Impact of Soil-Applied Thyme Oil on Strawberry Yield and Disease Abundance

ABSTRACT

Strawberry growers across the United States are looking for sustainable alternatives to conventional pesticides for management of soil-borne or crown-rot causing pathogens, such as plant-extracts like thyme oil. The objectives of this study were to determine the impact of soil-applied thyme oil fungicides on strawberry plant health, yield, sweetness, and foliar disease, and on the assemblage of soil microbial communities in strawberry plasticulture systems. In 2021 and 2022 the interaction of three soil-applied fungicides and three foliar fungicide programs were explored. Thyme oil was not found to improve strawberry plant growth or yield when compared to strawberries in the control. However, plants treated with mefenoxam exhibited an average winter crown weight of 2.11 g, significantly lower than control and thyme oil treatments at 3.14 g and 3.16 g, respectively. This translated into a significant yield loss where mefenoxam treated plants exhibited lowered yields at 416.04 g of fruit per plant compared to 523.57 g and 521.53 g per plant in the control and thyme oil treatments, respectively. A high incidence of foliar and fruit diseases was present; however, no interaction of soil and foliar fungicide treatments were observed for yield or fruit sweetness. Minimal root protection from pathogens was observed for all treatments and Fusarium, Pythium and Phytophthora fungi were regularly observed. Mefenoxam was observed to increase the recovery of oomycetes, which may cause Pythium and Phytophthora. No benefit from thyme oil was observed at our site and its addition as a soil fungicide would likely not benefit most growers.

KEYWORDS:

• Fragaria ananassa Duch
• thyme oil
• growth
• productivity
• Pythium
• Phytophthora

Introduction

Strawberries are produced on over 50,000 acres across the United States with the majority of production in California and Florida (United States Department of Agriculture, National Agricultural Statistics Service USDA-NASS, 2023). Strawberries across the US are primarily grown in an annual plasticulture system which historically rely on fumigation and other pest management tactics to manage several soil-borne pathogens that plague strawberry systems (Fernandez et al., 2001; Miles et al., 2018; Particka and Hancock, 2005; Sydorovych et al., 2006). This is especially true for Southeastern US production systems where higher soil temperature and rainfall often favor disease (Rysin et al., 2015; Safley et al., 2004). With the ban of methyl bromide, strawberry growers in many regions across the United States have looked to several alternatives for the management of soil-borne pathogens and weeds (Bolda et al., 2023; U.S. EPA, 2023).

Conventional and organic production of strawberries is limited by several diseases caused by soil-borne pathogens (Rysin et al., 2015). Phytophthora crown rot, often caused by Phytophthora cactorum, and the black root rot disease complex, often caused by Rhizoctonia spp., Pythium spp., and nematodes, are two of the most common diseases of strawberries in the US (Baggio et al., 2021; Maas, 1998; Marin et al., 2018). Conventional growers often rely on fumigant mixes of chloropicrin and 1,3-dichloropropene, as well as metam sodium, methyl isothiocyanate, to manage weeds and diseases such as the black root rot complex (Bolda et al., 2023; Duniway, 2002; Fennimore et al., 2003; Yuen et al., 1991). Although disinfestation by fumigants is effective on a wide range of pests, several pathogens, such as Phytophthora cactorum and Colletotrichum spp., are introduced into fields after fumigation (Baggio et al., 2021). Growers often look to fungicides applied at or after planting to reduce the impact of crown rot diseases often caused by pathogens present on plants at planting (Jeffers et al., 2004; Mertely et al., 2020; Rebollar-Alviter et al., 2007). Additionally, fumigants such as 1,3-dichloropropene (1,3-D) and chloropicrin require specific equipment or training to apply, are expensive and are conventional pesticides, which often limits their use. Many small-scale growers in the midsouth, including organic producers, do not rely on fumigation and must employ several forms of control to manage pathogen buildup in the soil and manage those brought in on transplants (Rysin et al., 2015).

