as sessing the same herbicide chemistries. Findings from our study are insufficient to make decisions for field-based herbicide applications, given the disparities in environment, rainfall dynamics, and the composition and volume of native soils. Furthermore, each herbicide in this trial has a distinct soil adsorption coefficient for soil and organic matter that may have been exaggerated in this trial, given the high organic matter of potting mix included in the substrate blend. Thus, it is critical to review the literature for previous field trial data or to generate new field data, using these findings only as a guide for which herbicides to investigate.

For the commercially registered herbicides, our observations are consistent with commercial recommendations, especially regarding caution not to apply mesotrione until they have been established for 1 year or more (Mitchem and Czarnota 2023). The lack of injury in response to pendimethalin and S-metolachlor is consistent with field trials of established ‘Marion’ blackberries, for which no injury or yield reduction was observed in response to the 1× and 2× rates of pendimethalin and S-metolachlor (1.41 kg⋅ha^–1 a.i.), similar to the current 1× rate (Peachey 2012). Our findings are consistent with the findings of Meyers et al. (2015), who reported no yield reduction in established ‘Navaho’, ‘Ouachita’, and ‘Arapaho’ blackberries with similar rates of flumioxazin, oryzalin, simazine, and S-metolachlor. Indaziflam has also been shown previously to inflict no injury on established, field-grown ‘Apache’ blackberries (Grey et al. 2021).

Early and rapid growth are critical for weed competition and plant development, particularly in first-year plantings, so the observed reductions in plant heights in response to herbicide treatments would likely make crops less weed competitive. The internode length of blackberries has been shown to vary by cultivar and in response to prohexadione-Ca, a plant growth regulator (Johns 2022). Results from our trial indicate that preemergence herbicides can alter internode length and thus the stature and architecture of the plant; its canopy may also be affected. However, the effect of the majority of selected herbicides in our study on internode length did not differ from the untreated control (Table 3).

Although flumioxazin did not cause a significant reduction of leaf chlorophyll content in our blackberry trial, Saladin et al. (2003) reported a negative impact on photosynthesis and a reduction in foliar chlorophyll and carotenoid content when flumioxazin was applied to young grapes (Vitis vinifera). Flumioxazin-treated plants exhibited slight interveinal necrosis in leaves, which would account for some injury and localized reductions in leaf chlorophyll content, although neither was different from the untreated control (Tables 2 and 3). Leaf chlorophyll content was most affected by herbicide treatments that caused bleaching (mesotrione) and plant death. Because mesotrione is a carotenoid biosynthesis inhibitor (HPPD inhibitor) and disrupts synthesis of chlorophyll, it is not surprising that this treatment is most prominent in the response variable that measures chlorophyll content (Shaner 2014; Syngenta Crop Protection LLC 2018). Considering SPAD measurements were taken on the newest, fully expanded leaves of blackberries, it is possible that diuron, a photosystem II inhibitor that exhibits symptoms primarily in old leaves (Shaner 2014), would not have registered based on this measurement. It is also possible that the potential risk of flumioxazin was underestimated because injury can occur on sensitive species as a result of rain-splashed soil onto foliage (United Phosphorus, Inc. 2014), and soil splashing did not occur in this greenhouse trial.

At the 2× rate, plants treated with diuron, simazine, and sulfentrazone exhibited reductions in total above ground biomass, which indicates a rate-dependent response for total above ground biomass for these particular herbicides. The difference in response between the 1× and 2× rates of diuron, simazine, and sulfentrazone demonstrates the necessity for proper calibration and adherence to product labels to avoid exceeding the 1× field rate and incurring avoidable damage to plants. These findings reinforce the importance of applying a reduced rate of simazine (1/2× rate for established blackberries) on first-year blackberries, which is the current commercial recommendation (Mitchem and Czarnota 2023; Syngenta Crop Protection LLC 2023b).

Reductions in leaf number were observed in response to mesotrione and diuron, two of the more highly injurious herbicides (Tables 2 and 5). Interestingly, pendimethalin-treated plants were observed to have increased leaf number and leaf area relative to the untreated control. This divergence from the expected pattern may be an example of hormesis, during which plants are stimulated to increase growth. Hormesis is a dose–response phenomenon during which otherwise inhibitory substances can stimulate plant growth at low rates and has been observed specifically in blackgrass (Alopecurus myosuroides) treated with pendimethalin (Belz and Duke 2014; Metcalfe et al. 2017).

Of the chemical compounds investigated, the mesotrione, halosulfuron, and diuron (2× rate) treatments incurred the greatest injury levels in blackberry and the greatest reductions in plant growth. Although mesotrione is labeled for use in blackberries, it is appropriately recommended for use in plants established 1 year or more (Mitchem and Czarnota 2023). Although the greenhouse screening indicated indaziflam caused no injury for newly planted blackberries, field data in true soils are necessary to assess crop safety before any serious consideration is given to expanding the indaziflam herbicide label to include newly planted blackberries. The 24(c) labeling for S-metolachlor in several states is supported by these findings and may be worth exploring for other states where alternative herbicides are not available. Flumioxazin, napropamide, oryzalin, pendimethalin, and simazine treatments sustained little or no damage, and corroborated their labeling designations and field use (Burgos et al. 2014; Meyers et al. 2015). Of the herbicides tested, it was observed that indaziflam and diuron may have additional utility beyond current labels or recommendations. Despite the current restrictions and recommendations, young plants exposed to indaziflam and diuron did not exhibit dramatic reductions in plant height or total aboveground biomass, although plants exposed to diuron at the 2× rate exhibited unacceptable levels of injury, which poses a risk in the event of improper calibration or spray pattern overlap.

Some important context for this research is the loss of some blackberry herbicide options either from regulation or discontinued production of the herbicide. A recent discontinuation of oryzalin occurred with little explanation, and the commercial product Surflan^® (4 lb/gal oryzalin; United Phosphorus, Inc) is no longer available for purchase (Neal 2021). A further limitation looms, as a US Environmental Protection Agency (2022) interim report discloses the intended discontinuation of diuron for use in food crops. Thus, the limited herbicide options for blackberry weed control are becoming more limited with the loss of two previously registered herbicides. Successful blackberry production without weed interference will be reliant on fewer chemistries and may necessitate the implementation of integrated weed management strategies. This characterization of blackberry responses to soil-applied herbicides in greenhouse conditions supports current commercial herbicide recommendations, reinforces the importance of proper calibration with potentially injurious chemicals, and offers insight into which herbicide chemistries could be considered for assessment for use in the field.

Mobility, degradation, and uptake of indaziflam under greenhouse conditions. HortScience. 55:1216–1221. https://doi.org/10.21273/HORTSCI14966-20.[/RAW_REF_TEXT]

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