Implementation of this project helped us to identify and demonstrate the efficacy of organically acceptable methods to manage tomato wilt disease through on-farm trials. Two separate field trials were conducted in consecutive two years 2015 and 2016 to determine the efficacy of biological control agents, bio-fumigants and grafted plants in relation to non-treated check. Both trials had six different treatments that included 1) ‘Caliente’ mustard cover crop incorporated in the soil; 2) Mustard meal; 3) Serenade soil (QST 713 strain of Bacillus subtilis); 4) Prestop (Gliocladium catenulatum); 5) Grafted tomatoes; and 6) Non-treated check. Treatments were replicated four times in a randomized complete block design with each replicate plot consisting of 30 ft long and 5 ft wide plastic mulched drip irrigated row. Treated plots were separated by a 5 ft length of non-treated bed within the bed and single non-treated beds between beds. Vascular wilt susceptible heirloom tomato (Solanum lycopersicum) cv Mortgage lifter was included in the study.
Four of the five treatments resulted in significant yield increase in 2015 compared to plants grown in infested soil without any treatment. This included plants grown in soil treated with (a) mustard meal; (b) mustard cover crop incorporated in soil followed by covering with plastic mulch; c) serenade treatment of seed, planting mix, and planting holes; (d) Prestop treatment of seed, planting mix, and planting holes; (e) grafted plants. Among these four treatments, grafted plants produced the highest yield, which was significantly higher than any other treatments and non-treated control. Other three treatments (serenade, prestop and mustard meal) resulted in similar fruit yield but significantly higher than non-treated control.
Grafted plants produced 20 lb tomatoes/plant compared with only 11 lb in non-treated check. The difference was higher in 2016 as grafted plants produced 22 lb tomatoes/plant compared with 11 lb in non-treated check. Following strict guidelines for biofumigation, ‘Caliente’ cover crop treatment out-yielded mustard meal treatment. Both biocontrol products were weakly effective but Serenade soil was more effective than Prestop as revealed by higher tomato yield.
As both mustard cover crop and mustard meal produced comparable results, mustard meal may be preferred by growers due to the ease of application and avoiding the early season cover crop growing operation especially in an area where frost free growing season is relatively short. However, use of grafted plants should be the best method for preventing soilborne disease and boost yield for economic benefit.
Results were presented at the 2nd NEIPM center webinar and will also be presented at the upcoming small farm center meeting. We prepared a poster and oral presentation at the WVU Extension service small farm conference and American Phytopathological society-Potomac Division meeting. In-season outreach activities included field day and local heirloom tomato growers. Altogether we could reach out to 200 tomato growers, 35 of them adopted grafted tomato and bio-fumigation for managing soilborne tomato diseases.
Vascular wilt diseases caused by soil-borne fungal and bacterial pathogens are among the most devastating plant diseases worldwide (Tjamos and Beckman 1989). These diseases have caused serious losses to vegetable growers in West Virginia for last few years, especially tomato growers experienced significant crop losses in many counties of the state due to Verticillium wilt. Other soil-borne pathogens such as Fusarium spp., Rhizoctonia spp. and Ralstonia solanaceurum were also linked to heirloom tomatoes and other susceptible cultivars to a lower extent. We identified the problems related to soil-borne pathogens by collecting wilted tomatoes followed by isolation and analyses of the causal agent at the WVU plant and pest diagnostic clinic. Our general recommendations for managing soil-borne pathogens are to rotate fields and use resistant variety.
However, sometime growers’ preference for their favorite variety (e.g. heirloom cultivar Mortgage Lifter) and unavailability of suitable land areas to rotate, makes multi tactics based IPM method more practical and useful. In many prime tomato production areas where crop rotation was not an option, application of methyl bromide would control fungi, bacteria, nematodes and weeds. As methyl bromide was phased out, many alternatives have been used with variable success although organic growers and small growers who can’t use chemical fumigants due to new regulations or lack of the availability of custom applicators left with few options. Small growers in the state who do not have enough acreage to rotate their crop or leave infested area fallow for a few years, in most cases attempt to grow tomatoes despite potential risks posed by soil-borne pathogens. In addition, backyard and organic growers prefer organically acceptable products especially those can be applied by themselves opposed to custom applicators for managing diseases.