Strawberry growers in several regions across the United States have looked to more sustainable alternatives to fumigation or conventional pesticides for the management of soil-borne or crown-rot causing pathogens (La Mondia et al., 2002; McWhirt, 2015; Shennan et al., 2018; Marin and Bull, 2002). This has been in part driven by a focus on organic pest management techniques as demand for organic and low-input produce has increased (Zhao et al., 2006). Additionally, research has indicated to many growers that protecting soil health and diversity is paramount when using sustainable pest management techniques, and many conventional fumigants and fungicides are known to be antagonistic to these efforts (Dangi et al., 2015; Doran and Zeiss, 2000). Growers often integrate techniques such as crop rotation, soil disinfestation, cover-cropping, compost, and soil-inoculants to help mediate many pathogens that are often controlled by conventional fungicides and fumigants (La Mondia et al., 2002; McWhirt, 2015; Shennan et al., 2018; Subbarao et al., 2007; Zavatta et al., 2014).

Another method of managing pathogens in low-input production that is becoming more common is the use of essential oils or plant extracts (Bakkali et al., 2008). Not only are plant extracts known to suppress pathogens, but they are also implicated in building soil health and biodiversity. One such plant extract that is currently being used to manage soil-borne or crown rot diseases in U.S. strawberry production is thyme oil. Thyme oil has been shown to exhibit efficacy in managing Botrytis spp., Colletotrichum spp., Fusarium spp., Macrophomina spp., Phytophthora spp., Pythium spp., Rhizoctonia spp., and Verticillium spp (Abd-AllA et al., 2011; Abdel-Monaim et al., 2011; Bi et al., 2012; Hosseni et al., 2020; Khaledi et al., 2015; Lee et al., 2007; McMaster et al., 2013; Morkeliūnė et al., 2021; Tanovic et al., 2004). However, other studies have found that thyme oil was not effective in managing some of these same pathogens (Mahmoud et al., 2013). Additionally, no published work has assessed the impact of thyme oil as a formulated soil-applied fungicide on strawberry growth and development. Several growers currently using thyme oil fungicides have also indicated that soil applications of thyme oil can increase strawberry sweetness and have reduced the impact of foliar diseases such as anthracnose or Botrytis fruit rot, although no research has explored these effects. Finally, no work has been done to analyze the effect of thyme oil applications on microbial soil communities, including soil-pathogens already present in strawberry fields.

The objectives of this study were to determine the impact of thyme oil fungicides on strawberry plant health, yield, sweetness, and foliar diseases such as Botrytis and anthracnose fruit rot, and on the assemblage of soil microbial communities, including known pathogen species, in strawberry plasticulture systems. Several growers across the U.S. currently utilize thyme-oil containing products as soil-applied fungicides, and these objectives hope to shed further light on the value of these plant extract-based fungicides to sustainable and low-input production systems.

Materials and Methods

Strawberry plants of the cultivar “Chandler” were established in a plasticulture production system in Kibler, Arkansas at the University of Arkansas System Division of Agriculture Vegetable Research Station in Roxana silt loam soil on Sept. 30th in 2020 and September 23rd in 2021. Strawberry plugs were established in two offset rows per bed, with 0.3 m of space between each plant in a row and diagonal to plants in the adjacent row. Urea was applied pre-plant at a rate of 60 units of nitrogen per acre and plants received 2.27 kg of nitrogen per week, using either potassium or calcium nitrate (KNO₃ or CANO₃), in the spring following planting once plants broke dormancy. Strawberry plots for the 2021–2022 season were established on the exact same location as 2020–2021, although rows were ripped, disced, and new beds were established after harvest in 2021 prior to planting in fall of 2021. Soil was not previously fumigated and pythium root rot and Phytophthora crown rot had previously been diagnosed on plants at this research farm.

