Effects of Soil Solarization and Grafting on Tomato Yield and Southern Root-knot Nematode Population Densities

Authors:
Rachel E. Rudolph Department of Horticulture, University of Kentucky, 1100 S. Limestone, Lexington, KY 40506, USA

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Victoria Bajek Department of Horticulture, University of Kentucky, 1100 S. Limestone, Lexington, KY 40506, USA

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Misbakhul Munir Department of Plant Pathology, University of Kentucky, 1405 Veterans Drive, Lexington, KY 40506, USA

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Abstract

High tunnel production has increased in the past 10 years in Kentucky with more than 1500 high tunnels constructed across the state. Tomato is the most popular and most valuable high tunnel crop per square foot. This has contributed to a lack of rotation and increased pressure from root-knot nematodes (RKN; Meloidogyne spp.). Infection by RKN leads to root galling and reduces the host plant’s ability to take up water and nutrients. Sustainable strategies are needed to manage increasing RKN populations for long-term health of high tunnel soils. Soil solarization is a nonchemical management strategy that has shown promise in other regions and in open field systems. Because tunnels are primarily used to produce high-value crops and are often used for season extension, solarizing during the off-season would be the most beneficial for growers because solarizing would require taking the tunnel out of production. The primary objective of this study was to determine whether springtime soil solarization in Kentucky high tunnels followed by use of resistant tomato cultivars is a viable and effective management strategy for RKN populations. Soil solarization was performed in two commercial high tunnels naturally infested with southern RKN (Meloidogyne incognita) for 2, 4, and 6 weeks and compared with a nonsolarized control. Soil temperatures reached during solarization were assessed at 7.6-, 15.2-, and 22.8-cm soil depth. After solarization, tomato was transplanted, including ‘Cherokee Carbon’ grafted onto RKN-resistant rootstocks ‘Fortamino’ and ‘Estamino’, RKN-resistant nongrafted ‘Caimon’, and susceptible ‘Cherokee Carbon’ as the control. The highest soil temperature achieved was 50 °C during 6 weeks of solarization at 7.6-cm soil depth compared with 38 °C reached in nonsolarized soil. Soil population densities of RKN increased each month after solarization and were generally lower after solarization with resistant tomato cultivars. The interaction of soil solarization and tomato cultivars was significant with respect to RKN densities in soil and roots. The mean RKN soil and root population densities in the nonsolarized, nonresistant treatment combination was significantly greater compared with all other treatments (P < 0.0001). Population densities of RKN were significantly higher in the nonsolarized control compared with solarized treatments (P = 0.0002). Nongrafted ‘Cherokee Carbon’ had significantly more RKN in surrounding soil compared with all other tomato treatments. Tomato yield was unaffected by soil solarization, but there were significant differences based on tomato cultivars alone; nongrafted ‘Cherokee Carbon’ yielded less than the resistant ‘Caimon’. Together, solarization and resistant cultivars reduced RKN population densities in soil and roots, which can provide growers with a nonchemical approach for long-term RKN management and high tunnel resiliency.

High tunnels are covered structures that provide a protected environment that extends the growing season and allows growers to capture premium prices through increased yield and improved quality (Carey et al. 2009; Lamont 2009). The use of high tunnels for extending the growing season and potentially growing crops year-round has increased dramatically in recent years. More than 7000 high tunnels operating in the southern region of the United States have been funded by the Natural Resource Conservation Service (NRCS; Ernst et al. 2020). Kentucky has been one of the most active adopters with more than 1500 high tunnels and a production capacity of more than 260,000 m2 (Wheby D, NRCS, personal communication).

A high tunnel production and marketing survey conducted in Kentucky and other southern states reported that 50% of the respondents had grown specialty crops such as tomato (Solanum lycopersicum L.), lettuce (Lactuca sativa L.), and pepper (Capsicum annuum L.) under high tunnels for at least 10 growing seasons (Ernst et al. 2020). More than 50% of growers use the high tunnel for year-round production. Additionally, 36% of the high tunnel operators practice organic production methods but are not certified, 33% incorporate organic along with conventional management methods, and 10% operate certified organic high tunnels. The survey found that 85% of the high tunnels were never moved, and only 9% of the high tunnels were movable by design. Soil samples were collected from 175 commercial high tunnels across Kentucky from 2019 to 2022. Out of the 175 high tunnels, crop rotation was not practiced in 68% of those tunnels (Bajek et al. 2023). These conditions—continuous cropping, lack of rotation out of the same crop family, and often limited options for prevention and management—can favor the development of many soil-related issues, including soilborne diseases and root-knot nematodes (RKN; Meloidogyne spp.). There are limited options to manage many of these issues once they are present, especially in high tunnels and especially if the tunnel is certified organic. There are increasing numbers of high tunnel growers, organic and conventional, who could use alternative soil management options.

In the previously cited soil survey conducted across Kentucky, RKN was observed in 55% of the high tunnels sampled (Bajek et al. 2023). Of those samples that contained RKN, 54% were identified as Meloidogyne incognita. RKN is one of the most destructive plant-parasitic nematodes worldwide with a host range of more than 3000 plants species (Abad et al. 2003). Approximately 5% of crop production worldwide is lost to Meloidogyne spp. every year (Karajeh 2008). The infective stage of RKN, second stage juvenile (J2), detects and penetrates suitable host roots with its piercing mouthpart, called a stylet (Ralmi et al. 2016; Williamson 1998). Infection of RKN causes root galling, inhibits water and nutrient uptake, and leads to wilting, chlorosis, crop yield loss, and plant death (Ireri et al. 2018; Mitkowski and Abawi 2003; Onkendi et al. 2014). Many vegetable crops, including tomato, are susceptible hosts for RKN (Ahmad et al. 2021; Ralmi et al. 2016) and RKN are difficult to manage because of their wide host range (Gill and McSorley 2011). This is particularly problematic in high tunnels, with intensive and repeated crop production, and little opportunity for non–host crop rotation and sanitation (Bruce et al. 2019).