The number of organic growers in the state has been growing steadily over years. There is a critical need for conducting trials in fields with known infestation levels to determine the efficacy of potential biofumigants and organic seed/transplant treatments. Biofumigation with isothiocyanates (ITCs) producing plants especially some mustard varieties have shown promising results (Hansen and Keinath, 2013) although inconsistency in the system and disease suppression has restricted its widespread adoption (Motisi et al., 2010). Among the variables, study conditions (e.g. site of the study) and stage of crop incorporation in the soil followed by covering with plastic can have profound effect on level of disease suppression (Matthiessen and Kirkegaard, 2006). Sarwar and Kirkegaard (1998) reported that glucosinolate (GSLs) concentration was the highest at the bud-raised growth stage prior to flowering and higher in spring-versus fall-seeded brassicas.
Edaphic factors can also greatly influence the soil-borne disease control from biofumigation (Gimsing et al. 2007) creating the need of site specific testing. Predominant soil texture in WV farms is clay. Efficacy of biofumigation and biologically based products for managing soil-borne pathogens in clay soils have not been researched sufficiently. Seed treatment or augmenting beneficial microbial population in soil was found to reduce seedling mortality from soil-borne diseases (Rahman and Punja, 2007). However, application methods and timing of application of biological control may often influence the efficacy of these products. Multiple lines of evidence suggest that early colonization of root system by biocontrol agents is necessary to overcome competition or prevent infection by harmful microbes. Sharma and Nowak (1998) tested plant growth and disease development on the disease-susceptible tomato cultivar Bonny Best after Verticillium dahliae infection of tissue culture plantlets bacterized in vitro (by co-culturing with the bacterium) and seedlings bacterized in vivo (after 3 weeks growth in the greenhouse). Significantly higher disease suppression and plant growth were obtained from co-cultured compared with in vivo bacterized plants. This finding is very significant indicating that biocontrol agents need to be on the plant rhizosphere or phyllosphere early enough to thwart pathogen invasion and colonization.
Use of antagonistic micro-organisms has been attempted to reduce the damage caused by soil-borne fungal pathogens in many horticultural and agronomic crops including strawberry for protection against Verticillium dahliae (Berg et al. 2005), in tomato for protection against Alternaria solani (Babu et al. 2015). In vitro-beneficially-bacterized plantlets of grapevine not only grew faster than non-bacterized controls but also were sturdier, with a better developed root system and significantly greater capacity for withstanding gray mold fungus (Barka et al. 2000). Similarly, banana plantlets treated with endophytic Pseudomonas and Bacillus species showed improved vegetative growth, physiological attributes and strong defense against bunchy top diseases in the field (Kavino et al. 2007, 2010). Utilization of grafted tomatoes on resistant root stocks have also been highly successful in controlling soilborne diseases where good combinations of root stocks and scions were established. For example, when Rivard and Louws (2008) grafted ‘German Johnson’on ‘Maxifort’ rootstock, no symptoms of Fusarium wilt were seen, but plants with ‘Robusta’ rootstock had an intermediate level of disease (29%).
Similar to disease incidence, yield can also be impacted by rootstock-scion combination and soil characteristics of a location. In the multi-site study Rivard and Louws (2008) found that ‘Maxifort’ rootstock significantly increased yield in one location but ‘Maxifort’ and ‘Robusta’ rootstock did not consistently impact yield at the other two locations. However, few studies compared efficacy of biofumigation, BCA and grafted plants in managing soilborne diseases such as Verticillium wilt on heirloom tomato.
Major objectives of the project were to:
- Assess the efficacy of soil treatment with bio-fumigants and biological antagonists in mitigating soilborne disease severity;
- Assess the usefulness of grafted tomato in managing wilt disease;
- Measure yield advantage due to treatment of soil with potential bio-fumigant and biological antagonists;
- Disseminate the technology to organic growers and small farmers through individual communication, grower meetings and conferences, annual field day events, newsletters and cooperative county extension programming.