Strawberry plots were arranged in a split-plot layout with a single strawberry row being considered the whole plot and 12-plant plots separated by 1.5 m buffers within each row considered sub plots. Three whole plot factors were used to analyze soil-applied fungicide treatments: no soil-applied fungicides (control), mefenoxam, and thyme oil. Within each whole plot, twelve subplots were established to analyze three foliar fungicide treatments: no foliar fungicides (control), captan, and a standard fungicide program. One row of each soil-applied fungicide was established that contained 144 strawberry plants containing twelve, twelve-plant plots. Four replications of each of the three foliar applied fungicide treatments were established within each whole plot using a randomized complete block design.

Soil-Applied Fungicides

Soil-applied fungicides were applied via injection through the drip irrigation system, which applied each treatment evenly across a row. The water system was flushed prior to injection, non-treated rows were excluded via a shutoff where the row’s line met the irrigation system, and the water system for the treated row was primed prior to injection of soil-fungicide treatments. After uptake of the soil-fungicide through a venturi injector, an equal amount of time was allowed for the system to flush this material through the row prior to any other treatment being applied. An equal amount of time was given for each application of soil fungicide. A single drip tape setup with 304 mm emitter spacing and a flow rate of 0.91 L h⁻¹ was used and is typical for the growing region. For the control treatment, no soil applied fungicides were applied in either 2021 or 2022. For the mefenoxam treatment, “Ridomil Gold SL” (Syngenta Agrichemical Company, Basel, Switzerland) was applied two times, with each application being at a rate of 1.17 L ha⁻¹ (0.227 kg of mefenoxam), which was recommended by the manufacturer. The first application occurred in the days after planting in the fall and the second application occurred after plants broke dormancy and began establishing new roots in the spring (Table 1). In the first season, mefenoxam was applied directly after planting in the fall; however, the first application of mefenoxam was delayed in the second season to allow plants to acclimate after observations of stunting in the first season (Table 1). The thyme oil soil-fungicide treatment was applied using “Promax®” (Bio Huma Netics, Gilbert, AZ) at a rate of 9.35 L ha⁻¹ (3.5% thyme oil). Thyme oil treatments were followed by an application of a stimulant based on recommendations by both growers and the company selling this thyme oil product. Fourteen days following every application of thyme oil, an application of “Zap®” (Bio Huma Netics, Gilbert, AZ) (8-0-0) was applied at a rate of 9.35 L ha⁻¹. One rotation of thyme oil followed by a stimulant was made prior to dormancy, then this product rotation was resumed when plants broke dormancy and continued throughout harvest, which is the recommended use schedule from the manufacturer (Table 1).

Table 1. Soil-applied fungicide application dates in the first (2020–2021) and second (2021–2022) strawberry growing seasons in Kibler, AR.

2020–2021		2021–2022
Date	Product	Date	Product
September 30^th	Mefenoxam, Thyme Oil	October 27^th	Mefenoxam, Thyme Oil
October 14^th	Stimulant	November 11^th	Stimulant
March 9^th	Mefenoxam, Thyme Oil	March 18^th	Mefenoxam, Thyme Oil
March 23^rd	Stimulant	April 6^th	Stimulant
April 6^th	Thyme Oil	April 20^th	Thyme Oil
April 20^th	Stimulant	May 4^th	Stimulant
May 4^th	Thyme Oil
May 18^th	Stimulant

Foliar-Applied Fungicides

Foliar-applied fungicide treatments were initiated each season when 5–10% of plants had flowers that would develop into fruit (Table 2). All applications were made with a CO₂ backpack sprayer at 701 L ha⁻¹. A single pass with a two-nozzle boom was used for all treatments until canopies began to thicken, then two passes with the two-nozzle boom (adding up to 701 L ha⁻¹) was used to ensure proper coverage. Control plots received no foliar applied fungicide during the spring fruiting season. The captan treatment received applications of Captan Gold 80 WDG (Adama Agricultural Solutions, Ashdod, Israel) at a rate of 4.20 kg ha⁻¹ each week through fruiting, for a total of seven applications in 2021 and eight applications in 2022 (Table 2). The standard spray schedule used a combination of sing-site and multi-site fungicides in rotation to maximize control of fruit rot including anthracnose fruit rot and Botrytis fruit rot. Products used included Captan Gold 80 WDG (Adama Agricultural Solutions, Ashdod, Israel) at a rate of 4.20 kg ha⁻¹, Switch 62.5WG (Syngenta Agrichemical Company, Basel, Switzerland) (cyprodinil and fludioxonil) at a rate of 0.98 kg ha⁻¹, Luna® Sensation (Bayer Crop Science, Leverkusen, Germany) (fluopyram and trifloxystrobin) at a rate of 0.56 L ha⁻¹ or 0.31 L ha⁻¹, or Elevate® 50 WDG (UPL Limited, Bandra West, Mumbai, India) (fenhexamid) at a rate of 1.68 kg ha⁻¹. Applications were made on the same date as the captan schedule, with seven applications being made in 2021 and eight in 2022 (Table 2). Weekly applications were attempted, but rain events often increased or decreased spray intervals.