Survival and pathogenicity rates of M. incognita are affected by a variety of parameters, but soil temperature is one of the most influential. For survival and growth, M. incognita prefers temperatures between 15 and 25 °C. The ideal temperature for egg hatching is 15 to 30 °C, with temperatures below 10 °C inhibiting egg hatching (Goodell and Ferris 1989). Tsai (2008) observed that when incubated at 15 °C, J2s lived for 380 d, which was the longest survival time. The shortest survival time was 3.5 h at 45 °C (98.8% mortality), followed by 60 d at 40 °C and 35 °C (both 100% mortality). Although one of the benefits of high tunnel production is increased soil temperatures for season extension and earlier crop production (Frey et al. 2020; Lamont 2009; Zhao and Carey 2009), this factor also provides a conducive environment for increased RKN reproduction (Bajek and Rudolph 2023).

Because soil temperatures affect RKN reproduction and mortality, soil solarization is one method that could be used to manage RKN populations. Solarization involves covering soil with a transparent plastic sheet to trap radiant energy from the sun to increase soil temperatures (Katan et al. 1976; Stapleton 2000). Pathogen suppression occurs when soil beneath the plastic film is heated to a temperature high enough to inhibit pathogen propagules from spreading (Stapleton 2000), or kill the pathogens through thermal processes (Katan et al. 1976). High tunnel air temperatures can often be 10 to 17 °C warmer compared with open field air temperatures (Rudolph RE, unpublished data), and soil temperatures at 5-cm depth can be 2 to 5 °C higher than those in open field throughout the year (Kumari et al. 2014). High tunnel growers can use this to their advantage to trap solar energy and suppress RKN without the use of chemicals. Solarization is compatible for both organic and conventional production systems and has low environmental and human health impact.

With more than 1500 high tunnels distributed across Kentucky, the polyethylene plastic used to cover them is readily available and can also be used for soil solarization. These polyethylene sheets are ∼152 µm thick. Although thinner plastic tarps (19 to 25 µm) are recommended for maximum transmission of solar radiation (Stapleton and DeVay 1983), the thicker plastic is a resource that many high tunnel growers already have or can easily obtain that could be used for solarization purposes. Previous studies have used plastic sheets of similar thickness to successfully solarize soil (Marenco and Lustosa 2000; Moura et al. 2012).

There are many challenges with conducting solarization in high tunnels, including timing of solarization and crop selection post-solarization. Because one of the main purposes of growing crops in a high tunnel is season extension, this often means that crops are planted earlier and kept in production longer compared with open field production. Kentucky high tunnels are often occupied from March to November. Solarization could be in direct conflict with this timing because it ideally would be implemented during the warmer months, which is April through October in Kentucky. It is unknown whether high tunnel solarization is effective during the late winter or early spring months, when high tunnels are less likely to be in production.

Another challenge is what to plant after solarization is completed. Complete eradication of RKN is not possible, which means that planting a susceptible crop should still be avoided. Plant resistance to RKN is defined as the ability to restrict growth and development, but there may still be evidence of RKN feeding and reproduction. Because tomato is the most high-value crop per square meter that is grown in a high tunnel (Galinato and Miles 2013), many growers will want to continue to grow tomatoes in their high tunnel after soil solarization. The resistance gene in tomatoes is the Mi gene, a single dominant gene that confers resistance to M. arenaria, M. javanica, and M. incognita (Jacquet et al. 2005). This gene is used in breeding programs for commercially available tomato cultivars to inhibit RKN infection at an early stage (Jacquet et al. 2005), including rootstocks. Resistant cultivars are often used as rootstocks (the root system), rather than bred for fruit production. Susceptible tomato cultivars that produce desirable fruit can be grafted onto resistant rootstocks by using a silicone tube to secure the union of the scion (the upper fruiting body) to the rootstock (Kubota et al. 2008). Although several RKN-resistant tomato cultivars that do not need to be grafted are commercially available, options are still relatively limited. The Mi gene can be turned off when soil temperatures exceed 28 °C (Williamson 1998), which can happen in high tunnels during the summer (Bajek and Rudolph 2023). However, previous research has demonstrated that RKN population densities can be managed in high tunnel soils using RKN-resistant tomato cultivars (Bajek and Rudolph 2023; Frey et al. 2020; Rivard et al. 2010a). Following soil solarization with resistant cultivars, grafted or not, may be a viable RKN management strategy with minimal disruption to high tunnel production schedules and other management methods.

The objective of this study was to evaluate the response of RKN soil and root populations following spring soil solarization of 2-, 4-, and 6-week durations using 152-µm polyethylene plastic tarping and planting RKN-resistant tomato cultivars (grafted and nongrafted) in a naturally infested high tunnel. We hypothesized that soil solarization and resistant cultivars would suppress RKN population densities and result in higher marketable yield compared with no solarization and planting a susceptible tomato cultivar.

Materials and Methods

Sites.

Two locations were identified on commercial farms with high tunnels in central Kentucky. The RKN populations in the Woodford County (lat. 38.052576°N, long. 84.729946°W, 276 m elevation) and Mercer County high tunnels (lat. 37.945270°N, 84.912900°W, 240 m elevation) were initially sampled on 27 Sep and 11 Oct 2021, respectively. The presence of M. incognita was confirmed using molecular diagnostics by the North Carolina Department of Agriculture and Consumer Agronomic Services (Ye et al. 2019). The population densities were 190 and 150 J2·500 cc−1 of soil at the Woodford and Mercer sites, respectively. Both high tunnels were ∼9 m × 29 m. The soil types for the Mercer and Woodford high tunnels are Elk silt loam and Maury-Bluegrass silt loam, respectively. Before beginning the experiment, soil samples were collected from each location and submitted to the University of Kentucky Division of Regulatory Services for analysis of soil nutrients and chemistry. The Mercer County high tunnel was previously planted with tomato and squash (Cucurbita pepo). Because the Mercer County high tunnel had been previously cropped differently on the north and south sides over the course of several years, two separate soil samples were collected. The results of the soil tests indicated soil pH 6.5 on north side of the high tunnel and 6.7 on south side of the high tunnel, and phosphorus and potassium levels were 514.5 kg·ha−1 and 285.8 kg·ha−1, respectively. Similarly, the soil cation exchange capacity was 19 meq·100 g−1 on the north side and 18 meq·100 g−1 on the south side. The previous crops grown in the Woodford County high tunnel were lettuce, carrot (Daucus carota var. sativus), and cilantro (Coriandrum sativum L.). The results of the soil test were soil pH of 6.6, 721 kg·ha−1 of P, 830 kg·ha−1 of K, and a CEC of 16.9 meq·100 g−1.