Determining efficacy of soil treatment with bio-fumigant and biological antagonists:
Both trials had six different treatments that included 1) ‘Caliente’ mustard cover crop incorporated in the soil; 2) Mustard meal; 3) Serenade soil (QST 713 strain of Bacillus subtilis); 4) Prestop (Gliocladium catenulatum); 5) Grafted tomatoes; and 6) Non-treated check. Treatments were replicated four times in a randomized complete block design with each replicate plot consisting of 30 ft long and 5 ft wide plastic mulched drip irrigated row. Treated plots were separated by a 5 ft length of non-treated bed within the bed and single non-treated beds between beds. Vascular wilt susceptible heirloom tomato (Solanum lycopersicum) cv Mortgage lifter was included in the study.
This involved the seeding of ‘Caliente mustard seed’ in the early spring with later incorporation in the soil at the flowering stage which was conducted in two locations in 2015. In another location, we did seeding in the late summer of 2015, incorporated the biomass in late fall and prepared the plots for spring 2016 planting (WVU certified organic farm in Morgantown and Shafer heritage farm near Bruceton Mills, WV) where we could reproduce the results of 2015 growing season with minor deviations.Two separate field trials were conducted in consecutive two years 2015 and 2016 to determine the efficacy of biological control agents, bio-fumigants and grafted plants in relation to non-treated check.
i) Mustard cover crop plot establishment and soil incorporation: Field plots were disked and compost was applied @10 ton/ha in the early spring. Plots were marked based on the random distribution of the treatments in each of the four blocks. ‘Caliente’ mustard seeds were seeded in plots that were marked to receive cover crop bio-fumigation treatments on April 29, 2015. Early spring rain and high soil moisture prevented timely (i.e. first week of April) land preparation and seeding. Seeding rate was 400 lb/acre that is normally used for cover crop in order to add biomass in soil and improve soil health. As these plants reached flowering stage (≥75%), these were mowed with a flail mower to macerate the tissues so that glucosinolate and mirosinase could react and produce isothiocyanate (ITCs). Flail mowed mustard residues were then immediately roto tilled with a walk behind roto tiller to bury under the soil followed by setting drip irrigation line in the middle of the bed. Plots were immediately covered with plastic mulch and irrigated through the drip line to bring the soil moisture to field capacity on June 25, 2016. Mustard meal was applied @ 4000 lb/acre to the plots that were marked to receive this treatment by random distribution on the same day prior to setting drip irrigation and covering with plastic mulch. Similar to mustard cover crop, mustard meal was also roto tilled thoroughly to mix with the soil followed by drip irrigation to expedite microbial degradation and activate release of ITCs. Holes were dug through plastic mulch ten days after incorporation of mustard cover crop and mustard meal to ensure release of residual toxic gas. Seedlings were transplanted in the holes 48 h after digging holes through the plastic.
ii) Treatment of seeds and plants with biological control agents: The selected biological control agents Bacillus subtilis strain QST 473 and Gliocladium catenulatum were used as the formulated product Serenade soil and Prestop, respectively. Required number of tomato seeds were treated separately with Serenade soil and Prestop prior to seeding @ 5 oz/lb seed prior to seeding in planting mix in plastic trays in the greenhouse. Organic planting mix (Johnny’s) was also treated separately with the products @ 1% (v/v) prior to seeding. Six-week-old seedlings were transplanted in the field plots. Planting holes were sprayed with 1% suspension of products just prior to planting of the seedlings up to field capacity. Plants in these two treatments were also irrigated manually twice with suspension of the biological control agents within first 30 days of transplanting.