Table 2. Foliar fungicide application dates and products (rate of product/hectare) for the standard fungicide program treatment administered to strawberries in 2021 and 2022 in Kibler, AR.

2021		2022
Date	Fungicide (Product/ha)	Date	Fungicide (Product/ha)
March 22^nd	Luna Sensation (0.56 L)	March 16^th	Captan 80WDG (4.20 kg) + Switch 62.5WG (0.98 kg)
March 29^th	Captan 80WDG (4.20 kg) + Elevate 50WDG (1.68 kg)	March 24th	Captan 80WDG (4.20 kg) + Elevate 50 WDG (1.68 kg)
April 6^th	Switch 62.5WG (0.98 kg)	April 1^st	Switch 62.5WG (0.98 kg)
April 12^th	Captan 80WDG (4.20 kg) + Elevate 50WDG (1.68 kg)	April 15^th	Luna Sensation (0.56 L)
April 21^st	Luna Sensation (0.56 L)	April 21^st	Switch 62.5WG (0.98 kg)
April 27^th	Captan 80WDG (4.20 kg) + Switch 62.5WG (0.98 kg)	April 26^th	Luna Sensation (0.56 L)
May 5^th	Luna Sensation (0.56 L)	May 3^rd	Captan 80WDG (4.20 kg) + Switch 62.5WG(0.98 kg)
		May 11^th	Captan 80WDG (4.20 kg) + Luna Sensation(0.31 L)

Assessment of Plant Health, Yield, and Sweetness

Plant dry weight biomass and crown counts were collected each year in the winter and spring to measure plant growth and development. One representative plant was pulled from each plot at two separate timings in 2020–2021 and 2021–2022: winter (Dec. 19, 2020 and Nov. 29, 2021) and spring (Mar. 29, 2021 and Apr. 15, 2022). Whole plants were removed from the soil using a spade and the roots were cut off 1 cm from the base of the crown. Plants were then placed in separate paper bags and dried at 70°C for 5–7 days. Dried plants were assessed for total plant dry weight (g), total weight of the crowns (g), and total crown number.

Ripe strawberry fruit was harvested one-two times per week from each plot to assess yield. Harvest began when the first ripe fruit could be picked and continued until fruit quality began to deteriorate due to environmental conditions. In total, 10 harvests occurred in 2021 for the first harvest season, starting Apr. 20th and ending on May 21st. In 2022, plots were harvested 11 times, starting on April 14th and ending on May 19th. Berries were separated into marketable and cull fruit at harvest and then weighed to determine marketable and cull yield in each plot. Cumulative yield per plot across all harvest dates were considered for analysis, and data were corrected for the number of plants that were harvested from each plot. Marketable yield (fruit weight) and percent cull per plant (percentage of cull yield from sum or marketable and cull) were used for analysis. Strawberry fruit samples consisting of 10 berries per plot were collected weekly from all treatment combinations and frozen for later analysis of fruit soluble solids. At the end of the season all samples were thawed, fruit was homogenized and juice was analyzed for soluble solids (expressed as percent) using an Atago Pocket Refractometer (Cole-Parmer, Vernon Hills, IL).