Experimental design.

Soil solarization was conducted in the spring of 2022. The main plot treatments included two weeks (2wk), four weeks (4wk), six weeks (6wk) of soil solarization, and no solarization as the control. The area of each plot was ∼50 m2 (6.09 m long and 8.23 m wide) with 0.6 m buffer in between plots. The subplot treatments within each main plot included four tomato cultivars: RKN-susceptible ‘Cherokee Carbon’ (Cornell University 2022) grafted onto RKN-resistant rootstock ‘Fortamino’ (Cornell College of Agricultural Sciences 2022), ‘Cherokee Carbon’ grafted onto RKN-resistant rootstock ‘Estamino’ (Bajek and Rudolph 2023; Cornell College of Agricultural Sciences 2022), nongrafted RKN-resistant ‘Caimon’ (Cornell College of Agricultural Sciences 2022), and nongrafted ‘Cherokee Carbon’ as the control. There were four replicates of each tomato cultivar at Mercer County with one replicate consisting of five plants planted 46 cm apart. Woodford County had five replicates of each treatment. Each replicate contained five plants with an in-row spacing of 30 cm.

Before solarization, both of the high tunnels were tilled to a depth of 8 cm, and all plant debris was removed from the surface. The soil was raked smooth to create an even surface. To establish the baseline population density of RKN J2, 15 soil cores were collected from each plot at 0- to 20-cm depth using a soil probe (diameter: 2.2 cm). We combined the cores to make one large sample for each treatment plot. Three soil moisture sensors (10HS Soil Moisture Smart Sensor; Onset Computer Corporation, Bourne, MA, USA) attached to a data logger (HOBO USB Micro Station Data Logger, Onset Computer Corporation) were installed at 15.2-cm soil depth in the high tunnels in both locations to measure the amount of irrigation needed to reach 70% of field capacity. The grower in Mercer County used drip irrigation and the grower in Woodford County used sprinklers to wet the soil before solarization.

Solarization began in Mercer County on 1 Apr 2022 and in Woodford County on 4 Apr. Three soil temperature data loggers (HOBO U23 Pro v2 Temperature Data Logger, Onset Computer Corporation) were randomly buried in the soil in each treatment plot at 7.6-, 15.2-, and 22.9-cm depths (three data loggers per depth). The data loggers were programmed to collect temperature data every hour. The daily maximum soil temperature, the average daily soil temperature, and the degree hours reached were determined by the data collected from the soil temperature data loggers. One data logger also recorded air temperature every hour inside the high tunnel. After burying the data loggers, a transparent plastic tarp (152-µm thickness) was pulled tightly across each solarized treatment plot, secured in place on the corners using sod staples, and the edges were covered in soil to create a seal. The high tunnels were kept closed for the duration of solarization. Plastic was removed from each main plot after 2, 4, or 6 weeks. Once plastic was removed from a main plot, the plot was left undisturbed until tomatoes were transplanted.

Crop management.

The Mercer County grower followed conventional practices for management and monitoring, including plant fertility as recommended by UK Vegetable Extension (Bessin et al. 2021). The Woodford County grower followed certified organic practices for management and monitoring but used the same recommendations for plant fertility. In both locations, the growers used drip irrigation to water plants as needed and black plastic tarps to cover the entire soil surface inside the high tunnel for weed management. Tomatoes were transplanted into the plastic mulch covering the soil. Nongrafted ‘Caimon’ and ‘Cherokee Carbon’ (High Mowing Organic Seeds, Wolcott, VT, USA) were seeded on 17 Mar in 50-cell trays and grown in a greenhouse at the University of Kentucky Horticulture Research Farm (UK-HRF) with ambient light and temperature between 22 and 24 °C. Twenty-day-old plants of ‘Cherokee Carbon’ were grafted onto ‘Fortamino’ and ‘Estamino’ (Johnny’s Selected Seeds, Winslow, ME, USA) rootstocks in 128-cell trays by Trihishtil (Mills River, NC, USA). Grafted plants arrived at the UK-HRF on 27 Apr. All plants were transplanted into larger pots (diameter = 6.68, depth = 8.89) with potting soil (Fort Lite, VT Compost Co., Montpelier, VT, USA) on 29 Apr and kept in the greenhouse until 5 d before transplanting to the high tunnels when they were placed outside to harden off. Plants were transplanted in the Mercer and Woodford County high tunnels on 17 and 18 May, respectively. After transplanting tomatoes, we installed data loggers at both locations at 15.24-cm depth to collect soil temperature hourly for the duration of the growing season.

Data collection.

Tomato yield for each treatment replicate was collected and recorded by the growers at each location, about two harvests per week. Fruit quality was assessed by the growers according to their market standards (farmers markets and direct sales for both growers).

After tomato transplanting until the project termination, we collected soil samples monthly from each subplot for analysis of RKN population densities in both sites. Approximately 500 g of soil were collected at 0- to 20-cm depth from the root zone of the tomato plants using a soil probe (diameter: 2.2 cm) ∼5 cm away from the stem. Soil samples from nonsolarized plots were also collected at each time point. The Baermann funnel method was used to extract RKN J2 from 100 g of soil (Hooper 1986). Collected M. incognita J2 were identified and counted using an inverted microscope (Leica DMI 1, Wetzlar, Germany) at 10× magnification. Another 100-g subsample of the same soil was dried in an oven at 60 °C and weighed after 5 d to establish the dry weight. Population densities of RKN are expressed as the number of M. incognita·100 g−1 of dried soil.