iii) Grafting tomatoes: In order to have grafted plants ready and plant at the same time in the field together with other treatments, seedlings from Maxifort and ‘Mortgage Lifter’ were grown in the greenhouse. Maxifort was used as the rootstock and ‘Mortgage Lifter’ as the scion. Maxifort seeds were seeded 3 days later than ‘Mortgage Lifter’ to achieve equal stem diameter and plant height for both at the time of grafting. Four-week-old seedlings were used for grafting following the protocol described in grafting guide (Kleinhanz et al. 2015). Briefly, a humid chamber measuring 6’ long, 3’ wide and 4’ high was prebuilt on the greenhouse bench with pvc pipe and plastic to make it leak proof and maintaining high humidity inside. In order to block light partially, top of the chamber was covered with black plastic. Four-week-old ‘Mortgage Lifter, scion was grafted on resistant root stock following slice grafting and using 2.5 mm silicone clips (Johnny’s selected seeds, Winslow, ME 04901). Grafted plants were immediately moved inside the partially shaded humid chamber at 75oF. Two small humidifiers (Crane Drop Ultrasonic Cool Mist Humidifier) were used in two sides of the chamber to raise the humidity close to 100% prior to moving grafted plants inside the chamber. This level of humidity was maintained for the first 7 days and lowered gradually after that. Humidifier was kept running round the clock or as needed until day eight when black turf was removed and plants were exposed to light with a photo period of 16 h/day. Plants were taken to the field 10 days after grafting and planted carefully to keep the graft joint above the soil line.
v) Inter cultural operations and harvest: Plants were irrigated through drip tape except application of biocontrol agents’ suspension manually around the plant with a watering can. Plants were trellised following ‘Florida weave system’ with stakes and twines. Foliar applications of fixed copper were made three times in 2015 and twice in 2016 to suppress foliar diseases (early blight, Septoria leaf spot). Tomatoes were harvested weekly from each replicate plot of each treatment and weighed for only six weeks in 2015 due to late planting. Final harvest included all tomatoes including any green or unripe tomatoes mostly on grafted plants and a few from other treatments. Cumulative total yield/ 3 plant was calculated for each treatment and replicate. However, in 2016 only ripe tomatoes were harvested for six weeks during harvest time. An economic analysis for yield advantage was conducted by taking the cost of each treatment/acre. Price of tomato/lb was considered from local farmers market where organic growers generally sell their produce.
vi) Determination of plant vigor: Thirty five days after planting, plants were visually assessed for expressed vigor on a 1-5 scale, where 1=plants are surviving but did not show any additional growth since planting and 5=plants showed enhanced vegetative growth compared with regular plant growth of the same age. Each plant was assessed again 60 days after planting on the same scale that specifically developed for ‘Mortgage Lifter’. Plant height and stem diameter was also considered and compared among treatments while determining plant vigor.
Data collection and analysis: Plots were scouted weekly for wilt symptom or stunting and foliar symptom for Verticillium or Fusarium root infections. Data were collected on the incidence and severity of wilt 35 days, 60 days after planting in the field and end of the season prior to frost kill of plants. Severity of Verticillium wilt was evaluated on a 0-5 scale where 0=no disease symptom; 1 = 1 to 5% of leaves with V shaped symptom; 2 = 6 to 10% of leaves with V shaped symptom; 3 = 11 to 25% of leaves with V shaped symptom; 4 = 26 to 50% of leaves with V shaped symptom and some wilting of the plant; 5 = 51 to 100% of leaves with V shaped symptom and totally wilted plant. Disease index for each replicate plot was calculated from number of plants with symptom and recorded severity on each plant. The linear mixed model was used to conduct the analysis of variance in SAS (SAS institute, Cary NC). As Verticillium wilt index from 60 days after planting and end of the season wasn non-significant, 35 days leaf symptom based data are presented. Total cumulative tomato yield was presented as means of 4 replicates/treatment and 3 plants/replicate.
Seed, planting mix treatment and grafting: Seed and planting mix treatment with formulations of biological control agents to facilitate early colonization of root system slightly (10%) reduced seed germination in 2015. However, 15 days after germination, seedlings from treated seeds showed enhanced vigor compared with non-treated (data not shown). Reduction of formulation rate from 10 oz/gallon to 1 oz/gallon (formulation/mix) in 2016 for planting mix treatment alleviated seed germination problem but yet enhanced seedling vigor, which was clearly visible 15 days after germination. Pasteurization of planting mix prior to seeding further enhanced seed vigor compared with non-pasteurized and non-treated (Fig. 1). Grafting ‘Mortgage Lifter’ on Maxifort root stock did not show any incompatibility and had above 90% success rate in both years. Grafted plants healed within 10 days and were ready for field transplantation. Although grafting and healing process delayed establishment of grafted plants in the field by 10 and 7 days in 2015 and 2016, respectively, these plants outgrew non-grafted plants within a month of planting.