Plant Collection and Disease Severity Assessment

Plant samples were collected after harvesting at the end of the season to examine crown and root tissue in the 2021 and 2022 seasons. Two random plants were collected carefully recovering roots and placed in a cooler for transportation to the laboratory. Plant samples were washed under tap water to remove extra soil and dry at room temperature before assessing root and crown symptoms. Plants were split along the crown and roots from top to bottom and the severity of the root and crown was assessed based on a 0 to 5 scale as described in Fang et al. (2011). Briefly, both crown and root plants were scored as 0 = no tissue discolored; 1 = <25% tissue discolored; 2 = ≥25%, <50% tissue discolored; 3 = ≥ 50%, <75% tissue discolored; 4 = ≥ 75% tissue discolored; 5 = all tissue discolored (rotted), plant death.

Pathogen Isolation from Diseased Plants

Plant tissue rated was also processed for isolation of plant pathogens. Crown and root sections were taken from diseased plant material using 15 samples per treatment: Control, mefenoxam, and thyme oil. Two sections of crown and two sections of root tissue were taken from each sampled plant and were surface sterilized for isolation of fungi by submerging the sections for one minute in a 3% bleach solution followed by two washes on sterile distilled water for one minute each. Sterile tissue was placed on a sterile towel paper and sectioned into 3 mm pieces and plated on PDA. For oomycetes, root tissue was washed three times in sterile distilled water and placed on corn meal agar amended with antibiotics: pentachloronitrobenzene (PCNB) (50 mg L⁻¹), ampicillin (250 mg L⁻¹), rifampicin (10 mg L⁻¹), pimaricin (5 mg L⁻¹), and benomyl (10 mg L⁻¹) (Jeffers, 1986). Isolates were classified based on morphotypes and spore morphology to confirm their identity, and the ITS gene was used for classification.

Statistical Analysis

Data for total plant weight, total weight of crowns, and total number of branch crowns were compared independently for winter and spring sampling dates using a linear mixed model with a normal distribution in PROC GLIMMIX in SAS v 9.4. Only soil-fungicide treatments were considered when assessing plant growth and development. Data from the two years were pooled. year and replication nested within Year were considered random variables. Data from 12 plants were assessed each year for each soil treatment and timing. Data for marketable yield per plant, percent cull yield, and soluble solids were compared using a linear mixed model with a normal distribution in PROC GLIMMIX in SAS v 9.4. Data from the two years were pooled. The interaction of soil fungicide and foliar fungicide treatments were first considered, and main effects alone were explored if no significant interaction was observed. Year, replication nested within year, and date nested within replication x year were considered random variables. Denominator degrees of freedom for all analyses were adjusted using a Kenward – Rogers approximation (Kenward and Roger, 1997). A Tukey’s honestly significant difference post hoc analysis was used to separate means for all analyses at α = 0.05. For disease severity assessment for strawberry plants collected, values were plotted by treatment and by year to represent the distribution by specific rating scores. Statistical analysis was conducted using rank-based comparison using non-parametric methods using the Kruskal–Wallis test in the package agricolae in R (Mendiburu de, 2021; R Core Team, 2022).

Results

Soil application of mefenoxam was observed to significantly impact the growth and development of strawberries (Table 3). Strawberry plants treated with soil applications of mefenoxam exhibited winter plant weights lower than control plants and those treated with soil applications of thyme oil and a biostimulant respectively. This reduction in plant mass for mefenoxam treated plants also reduced strawberry crown dry weights, where crowns from strawberry plants for mefenoxam treated, control, and thyme oil treated plants, respectively weighed 2.11 g, 3.14 g and 3.16 g. No significant impact to crown number was observed for any soil fungicide treatment during winter plant measurements (Table 3). Additionally, when considering spring plant weight, spring crown weight, and spring branch crown number, no significant difference was observed when comparing soil fungicide treatments (Table 3).

Table 3. Average number of branch crowns, crown weight, and plant dry weight assessed from plants in the winter in December of 2020 and 2021 and spring in April of 2021 and 2022 in Kibler, Arkansas.