Tomato roots were collected at the end of the experiment in Mercer and Woodford Counties on 23 Sep and 7 Oct, respectively. The roots of the middle three plants in each treatment plot were uprooted from the soil, cut from the stem, placed in a bag, and kept at 4 °C. Within 2 d of sampling, roots were prepared for extraction by washing the roots and cutting them into 5-cm pieces. Eggs were extracted from the roots and counted using a 10% bleach solution method as described by Hussey and Barker (1973). A sugar centrifugation method (Jenkins 1964) was used for samples that still contained sediment. After extraction, the roots were oven dried for 5 d at 60 °C and weighed. Eggs were identified as described above and population densities were expressed as the number of M. incognita eggs·g−1 of dried root.

Data analyses.

The GLIMMIX procedure of SAS (SAS Institute Inc., Cary, NC, USA) was used to analyze treatment effects on soil (J2) and root (eggs) RKN population densities, marketable tomato yield, and tomato root biomass according to generalized linear mixed modeling. Factors considered fixed effects included solarization duration, tomato cultivar, and their interactions. Random effects included replication in each individual site+ site (model over multiple sites). Where random effects were estimated to be zero or did not improve fit of the model, they were excluded. Fisher’s protected least significant difference (LSD) was used for pairwaise comparison at α ≤ 0.05. For RKN population densities in soil, each sampling date was analyzed separately.

Results

Soil temperature.

For each solarization duration, the average daily maximum soil temperature (i.e., the average of all daily maxima during each solarization event) and average daily soil temperature were calculated. Average daily and average daily maximum soil temperatures did not reach or exceed 30 °C at any depth during the 2-week soil solarization treatment in early April (Table 1). There were only 2 h in which temperatures exceeded 35 °C. The 2-week solarization treatment reached daily maximum temperatures that were at least 74% higher compared with the nonsolarized control during that same time at all three soil depths. Temperatures differences were similar when comparing nonsolarized soil temperatures to the 4- and 6-week solarization treatments at all three soil depths. The 4- and 6-week soil solarization treatments resulted in average daily maximum soil temperatures that were 5 and 8 °C higher, respectively, at each soil depth compared with the 2-week solarization treatment. The 4- and 6-week solarization treatments achieved 53 and 124 more h, respectively, above 35 °C at 7.6-cm soil depth compared with the 2-week treatment. The 4-week solarization achieved 27 h above 40 °C at 7.6-cm soil depth, but at greater depths, temperatures did not exceed 40 °C. The 6-week solarization treatment achieved 20 more h over 40 °C at 7.6-cm depth compared with the 4-week treatment. It also had 14 h over 45 °C at the same depth. At 15.2- and 22.9-cm depths, 6-week solarization had 32 and 9 h over 40 °C, respectively.

Table 1.

Soil temperatures and degree hours achieved at three soil depths during soil solarization in commercial high tunnels naturally infested with southern root-knot nematode (Meloidogyne incognita) in Mercer and Woodford Counties, Kentucky in 2022.

Table 1.

After tomatoes were transplanted, average soil temperatures started at 25.1 °C in May, reached the highest in mid-June at 29.6 °C. By late September, the average daily soil temperature was 23.3 °C (Fig. 1). The daily maximum temperature was at the highest in mid-June at 31.3 °C and lowest in mid-September at 22.4 °C (Fig. 1).

Fig. 1.
Fig. 1.

Soil temperatures [daily average, daily maximum (max), and daily minimum (min)] in Kentucky high tunnels at 15-cm soil depth in 2022. Temperature data were collected from tomato transplanting through termination and final harvest. Temperature readings are averaged across two sites.

Citation: HortScience 58, 11; 10.21273/HORTSCI17396-23

Tomato yield.

The interaction between solarization duration and cultivar on marketable tomato yield was not significant (P = 0.38; data not shown), nor was the main effect of solarization duration (P = 0.51; Fig. 2). When evaluating the effects of cultivar alone, there were significant differences among treatments (P = 0.013; Fig. 2). ‘Caimon’ had significantly higher yield compared with nongrafted ‘Cherokee Carbon’ and yielded 20% more fruit compared with nongrafted ‘Cherokee Carbon’. The yields of both grafted treatments were not significantly different from nongrafted ‘Cherokee Carbon’ but produced at least 12% more fruit. The grafted ‘Fortamino’ treatment had the lowest yield within the nonsolarized soil.

Fig. 2.
Fig. 2.

Marketable tomato fruit yield harvested from tomato plants grown after soil solarization in commercial high tunnels naturally infested with southern root-knot nematode (Meloidogyne incognita) in Mercer and Woodford Counties, Kentucky in 2022. i Yield was collected from each replicate and site and combined to determine mean yield per plant. ii Treatments included 2 weeks (2wk solar), 4 weeks (4wk solar), and 6 weeks (6wk solar) of soil solarization, and the nonsolarized control. iii Values (means ± standard error) are pooled from two different locations (Mercer and Woodford Counties). Statistical analysis includes replication within site + site as random effect. Any two means not followed by the same letter are significantly different at α ≤ 0.05. iv Treatments included nongrafted ‘Caimon’ (C), ‘Cherokee Carbon’ grafted onto ‘Estamino’ rootstock (CC + E), ‘Cherokee Carbon’ grafted onto ‘Fortamino’ rootstock (CC + F), and nongrafted ‘Cherokee Carbon’ (CC).

Citation: HortScience 58, 11; 10.21273/HORTSCI17396-23

RKN population densities.