Fig 1. Heirloom tomato variety ‘Mortgage Lifter’ seeds were seeded in autoclaved (left) and non- autoclaved amended planting mix; photos were taken 25 days after seeding
Plant vigor after field set: Thirty five days after transplanting of seedlings in the field in 2015, grafted plants showed significantly higher vigor compared with non-treated control. Mustard cover crop and mustard meal treatment also had higher plant vigor but it was not statistically different than that of non-treated control. Plants treated with biological control agents Serenade soil (Bacillus subtilis) and Prestop (Gliocladium catenulatum) showed enhanced vigor during greenhouse transplant production stage. However, after field setting, these plants did not continue adding proportional vigor to supersede grafted plants. Estimation of vigor 60 days after planting indicated that all treatments had significantly higher vigor compared with non-treated control, grafted plants being the highest (Fig. 2). Similar vigor enhancement was observed in 2016 as well with minor differences. Despite a little slow recovery and establishment of grafted plants in the field, these plants showed highest vigor within 40 days of field set followed by mustard cover crop bio-fumigation. Despite early plant vigor enhancement by biological control organism G. catenulatum, there was no significant vigor differences compared with control at the end of the season.
Fig 2. Plant vigor estimated 60 days after field set of transplants in different treatments compared with non-treated control on a 1-5 scale. Vertical bars with same letter on the top are not significantly different based on Fisher’s protected LSD test (p=0.05).
Disease symptom on lower leaves: Within 35 days of transplanting tomato seedlings in the infested soil, typical v shaped lesion indicative of leaf symptom of Verticillium wilt started showing up on the lower leaves. All treatments had significantly lower number of symptomatic leaves compared with non-treated control (Fig. 3). Despite early symptom appearance on lower leaves, number of wilted plants remained low in 2015. Only one plant from each of Serenade soil, Prestop and non-treated control fully wilted during second week of harvest. However, plants in the non-treated check and biological control treatment had a wilty appearance during the late summer hot and dry weather condition and the differences among the treatments were clearly visible.
Fig. 3. Verticillium wilt severity as expressed on lower leaves and assessed 35 days after transplanting. Vertical bars with same letter on the top are not significantly different based on Fisher’s protected LSD test (p=0.05).
Yield advantage .
Four of the five treatments resulted in significant yield increase in 2015 compared to plants grown in infested soil without any treatment. This included plants grown in soil treated with (a) mustard meal; (b) mustard cover crop incorporated in soil followed by covering with plastic mulch; c) serenade treatment of seed, planting mix, and planting holes; (d) prestop treatment of seed, planting mix, and planting holes; (e) grafted plants. Among these four treatments, grafted plants produced the highest yield, which was significantly higher than any other treatments and non-treated control. Other three treatments (serenade, prestop and mustard meal) resulted in similar fruit yield but significantly higher than non-treated control. Grafted plants produced 20 lb tomatoes/plant compared with only 11 lb in non-treated check. The difference was higher in 2016 as grafted plants produced 22 lb tomatoes/plant compared with 11 lb in non-treated check (Fig. 4). There was no significant difference in yield between mustard cover crop treatment and non-treated plants in 2015 likely due to escape of gas through holes in plastic mulch caused by stumps from of mustard plants. However, in 2016, all treatments including mustard cover crop significantly increased yield compared to non-treated control likely due to additional care in stump removal, thorough incorporation in soil and keeping plastic mulch leak proof. Serenade treatment was more effective than Prestop in both years and the yield difference between these two treatments was not statistically different. Most interesting and noteworthy finding in 2016 was that both mustard meal and mustard cover crop treatments produced comparable results. Both treatments had exactly the same total amount (16 lb) of tomatoes/plant.
Fig. 4. Average tomato yield/plant from four replicated plots (n=12) in treated and non-treated plots in 2015 and 2016 field trials.