	Winter			Spring
Soil Treatment	Branch Crowns (#)	Crown Weight (g)*	Plant Dry Weight (g)	Branch Crowns (#)	Crown Weight (g)	Plant Dry Weight (g)
Control	2.58 a	3.14 b	12.57 b	5.33 a	8.71 a	34.45 a
Mefenoxam	2.08 a	2.11 a	8.89 a	4.33 a	6.45 a	28.62 a
Thyme Oil	2.50 a	3.16 b	13.48 b	4.79 a	8.70 a	30.91 a
P-value	0.91	<0.01	<0.01	0.63	0.72	0.63

*Values followed with a different letter within a specific measurement were found to be significantly different.

No significant interaction was observed between foliar and soil fungicide treatments when considering marketable yield, percent cull, and soluble solids. Strawberry plants treated with soil applications of mefenoxam were found to produce significantly lower yield per plant (416.04 g) compared to control plants and plants treated with thyme oil (523.57 g and 521.53 g per plant, respectively) (Table 4). No significant difference was observed for marketable yield when comparing control strawberry plants and those with soil treatments of thyme oil. Additionally, no significant difference was observed in the percent yield that was culled or for soluble solids when comparing soil fungicide treatments (Table 4).

Table 4. Average marketable yield per plant, rate of cull yield (% of total yield), and soluble solids (%) for strawberry plants with different soil and foliar fungicide treatments in Kibler, Arkansas in 2021 and 2022.

Soil Fungicide	Marketable Yield per Plant (g)*	Cull Yield (%)*	Soluble Solids (%)
Control	523.57 a	20.79 a	6.24 a
Mefenoxam	416.04 b	29.01 a	6.42 a
Thyme Oil	521.53 a	24.20 a	6.21 a
P-value	<0.01	0.26	0.38
Foliar Fungicide
Control	399.16 b	36.69 a	6.20 a
Captan	535.42 a	15.13 c	6.27 a
Standard Spray Schedule	526.57 a	22.19 b	6.36 a
P-value	<0.01	<0.01	0.65
Soil x Foliar Fungicide P-value	0.57	0.16	0.85

*Values followed with a different letter within a specific yield metric were found to be significantly different.

Foliar fungicides were observed to significantly impact strawberry yields (Table 4). Strawberry plants in the control exhibited significantly lower marketable yields when compared to plants that received either captan or a standard spray schedule (399.16 g, 535.42 g and 526.57 g per plant, respectively). Additionally, control plants were found to have the highest percentage of culled fruit at 36.69% when compared to other foliar fungicide treatments. Although lower than control plants, plants treated with a standard foliar spray schedule were found to have a higher percentage of culled fruit than plants treated with foliar applications of captan, at 22.19% and 15.13% respectively. No significant difference in soluble solids was observed when comparing any foliar fungicide treatments (Table 4).

Strawberry plants collected at the end of the season were assessed to examine if there was root rot or crown rot present in the tissue on the different treatments. Symptoms included the browning and discoloration of vascular tissue in the crown and dark damaged roots with minimal adventitious roots. In general, 2022 had a higher range of severity values in comparison to the 2021 season (Figure 1). Mean ranks for crown severity using year by soil-applied fungicide resulted in significant differences where control treatments for both years and the thyme oil for 2021 had the higher ranks. Likewise, in root rot ratings, merging soil fungicide by year in a variable indicated significant differences mainly broken by year. Higher diseases ratings were observed in 2020 in comparison to treatments in 2021. Overall, there were significant differences in crown rot severity ratings when combining 2021 and 2022 seasons, where the control was the highest followed by mefenoxam and thyme oil (Table 5). In the case of root rot severity ratings, there were no significant differences. When considering root rot severity ratings, control strawberries averaged a 1.47 severity rating, followed by 1.29 for thyme oil and 1.26 for mefenoxam. In comparison, crown severity average ratings were 1.60 for the control, 1.37 for mefenoxam, and 1.19 for thyme oil.