Population densities of RKN in soil in nonsolarized plots increased throughout the season and were 2 or 3 times greater compared with population densities within solarized treatments at the end of the season. There was a significant interaction effect of solarization and cultivar in the RKN soil populations in August and September sampling dates (P = 0.0001 and 0.0003, respectively), but not for the June or July sampling dates (Table 2). At the June and July soil samplings, M. incognita J2 populations were significantly different among the solarization treatments, and in July they were also significantly affected by tomato cultivar (Table 2). In June, plots solarized for 4 or 6 weeks had significantly fewer RKN compared with the nonsolarized control. By July, only plots solarized for 6 weeks had smaller RKN population densities than the nonsolarized control. At this time, soil surrounding ‘Estamino’ roots had higher RKN population densities compared with soil surrounding ‘Fortamino’ (Table 2). By August and September (the last sample date), nonsolarized soil surrounding nongrafted ‘Cherokee Carbon’ had a mean RKN J2 population density that was more than 6 times that of the other treatments (Table 2).

Table 2.

Monthly root-knot nematode (RKN; Meloidogyne incognita) population densities in soil surrounding grafted and nongrafted tomato plants grown after soil solarization in naturally infested commercial high tunnels in Mercer and Woodford Counties, Kentucky in 2022.

Table 2.

The number of M. incognita eggs per gram of dried root was affected by the interaction between solarization and cultivar (P < 0.0001; Table 3). Nongrafted ‘Cherokee Carbon’ grown in nonsolarized soil had at least 6 times the density of RKN eggs per gram of dried root compared with the other cultivars in all other solarization treatments. Generally, as solarization duration increased, egg densities in tomato roots decreased, even in nongrafted ‘Cherokee Carbon’. ‘Cherokee Carbon’ grown in 2-week solarization plots had the next highest mean RKN egg density in roots which was almost 10 times more than the egg density in ‘Cherokee Carbon’ planted in 4-week solarization plots. With 6-week solarization, nongrafted ‘Cherokee Carbon’ had an average RKN egg density 3 times greater than that of ‘Caimon’, which had more than twice the egg density compared with ‘Fortamino’ and 4 times the egg density of ‘Estamino’ (Table 3).

Table 3.

Root-knot nematode (RKN; Meloidogyne incognita) egg population densities in soil surrounding grafted and nongrafted tomato plants grown after soil solarization in naturally infested commercial high tunnels in Mercer and Woodford Counties, Kentucky in 2022.

Table 3.

Discussion

High tunnel growers with RKN infestations have limited management options. Rotation to non–host crops that are as profitable as tomato is limited. Maintaining profitability is likely the main reason high tunnel growers frequently grow tomato year after year because it is the most high-value high tunnel crop per square foot (Galinato and Miles 2013). Utilization of grafted resistant rootstock has been shown to improve tomato yields in high tunnels with RKN pressure (Bajek and Rudolph 2023; López-Perez et al. 2006; Rivard et al. 2010a). However, grower adoption of grafting can be limited because of the cost (Barrett et al. 2012; Rivard et al. 2010b). Resistant rootstock seed can cost up to four times more than the seed of scion cultivars (Barrett et al. 2012). Additionally, if a grower is grafting plants on their farm, there is the additional cost of growing and maintaining extra plants, and the labor to graft those plants successfully. There are companies that will graft plants for growers, which reduces the time and labor of grafting on-farm. However, those plants can cost anywhere between $1.45 and $3.00 each, depending on the cultivar combinations of scion and rootstock and whether the seed is certified organic (Hinson B, Tri-histil, personal communication). Additionally, those plants often need to be shipped quickly and with special packaging, which increases the expense.

Despite these limitations, grafting provides many benefits, including allowing growers to continue to offer their preferred tomato cultivars for their market, regardless of the RKN resistance of those cultivars. However, nongrafted resistant tomato cultivars may provide a more affordable option for many growers who have more flexibility with regard to which tomato cultivar they can offer their customers. In our study, nongrafted RKN-resistant ‘Caimon’ performed better than nongrafted, susceptible ‘Cherokee Carbon’ and similar to grafted treatments in nearly all measured parameters. The success of this nongrafted resistant cultivar should encourage development and research into more cultivar options with RKN resistance that do not require grafting.

Although RKN-resistant tomato cultivars demonstrated significantly higher yield compared with the RKN-susceptible control, this was not the case with respect to soil solarization. There were no significant yield differences among solarization treatments and the nonsolarized control, even when accounting for the interaction of solarization and tomato cultivar treatments. Previous solarization research involving RKN has had mixed results on yield differences. In a greenhouse experiment in which tomatoes were planted in soil naturally infested with M. incognita, Oka et al. (2007) observed significantly higher tomato crop yield compared with the nonsolarized control when M. incognita-infested soil was solarized for 50 d, but not the following year when soil was solarized for 44 d. In a separate experiment, bell pepper (Capsicum annuum L.) yield was significantly higher when grown in soil that was solarized for 30 d compared with pepper grown in nonsolarized soil. The authors did not report soil temperatures. Initial RKN populations were reported as 3.2 ± 3.1 and 1.8 ± 2.7 J2·50 g−1 of soil for the tomato and pepper experiments, respectively, and varied from less than 50 to ∼700 J2·50 g−1 of soil across treatments throughout the season. These are larger differences in population densities compared with our study and could be one reason for the observed yield differences. Stevens et al. (2003) observed significantly higher tomato yield after 75 d of solarization during the summer months in Alabama compared with the plants grown in nonsolarized soil naturally infested with M. incognita over 2 consecutive years. The authors also observed significantly higher sweetpotato [Ipomoea batatas L. (Lam.)] yield after 84 d of solarization in the same field compared with sweetpotato grown in nonsolarized soil. The duration of solarization treatments in this study was about twice as long as our longest solarization time period and occurred during the summer in a state that is further south, which likely resulted in reaching higher temperatures for more hours. This could account for the significant yield differences observed by the authors.