Management of soil-borne diseases is often difficult due to the persistence of the pathogen in soil and plant debris from one year to another. Use of fungicides and fumigants especially methyl bromide alternatives do not always provide expected control that used to be obtained with methyl bromide (MB). Despite good efficacy against soilborne pests, MB was banned under the Montreal protocol (Minuto et al. 2006; Omar et al. 2006) due to its ozone-depleting properties. In addition, use of chemicals for soil fumigation is not feasible to small and organic growers due to cost, EPA regulations, and concerns about food safety, environmental quality and pesticide resistance, which have dictated the need for alternative pest management techniques. Apart from having significant negative effects on environment and partial control of harmful organisms, fumigation with methyl bromide alternatives such as Telone C17/35, Pic-clor 60, Vapam etc. can also represent a significant variable cost (costing approximately 2,000 to 3,000 $ ha-1, depending on the product and system used). The potential to replace chemical soil disinfestation with the soil amendment-based protocols shown in this study could therefore provide both environmental and economic benefits to farmers. However, refinement of the protocol and more result demonstration to growers would be necessary to increase adoption of these alternative methods. Many growers in the Mid-Atlantic region practice cover crop for nutrient management and adding biomass in the soil. Minor changes in the cover crop selection, incorporation and use of turf to trap gas can provide pest management benefit without any significant amount of additional cost. In this study, both mustard meal and mustard cover crop biogumigation provided significant yield advantage in 2016 while only mustard meal provided similar yield advantage in 2015. The inconsistency of the result between these two years indicated that both right incorporation of biomass and trapping gas from their degradation plays an important role in producing positive results. In the first year of the study, stump removal was incomplete, which poked plastic mulch and most likely released ITC as the gas formed. Due to extra care during flail mowing and roto tilling of mustard biomass there was no remaining stump to poke the plastic mulch that may have ensured entrapment of gas and better management of soil-borne pathogens especially Verticillium spp. The disease suppressive effect of incorporating Brassica green manures, crop residues and/or processing waste into soil has also been tested extensively, and the efficacy was linked to their glucosinolate contents (Sarwar and Kirkegaard 1998; Kirkegaard and Sawar 1998; Xiao et al. 1998; Matthiessen and Kirkegaard 2006). ‘Caliente 199’ is known to contain high glucosinolate and the contents reach the highest level during full bloom. Many reports indicated that hydrolysis of glucosinolates present in Brassica tissue result in the release of isothiocyanate, which can provide biofumigation effect on a range of soil-borne pathogens (Morra and Kirkegaard 2002; Matthiessen and Kirkegaard 2006). The use of Brassica tissue based organic matter inputs were also found to result in additional suppressive effects compared to other organic matter inputs such as green waste or cattle manure compost (Matthiessen and Kirkegaard 2006).
Another alternative to fungicides is biocontrol with beneficial microorganisms (Weller 1988; Van Loon 2007). In the recent decades, many microorganisms have been tested for their capacity to suppress soil-borne pathogens in agriculture (Siddiqui and Shakeel 2007, Li et al. 2008). However, microorganisms that show biocontrol potential during in vitro tests and/or in bioassays often exhibit inconsistent behavior under field conditions. This has been the major impediment to large-scale use of biocontrol agents (BCA) in agriculture (Cook and Baker, 1983; Weller, 1988), and warranted microclimate based determination of efficacy for a specific host-pathogen combination. In the current study, we tested two formulations of biological control agents in vivo for managing wilt diseases on heirloom tomato cv. Mortgage Lifter for consecutive two years in an artificially infested certified organic farm that belongs to West Virginia University. In 2016, we also used a naturally infested farm and obtained similar results. Consistent with the recommendations for biological control to occupy root surface of a plant prior to pathogen infection, we applied BCAs as seed and planting mix treatment as well as in the planting holes and twice during manual watering with a watering can. Antagonistic micro-organisms have been successfully used to reduce damage caused by fungal pathogens in many horticultural and agronomic crops including tomato for protection against Alternaria solani (Babu et al. 2015), in strawberry for protection against Verticillium dahliae (Berg et al. 2005). It was reported that in vitro-beneficially-bacterized plantlets of grapevine not only grew faster than non-bacterized controls but also were sturdier, with a better developed root system and significantly greater capacity for withstanding gray mold fungus (Barka et al. 2000). Similarly, banana plantlets treated with endophytic Pseudomonas and Bacillus species showed improved vegetative growth, physiological attributes and strong defense against bunchy top diseases in the field (Kavino et al. 2007, 2010). Seed treatment or augmenting beneficial microbial population in soil was also found to reduce seedling mortality from soilborne diseases (Rahman and Punja, 2005). In addition, biological agents such as Trichoderma, Serratia, Pseudomonas, different plant extracts and bio-fumigation are some of the alternative strategies that were found effective in variable levels in reducing the number of microsclerotia or wilt symptoms on multiple crops (Berg et al. 2001; Kurze et al. 2001; Steffek et al. 2006; Tahmatsidou et al. 2006; Meszka and Bielenin 2009). Despite facilitation of early colonization of root system in the current study, both biocontrol agents showed only moderate but significant efficacy in reducing disease severity, improving plant vigor and yield. An interesting observation from this study is that treatment of autoclaved planting mix with beneficial microbes significantly improved plant vigor compared with non-autoclaved likely due to higher colonization of root system in the absence of competition from other microbes that otherwise would have been in the mix. Any transplant producer in the greenhouse utilizing pasteurized planting mix can easily use this for boosting seedling vigor that can have a profound impact when those seedlings are transplanted in a contaminated soil. We did not add this variable in this study to check if those seedlings could maintain this vigor after field set and up to harvesting of fruit. But similar studies should explore if early enhanced vigor can provide additive effect with other probiotic treatments.
In this study, we used commercially available formulation (Serenade soil) of Bacillus subtilis QST 713. While strain QST 713 of Bacillus subtilis was found effective against tomato wilt diseases caused either by fungal pathogen Verticillium spp. or Fusarium spp. or bacterial pathogen Ralstonia solanacearum, other strains of the same BCA in other studies were found more effective against soil borne disease on tomato. Baysal et al. (2008) tested B. subtilis strain EU07 and QST 713 for controlling crown and root rot disease of tomato caused by Fusarium oxysporum) f. sp. radicis-lycopersici at 106 CFU/ml. Strain EU07 reduced disease incidence by 75% whereas QST 713 reduced only 52% when applied as an inoculant compared with non-treated. This example shows the potential and prospect of biocontrol if right strain can be selected and utilized. However, it is not known at this time, if and when some of these strains will be available to users as commercial products. Potential use and commercialization of Bacillus based products have been discussed by researchers in recent reviews (Rahman 2017) that described the importance of proactive and special methods of application of biocontrol agents in different host-pathogen interactions. Application of these products at growers’ level thus require method demonstration by extension professionals. In our study we achieved efficacy of biocontrol agents similar to biofumigation with mustard cover crop or mustard meal. Although application methods for both bio-fumigation and BCAs have technical components, our method and result demonstration through on-farm trial generated much interest and changed growers’ perception how non-chemical methods work and how much additional efforts are involved with it. However, not much information available on substrate inoculation after autoclaving to facilitate quick rhizosphere colonization in a less competitive environment. More research is needed to unravel the mechanism how inoculation of pasteurized medium enhances seedling vigor and if that vigor could translate to enhanced disease resistance or fruit yield. An added advantage of managing soilborne fungal disease with bacterial biocontrol agent is that conventional growers can combine these agents with fungicides for additive effects as found in other studies (Omar et al. 2006, Sundheim 1986)) Although species of Bacillus have been found effective against soilborne pathogens by many authors worldwide, from the practical point of view it is interesting to note that the application method of the biocontrol agent is a key factor for the efficacy of Bacillus amyloliquefaciens FZB42. An effective suppression of R. solani was found only after two applications of FZB42 before and after transplanting. For the settlement of the inoculated strain in the rhizosphere in a sufficient high number, it might be important that the microflora in the rhizosphere of young plants is stabilized (Berendsen et al. 2012).These studies also identified multiple modes of actions or mechanisms of pathogen suppression either by direct competition for space or nutrients on root surface, production of antibiotic metabolites and volatile organic compounds (VOCs). Raza et al. (2016) found that the VOCs of Bacillus amyloliquefaciens strain T-5 significantly inhibited the growth of Ralstonia solanacearum (causal agent of tomato bacterial wilt) in agar medium and in soil.