Figure 1. The proportion of plant crowns and roots per rating score assessing the severity of tissue discoloration in the field trial for seasons 2021 and 2022. Scales are based on Fang et al. (2011). Levels ranging from 0 = no tissue discolored to 5 = all tissue discolored (rotted), plant death.

Table 5. Disease severity rank sums for crown and root rot in strawberry plants with different soil and foliar fungicide treatments in Kibler, Arkansas in 2021 and 2022.

Soil Treatment	Crown rot Mean rank	Root rot Mean rank
Control	82.40 a	78.30 a
Mefenoxam	62.26 b	69.07 a
Thyme Oil	69.46 ab	67.03 a
P-value	0.02	0.27

*Values followed with a different letter within a specific yield metric were found to be significantly different.

In terms of isolation, common soilborne pathogens of strawberries were isolated from symptomatic plants. While none of these recovered fungi have been re-inoculated in healthy plants to confirm their pathogenicity, most of the genera/species found were from either crown rot or root rots. The most abundant genus was Fusarium (Table 6), which is a common vascular pathogen, followed by the oomycetes, Pythium and Phytophthora, which were abundant in all treatments, especially in mefenoxam-treated plots. Other pathogens found were Rhizoctonia, Macrophomina and Neopestaliotiopsis.

Table 6. Isolation count of soil-borne pathogens from strawberry crown and roots per soil-fungicide application treatment in Kibler, Arkansas in 2021 and 2022.

Species	Control	Thyme oil	Mefenoxam	Total
Fusarium	21	24	17	62
Oomycetes (Pythium and Phytophthora)	14	14	22	50
Neopestalotiopsis	6	9	9	24
Macrophomina	6	3	6	15
Rhizoctonia	0	0	1	1

Discussion

No impact on strawberry growth and development was observed when applying thyme oil containing fungicides and the accompanying biostimulant product through drip irrigation. Although the stimulant contained several forms of fertilizer, no increase in plant size, crown weight, or crown number was observed when compared to plants that received no soil fungicides across two years. However, in our study mefenoxam was found to negatively impact strawberry plant and crown weight prior to winter dormancy, which ultimately reduced marketable yield. Although mefenoxam is a valuable tool for Phytophthora crown rot management, plant establishment and development from planting until winter dormancy is vital in Southeastern strawberry production for optimized spring production. Any delays in fall crop establishment have been found to negatively impact spring yield (Poling and Monks, 1994). Several strawberry growers in the Southeastern growing region use mefenoxam every year indiscriminately, and further studies should seek to evaluate whether this could be yield limiting in well-drained soils where Phytophthora crown rot may not be a common issue. Other studies have not previously noted the capacity of mefenoxam to inhibit the growth and development of strawberry plug plants (Marin and Peres, 2021). However, the combination of under-sized or stressed plugs at the beginning of the season could have exacerbated phytotoxic effects of mefenoxam on the plants (Foster and Hausbeck, 2010; Singh et al., 2003). It has been observed before that repeated applications could result in phytotoxicity and reduce plant growth, especially at a higher rate (Hao et al., 2019). While our study did use a high rate for mefenoxam (1.17 L ha⁻¹), plugs were smaller than average in 2020 due to delays in production, increasing chances of effects of mefenoxam on plant growth. However, applications were delayed in 2021 to allow for plant growth prior to application of the high rate, and significant stunting was still observed even though plug size was ideal at planting.

Soil applications of thyme oil in conjunction with a biostimulant were not found to augment foliar fungicide success in this study. Although previous studies indicated that thyme oil can impact anthracnose and Botrytis, the two main drivers of fruit rot in strawberry, soil applications of the evaluated product did not augment foliar fungicide treatment efficacy (Abd-AllA et al., 2011; Hosseni et al., 2020; Lee et al., 2007; Morkeliūnė et al., 2021). This is likely due to the large amount of latent infections of anthracnose and Botrytis that are present on plants prior to planting (Oliveira et al., 2013, 2017; Rahman et al., 2015). Although pathogens that cause these diseases are known to survive in sclerotia in the soil, more than two years of growing strawberries in the same place would be necessary to determine if thyme oil could impact soil-buildup of these pathogens (Maas, 1998). Additionally, crop rotation, cover crops, and other forms of cultural control have been found to successfully reduce the impact of pathogen buildup in soils (La Mondia et al., 2002; Subbarao et al., 2007; Zavatta et al., 2014).