Although solarization did not result in significant tomato yield differences in our study, it did decrease RKN population densities in both roots and soil. Similar results were also observed by Oka et al. (2007) when soil was solarized for 50, 30, and 44 d during different experiments. Wang et al. (2006) observed no significant differences in RKN population densities between solarization and the weedy nonsolarized control. Possible reasons for this may have been because the soil moisture content was 6% before solarization. Solarization is most effective when soil is moist, at least 70% field capacity (Elmore et al. 1997). Additionally, the experiments were conducted in sandy soil. Sandy soil holds less water and heat transfer is less throughout the soil profile compared with clay soil (Abu-Hamdeh 2003; Wilson et al. 1987). Although there were 29 d when temperatures reached or exceeded 42 °C, the authors did not report the number of hours at temperatures where RKN could be killed or suppressed. In a laboratory experiment, M. incognita egg suppression was achieved with a minimum of 13.1 h at 42 °C, whereas complete J2 death was observed after 13.8 h at the same temperature (Wang and McSorley 2008). The authors also evaluated M. incognita J2 survival under oscillating temperature conditions. After 15 h of oscillating between 42 °C and 22 °C for 3 h at a time, there was 0% J2 survival. This is valuable for high tunnel or open field conditions where the soil temperature cannot be maintained at a constant, often fluctuates greatly during a single day, and is dependent on weather and solar radiation. Wang and McSorley (2008) noted that more hours at lower temperatures could also achieve egg suppression and J2 death. This, in addition to the high tunnels both containing silt loam soils, may explain significant differences between solarization treatments and the nonsolarized control in our experiments. Silt loam soils maintain moisture for longer and transfer heat better throughout the soil profile compared with coarser textured soils (Schoonover and Crim 2015). We observed significantly lower RKN population densities in soil and tomato roots which could be the result of achieving 60 and 30 h over 40 °C at 7.6- and 15.2-cm soil depths, respectively, in the 6-week solarization treatment. Even the nongrafted ‘Cherokee Carbon’ grown in the 4- and 6-week solarization treatments had significantly fewer RKN eggs·g−1 of dried root compared with nongrafted ‘Cherokee Carbon’ in the nonsolarized control and the 2-week solarization treatments. Soil temperatures in the nonsolarized control never reached 40 °C and only exceeded 35 °C for 19 h in the last 2 weeks of the 6-week timeframe. The 2-week solarization treatment had only 2 h over 35 °C, which was due to it being implemented during the coldest days of the entire project.

Combining soil solarization with RKN-resistant cultivars significantly reduced RKN population densities in roots and soil but did not eradicate RKN. This was not unexpected because previous work has observed resistance of certain tomato cultivars to be on a spectrum; they are not complete nonhosts and do allow for a certain rate of RKN infection and reproduction (Bajek and Rudolph 2023; López-Perez et al. 2006; Rivard et al. 2010a). High soil temperatures in the high tunnels from May to September likely contribute to increased RKN reproduction as well as increased infection of RKN-resistant tomato cultivars. In our study, the average daily maximum soil temperature reached or exceeded 28 °C (the temperature at which the Mi gene can be turned off) on 37 d during the growing season. There were ∼314 h where soil temperatures were greater than or equal to 28 °C.

Although growers can manage high tunnel temperatures in the warmest months by opening the side and end walls, installing larger end wall doors to allow more air flow, and draping shadecloth over the roof of the structure, complete control over the temperature in a high tunnel is not possible and is likely to affect the performance of RKN-resistant cultivars. This information is something growers should keep in mind for long-term effective management of RKN.

Conclusion

Utilization of RKN resistant cultivars, whether grafted or nongrafted, resulted in higher tomato yield compared with the nonresistant control. This approach alone was shown to be beneficial for managing M. incognita population densities in tomato roots and surrounding soil. However, in combination with soil solarization, RKN-resistance may be a better approach for long-term high tunnel soil health and resiliency. Although yield differences were not demonstrated after one season of soil solarization, this strategy is relatively inexpensive and simple for growers to employ and may provide more benefits after several years. On the basis of the performance of the nongrafted RKN-resistant ‘Caimon’, more RKN-resistant cultivars that do not require grafting are needed to help growers reduce production costs. Genetic resistance to RKN is an effective nonchemical management approach, and it is strengthened by combining it with soil solarization for improved integrated pest management. These two strategies will likely appeal to growers operating on various scales and production philosophies.

References Cited

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    • Search Google Scholar
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    • Search Google Scholar
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    • Search Google Scholar
    • Export Citation
  • Bajek V, Rudolph RE. 2023. Managing southern root-knot nematode in Kentucky high tunnels using grafted tomato. HortScience. 58(6):704713. https://doi.org/10.21273/HORTSCI17141-23.

    • Search Google Scholar
    • Export Citation
  • Barrett CE, Zhao X, Hodges AW. 2012. Cost benefit analysis of using grafted transplants for root-knot nematode management in organic heirloom tomato production. HortTechnology. 22(2):252257. https://doi.org/10.21273/HORTTECH.22.2.252.

    • Search Google Scholar
    • Export Citation
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  • Bessin R, Gauthier N, Fealko E, Rudolph R, Wright S. 2021. Vegetable production guide for commercial growers 2022–23. Univ Kentucky Coop Ext Serv. ID-36. http://www2.ca.uky.edu/agc/pubs/id/id36/id36.pdf. [accessed 17 Feb 2022].

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    • Search Google Scholar
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    • Search Google Scholar
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    • Search Google Scholar
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    • Search Google Scholar
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    • Search Google Scholar
    • Export Citation
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    • Search Google Scholar
    • Export Citation
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    • Search Google Scholar
    • Export Citation
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    • Search Google Scholar
    • Export Citation
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    • Search Google Scholar
    • Export Citation
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    • Search Google Scholar
    • Export Citation
  • Kumari P, Ojiha RK, Abhivyakti AW, Rajesh RP. 2014. Microclimatic alteration through protective cultivation and its effect on tomato yield. J Agrometerol. 16(2):172177. https://doi.org/10.54386/jam.v16i2.1508.

    • Search Google Scholar
    • Export Citation
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    • Search Google Scholar
    • Export Citation
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    • Search Google Scholar
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  • Fig. 1.

    Soil temperatures [daily average, daily maximum (max), and daily minimum (min)] in Kentucky high tunnels at 15-cm soil depth in 2022. Temperature data were collected from tomato transplanting through termination and final harvest. Temperature readings are averaged across two sites.