Comparison of a few feasible options for managing wilt disease on heirloom tomatoes indicated that grafted tomatoes provided superior resistance and yielded the highest in both years. Many investigators reported that grafting with resistant rootstocks could play an important role in management of soilborne diseases in high tunnel and field tomato production and unraveled the mechanism of resistance (Rivard and Louws 2010; Rivard et al. 2008; Grimault and Prior 1994; Peregrine and Binahmad. 1982, Tikoo et al. 1979). Another advantage of grafted tomato is that the technology can be quickly deployed without causing significant changes in farming operations although it can add some extra cost for transplants. Transplant production cost can be minimized provided small scale producers or organic growers quickly learn the technique for producing transplants on their own. However, selection of root stocks would require some knowledge on the specific organism that may be present in a specific farm. For example, in this study we used Maxifort root stock that has resistance against multiple soilborne diseases such as corky root rot, all races of Fusarium, nematode, tomato mosaic virus in addition with the major target disease Verticillium wilt but it doesn’t possess resistance against bacterial wilt. If major problem is bacterial in a field, one should use either RT-04-105-T or RT-04-106-T root stock. In this study, we compared multiple nonchemical strategies to manage soilborne diseases in heirloom tomatoes under certified organic production system and demonstrated results to potential users of these technologies. Yield increase (up to 85%) in grafted tomatoes over non-treated check is a clear indication that grafting will be an important component together with a few others for U.S. tomato growers that are seeking nonchemical strategies to manage soilborne diseases.
Growing heirloom tomatoes in an organic system can be challenging in the absence of an effective rotation plan. Heirloom tomatoes lack resistance against biotic stress and can easily be decimated by soilborne diseases. Although many different efforts have been made separately to evaluate efficacy for controlling diseases in organic system, we assessed a set of diverse methods in an experimental setting and obtained results in a reproducible manner that growers can adopt based on their situation. While utilization of tomatoes grafted on resistant root stocks can be easily established in contaminated soil, method and timing of biocontrol agents’ application for the same purpose seem to play an important role and needs to be strictly followed. Methods that ensure early colonization of root system is more likely to prevent infection by pathogenic microbes in the field. As both mustard cover crop and mustard meal produced comparable results, mustard meal may be preferred by growers due to the ease of application and avoiding the early season cover crop growing operation especially in an area where frost free growing season is relatively short. However, use of grafted plants should be the best method for preventing soilborne disease and boost yield for economic benefit.
Education & Outreach Activities and Participation Summary
Results from this demonstration and research were used for presenting two webinars organized by Northeast IPM center. One Plant Disease Management Report was published in American Phytopathological Society journal. I also made five presentations in Extension small farm conference, field days and divisional meetings of American Phytopathological Society. One poster presentation was also made at the APS national meeting in Tampa, FL and abstract was published. Grower cooperators gave talk during the field days on the benefit of the project to other growers who showed interest in adopting the technology. One peer reviewed full length manuscript is in the process of submission.
Soil-borne diseases on tomatoes; Bio-fumigation with “Caliente mustard”; Grafting tomatoes and bio-control.
1) In two locations where 2016 trials were conducted, we could reproduce the results of 2015 growing season with minor deviations. 2)Due to following strict guidelines for biofumigation, ‘Caliente’ cover crop treatment out-yielded mustard meal treatment. 3) Both biocontrol products were weakly effective but Serenade soil was more effective than Prestop as revealed by higher tomato yield. 4)Results were presented at the 2nd NEIPM center webinar and will also be presented at the upcoming small farm center meeting. 5)In-season outreach activities included field day and local heirloom tomato growers. 6) Altogether we could reach out to 200 tomato growers, 35 of them adopted grafted tomato and bio-fumigation for managing soilborne tomato diseases.