Thyme oil applications were also not found to impact strawberry soluble solids in this study. In fact, no impact on fruit soluble solids content was observed across all soil and foliar fungicide treatments. Liu et al. (2023) previously suggested that fungicide use can deteriorate strawberry fruit flavor and sugar content, and may be the reason that consumers feel that strawberry flavor has declined. We did not find any impact on soluble solids when comparing similar fungicide treatments (FRAC Group 7 – “Luna Sensation” in this study). However, strawberry cultivar and growing systems significantly impact fruit ripening and flavor, which differed in our study compared to Liu et al. (2023) (Ménager et al., 2004; Voca et al., 2009).

The response observed from the use of thyme oil in comparison to mefenoxam and control plants for soil applications indicated that neither thyme oil or mefenoxam increases resistance against soilborne pathogens. The control plants exhibited fewer symptoms than those from the other treatments in both crowns and roots. There were no differences in root protection across treatments, which suggests minimal protection from all treatments. Linking isolation data with symptoms observed, it seems that mefenoxam increased the recovery of oomycetes, which includes Pythium and Phytophthora major pathogens of strawberries (Maas, 1998). Resistance among those isolates could also explain issues controlling those pathogens. There are reports of increased growth under sub-lethal doses of mefenoxam in oomycetes (Pradhan et al., 2017), and this is very likely to happen during applications since soil and environmental conditions could affect delivery rates in drip or drench applications. While other species found in this study are also common pathogens of strawberry: Fusarium, Neopestaliotiopsis, Macrophomina and Rhizoctonia, it is necessary to test their pathogenicity. All of these isolates came from symptomatic tissue and there are no striking patterns in terms of abundance for other groups different from the oomycetes. This suggests that thyme oil does not affect the prevalence of soilborne pathogens infecting tissue. However, the question remains on the levels of the soil community within those systems and the role of sustainable practices in a strawberry cropping system.

Thyme oil was not found to negatively impact strawberry growth and development and no impact on pathogen presence or disease on plants was observed when compared to strawberries in the control. Thyme oil appears to be a safe fungicide for strawberry when applied to the soil through drip irrigation. However, mefenoxam was found to significantly stunt strawberries and ultimately lower yields. Although mefenoxam likely would result in yield increases with high Phytophthora crown rot pressure, growers should likely avoid indiscriminate use in well-drained soils. Growers who take advantage of crop rotation and cover crops likely won’t observe a lasting buildup of soil pathogens that cause fruit rots. Plants are likely to be obtained with latent infections of pathogens that cause anthracnose fruit and crown rot, Botrytis fruit rot, and Phytophthora crown rot, minimizing the effectiveness of several soil-applied pesticides. Soil applications of thyme oil were not found to impact foliar fungicide treatments. Our study indicates that future work with thyme oil should focus on its effectiveness in suppressing soil-borne pathogens in soil with known high-levels of inoculum. Our data does not suggest that soil-applications of thyme oil is a valuable integrated pest management strategy for low-input or organic growers, but ultimately more work is necessary to determine if there is any value in high-risk situations. Currently, we would not recommend growers use thyme oil as a soil fungicide, although future studies could provide further insight.

Acknowledgments

We would like to thank University of Arkansas System Division of Agriculture Vegetable Research Station staff, Steve Eaton, Alden Hotz, Lesley Carr, Lizzy Herrera, Qiurong Fan, Ashley Roush, Rafael Zaia and Ryan Keiffer for their assistance with this project. This research was supported by the Mid America Strawberry Growers Association.

Disclosure Statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

The work was supported by the Mid america strawberry growers association.