  • Fig. 2.

    Marketable tomato fruit yield harvested from tomato plants grown after soil solarization in commercial high tunnels naturally infested with southern root-knot nematode (Meloidogyne incognita) in Mercer and Woodford Counties, Kentucky in 2022. i Yield was collected from each replicate and site and combined to determine mean yield per plant. ii Treatments included 2 weeks (2wk solar), 4 weeks (4wk solar), and 6 weeks (6wk solar) of soil solarization, and the nonsolarized control. iii Values (means ± standard error) are pooled from two different locations (Mercer and Woodford Counties). Statistical analysis includes replication within site + site as random effect. Any two means not followed by the same letter are significantly different at α ≤ 0.05. iv Treatments included nongrafted ‘Caimon’ (C), ‘Cherokee Carbon’ grafted onto ‘Estamino’ rootstock (CC + E), ‘Cherokee Carbon’ grafted onto ‘Fortamino’ rootstock (CC + F), and nongrafted ‘Cherokee Carbon’ (CC).

  • Abad P, Favery B, Rosso MN, Castagnone-Sereno P. 2003. Root-knot nematode parasitism and host response: Molecular basis of a sophisticated interaction. Mol Plant Pathol. 4:217224. https://doi.org/10.1046/j.1364-3703.2003.00170.x.

    • Search Google Scholar
    • Export Citation
  • Abu-Hamdeh NH. 2003. Thermal properties of soils as affected by density and water content. Biosyst Eng. 86(1):97102. https://doi.org/10.1016/S1537-5110(03)00112-0.

    • Search Google Scholar
    • Export Citation
  • Ahmad G, Khan A, Khan AA, Ali A, Mohhamad HI. 2021. Biological control: A novel strategy for the control of the plant parasitic nematodes. Antonie van Leeuwenhoek. 144(7):885912. https://doi.org/10.1007/s10482-021-01577-9.

    • Search Google Scholar
    • Export Citation
  • Bajek V, Munir M, Rudolph RE. 2023. Soil census of Kentucky high tunnels reveals statewide distribution of two Meloidogyne species. Plant Health Prog. https://doi.org/10.1094/PHP-05-23-0052-S.

    • Search Google Scholar
    • Export Citation
  • Bajek V, Rudolph RE. 2023. Managing southern root-knot nematode in Kentucky high tunnels using grafted tomato. HortScience. 58(6):704713. https://doi.org/10.21273/HORTSCI17141-23.

    • Search Google Scholar
    • Export Citation
  • Barrett CE, Zhao X, Hodges AW. 2012. Cost benefit analysis of using grafted transplants for root-knot nematode management in organic heirloom tomato production. HortTechnology. 22(2):252257. https://doi.org/10.21273/HORTTECH.22.2.252.

    • Search Google Scholar
    • Export Citation
  • Bessey EA. 1911. Root-knot and its control. USDA Bureau Plant Industry Bul 217.

  • Bessin R, Gauthier N, Fealko E, Rudolph R, Wright S. 2021. Vegetable production guide for commercial growers 2022–23. Univ Kentucky Coop Ext Serv. ID-36. http://www2.ca.uky.edu/agc/pubs/id/id36/id36.pdf. [accessed 17 Feb 2022].

  • Bruce AB, Maynard ET, Farmer JR. 2019. Farmers’ perspectives on challenges and opportunities associated with using high tunnels for specialty crops. HortTechnology. 29(3):290299. https://doi.org/10.21273/HORTTECH04258-18.

    • Search Google Scholar
    • Export Citation
  • Carey EE, Jett L, Lamont WJ, Nennich TT, Orzolek MD, Williams KA. 2009. Horticultural crop production in high tunnels in the United States: A snapshot. HortTechnology. 19(1):3743. https://doi.org/10.21273/HORTSCI.19.1.37.

    • Search Google Scholar
    • Export Citation
  • Cornell College of Agricultural Sciences. 2022. Disease-resistant tomato varieties. https://www.vegetables.cornell.edu/pest-management/disease-factsheets/disease-resistant-vegetable-varieties/disease-resistant-tomato-varieties/. [accessed 26 May 2023].

  • Elmore CL, Stapleton JJ, Bell CE, DeVay JE. 1997. Soil solarization: A nonpesticidal method for controlling diseases, nematodes, and weeds. Univ California Div Agric Nat Resour. Publication 21377. https://vric.ucdavis.edu/pdf/soil_solarization.pdf. [accessed 20 Feb 2022].

  • Ernst M, Woods T, Butler A, Wolff B, Jacobsen K. 2020. High tunnel production and marketing survey: Data summary. Univ Kentucky Cent Crop Diversification. CCD-SP-17. https://www.uky.edu/ccd/sites/www.uky.edu.ccd/files/HTsurvey.pdf.

  • Frey CJ, Zhao X, Brecht JK, Huff DM, Black ZE. 2020. High tunnel and grafting effects on organic tomato plant disease severity and root-knot nematode infestation in a subtropical climate with sandy soils. HortScience. 55(1):4654. https://doi.org/10.21273/HORTSCI14166-19.

    • Search Google Scholar
    • Export Citation
  • Galinato SP, Miles CA. 2013. Economic profitability of growing lettuce and tomato in Western Washington under high tunnel and open-field production systems. HortTechnology. 23(4):453461. https://doi.org/10.21273/HORTTECH.23.4.453.

    • Search Google Scholar
    • Export Citation
  • Gill HK, McSorley R. 2011. Cover crops for managing root-knot nematodes. Univ Florida Coop Ext Serv. https://doi.org/10.32473/edis-in892-2011.

  • Goodell PB, Ferris H. 1989. Influence of environmental factors on the hatch and survival of Meloidogyne incognita. J Nematol. 21:328334.

    • Search Google Scholar
    • Export Citation
  • Hooper DJ. 1986. Handling, fixing, staining, and mounting nematodes, p 58–80. In: Southey JF (ed). Laboratory methods for work with plant and soil nematodes. HM Stationery Office, London, UK.

    • Search Google Scholar
    • Export Citation
  • Hussey RS, Barker KR. 1973. A comparison of methods of collecting inocula for Meloidogyne spp., including a new technique. Plant Dis Rep. 57(12):10251028.

    • Search Google Scholar
    • Export Citation
  • Ireri DF, Murungi LK, Ngeno DC. 2018. Farmer knowledge of bacterial wilt and root knot nematodes and practices to control the pathogens in high tunnel tomato production in the tropics. Int J Veg Sci. 25(3):213225. https://doi.org/10.1080/19315260.2018.1499690.

    • Search Google Scholar
    • Export Citation
  • Jacquet M, Bongiovanni M, Martinez M, Verschave P, Wajnberg E, Castagnone‐Sereno P. 2005. Variation in resistance to the root‐knot nematode Meloidogyne incognita in tomato genotypes bearing the Mi gene. Plant Pathol. 54(2):9399. https://doi.org/10.1111/j.1365-3059.2005.01143.x.

    • Search Google Scholar
    • Export Citation
  • Jenkins WRB. 1964. A rapid centrifugal flotation method for separating nematodes from soil. Plant Dis Rep. 48(9):692695.

  • Karajeh M. 2008. Interaction of root-knot nematode (Meloidogyne javanica) and tomato as affected by hydrogen peroxide. J Plant Prot Res. 48(2):181187. https://doi.org/10.2478/v10045-008-0021-x.

    • Search Google Scholar
    • Export Citation
  • Katan J, Greenberger A, Alon H, Grinstein A. 1976. Solar heating by polyethylene mulching for the control of diseases caused by soilborne pathogens. Phytopathology. 66:683688.

    • Search Google Scholar
    • Export Citation
  • Kubota C, McClure MA, Kokalis-Burelle N, Bausher MG, Rosskopf EN. 2008. Vegetable grafting: History, use, and current technology status in North America. HortScience. 43(6):16641669. https://doi.org/10.21273/hortsci.43.6.1664.

    • Search Google Scholar
    • Export Citation
  • Kumari P, Ojiha RK, Abhivyakti AW, Rajesh RP. 2014. Microclimatic alteration through protective cultivation and its effect on tomato yield. J Agrometerol. 16(2):172177. https://doi.org/10.54386/jam.v16i2.1508.

    • Search Google Scholar
    • Export Citation
  • Lamont WJ. 2009. Overview of the use of high tunnels worldwide. HortTechnology. 19(1):2529. https://doi.org/10.21273/HORTSCI.19.1.25.

    • Search Google Scholar
    • Export Citation
  • López-Pérez JA, Le Strange M, Kaloshian I, Ploeg AT. 2006. Differential response of Mi gene-resistant tomato rootstocks to root-knot nematodes (Meloidogyne incognita). Crop Prot. 25(4):382388. https://doi.org/10.1016/j.cropro.2005.07.001.

    • Search Google Scholar
    • Export Citation
  • Marenco RA, Lustosa DC. 2000. Soil solarization for weed control in carrot. Pesqui Agropecu Bras. 35(10):20252032. https://doi.org/10.1590/S0100-204X2000001000014.

    • Search Google Scholar
    • Export Citation
  • Mitkowski NA, Abawi GS. 2003. Root-knot nematodes. Plant Health Instructor. https://doi.org/10.1094/PHI-I-2003-0917-01.

  • Moura L, Queiroz I, Mourão I, Brito LM, Duclos J. 2012. Effectiveness of soil solarization and biofumigation for the control of corky root and root knot nematode Meloidogyne spp. on tomato. Acta Hortic. 933:399405. https://doi.org/10.17660/ActaHortic.2012.933.51.

    • Search Google Scholar
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Rachel E. Rudolph Department of Horticulture, University of Kentucky, 1100 S. Limestone, Lexington, KY 40506, USA

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Victoria Bajek Department of Horticulture, University of Kentucky, 1100 S. Limestone, Lexington, KY 40506, USA

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Misbakhul Munir Department of Plant Pathology, University of Kentucky, 1405 Veterans Drive, Lexington, KY 40506, USA

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Contributor Notes

This research was supported by the US Department of Agriculture National Institute of Food and Agriculture, Hatch project 1021069.

We thank Martín Polo, Henry Smith, Erin Maines, Lauren Irwin, Erin Haramoto, and Inga Zasada for their assistance. We also thank the grower cooperators for donating the land, labor, and time for this project.

Mention of trade names or commercial products in this publication is solely for the purpose of providing specific information and does not imply recommendation or endorsement.

R.E.R. is the corresponding author. E-mail: rachel.rudolph@uky.edu.

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  • Fig. 1.

    Soil temperatures [daily average, daily maximum (max), and daily minimum (min)] in Kentucky high tunnels at 15-cm soil depth in 2022. Temperature data were collected from tomato transplanting through termination and final harvest. Temperature readings are averaged across two sites.

  • Fig. 2.

    Marketable tomato fruit yield harvested from tomato plants grown after soil solarization in commercial high tunnels naturally infested with southern root-knot nematode (Meloidogyne incognita) in Mercer and Woodford Counties, Kentucky in 2022. i Yield was collected from each replicate and site and combined to determine mean yield per plant. ii Treatments included 2 weeks (2wk solar), 4 weeks (4wk solar), and 6 weeks (6wk solar) of soil solarization, and the nonsolarized control. iii Values (means ± standard error) are pooled from two different locations (Mercer and Woodford Counties). Statistical analysis includes replication within site + site as random effect. Any two means not followed by the same letter are significantly different at α ≤ 0.05. iv Treatments included nongrafted ‘Caimon’ (C), ‘Cherokee Carbon’ grafted onto ‘Estamino’ rootstock (CC + E), ‘Cherokee Carbon’ grafted onto ‘Fortamino’ rootstock (CC + F), and nongrafted ‘Cherokee Carbon’ (CC).

 

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