An on-farm study was conducted to address weed and root-knot nematode (RKN) infestations in an organic high tunnel in central Florida. A randomized complete block design with four replications was used to assess the effect of off-season management with four cover crop treatments: Iron Clay (IC), US-1136, US-1137, and US-1138 cowpeas (Vigna unguiculata (L.) Walp.), a soil solarization treatment (SS), and US-1137 cowpea followed by soil solarization (US-1137+SS) in comparison with a weedy control (W), a weed-free (WF) treatment. A fall cucumber (Cucumis sativus L.) crop was followed by a winter fallow, then a romaine lettuce crop (Lactuca sativa L.). The cowpea treatments did not differ in the amount of biomass produced. The dominant weed group by mass consisted of grasses and all off-season fallow treatments resulted in lower grass and total weed biomass than the weedy control. Although the quantities of marketable cucumbers with the W and WF treatments were lower than with the summer fallow treatments, no difference in marketable cucumber weight was apparent. Both the W and the WF treatments had the highest marketable lettuce weights compared to the other treatments. The US-1137+SS treatment did not improve cucumber or lettuce yields over SS alone. Root-knot nematode populations in December after cucumber and in April after lettuce averaged over treatments were 5 per 100 cc and 7 per 100 cc, respectively, compared to the baseline 82 per 100 cc obtained in July prior to the off-season treatments. Root-knot nematode galling indices in cucumber and lettuce plants also were not different from those with the weedy control. Therefore, decreases in root-knot nematode occurrence were not the result of suppression of weeds with summer or winter fallow treatments.
Earliness, extended production season, increased productivity, and reduced pest and disease problems are some of the advantages of using high tunnels for crop production over open field production (Lamont et al., 2003; Wells and Loy, 1993; Wells, 1996). Because of the season extension capability, high tunnels allow growers to offer a wider range of produce and obtain premium prices for produce that is in scarce supply during the winter. Although estimated to be only about 100 acres in 2009, high tunnel acreage in Florida has been growing rapidly and is currently estimated to be 1500 acres (Burfield, 2013).
Despite the many benefits, weed and root-knot nematode (Meloidogyne spp.) management have proven to be challenging for high tunnel growers (Larson, 2009; Oloo et al., 2009; Sanchez, 2008; Santos et al., 2008). Whereas herbicides and soil fumigants can be used in conventional high tunnels, organic growers have fewer available options. Therefore, ensuring that organic growers have effective pest management measures is a priority. Nonchemical alternatives to soil fumigants designed for managing soil borne pests in organic high tunnels can also be applicable to conventional high tunnels in urban and peri-urban areas where buffer zones preclude the use of soil fumigants. In considering nonchemical approaches with applicability for organic high tunnels, we hypothesized that a root-knot nematode-resistant cover crop and soil solarization could be effective tools for suppressing weeds and root-knot nematodes during the summer off-season.
Cowpea (Vigna unguiculata (L.) Walp.) is a legume that is widely recognized for its ability to serve as a cover crop and green manure and is well adapted to high temperatures and sandy soil. In addition to these attributes, some cowpea cultivars exhibit vigorous growth and rapid canopy closure that are effective for suppressing weeds. Iron Clay cowpea has also shown potential for allelopathic inhibition of weeds (Adler and Chase, 2007) and for effectively suppressing root-knot nematodes (McSorley et al., 1999).
Whereas soil solarization has been shown to be an effective means of suppressing weeds, the control of root-knot nematodes has been less consistent. Unlike nematode control, excellent weed suppression has been reported in high tunnels with soil solarization (Larson, 2009; Medina et al., 2009; Santos et al., 2008). In open-field studies, after an initial reduction in soil infestation by root-knot nematodes with soil solarization, populations may recover when subsequent susceptible crops are planted (Wang and McSorley, 2008). These authors proposed that improved soil heating at greater soil depths may extend the suppressive effects of soil solarization. Although Larson (2009) demonstrated enhanced soil heating and more hours of lethal soil temperatures when soil solarization is performed in tunnels than in adjacent uncovered fields, Santos et al. (2008) reported that root-knot nematodes in a hot pepper crop had not been effectively controlled by soil solarization conducted in the high tunnel prior to the crop. A combination of measures can sometimes be more effective than either measure used by itself. Wang et al. (2006) found that persistence of control with a cowpea cover crop followed by soil solarization was equivalent to that obtained with methyl bromide soil fumigation.
The objective of this on-farm study was to evaluate cover cropping and soil solarization for the suppression of weeds and root-knot nematodes during the off-season in an organically managed high tunnel.
The experiment was conducted at the Organic Country Farm, Osceola County, Florida with soil type Cassia fine sand in an organic high tunnel with an east-west orientation, in an area measuring 24 ft x 200 ft. The experimental design was a randomized complete block with eight treatments and four replications. The off-season summer treatments consisting of four cowpea cover crops, soil solarization alone, a cowpea cover crop followed by soil solarization, a weed-free treatment managed with black polyethylene mulch, and a weedy control in which no weed management was performed are summarized in Table 1. Treatments were initiated in 3-ft x 20-ft plots on 18 Jul. 2012. The cowpea cover crops were seeded by hand-broadcasting at a seeding rate of 25 seeds m-2 and plots were raked to incorporate the seeds. Overhead irrigation was provided as needed to promote cover crop growth. The US-1137+SS treatment was string mowed on 15 Aug. 2012 and transparent polyethylene film was installed for soil solarization over the crop residue. The 8-wk cowpea cover crops were terminated by incorporating with a disk harrow on 12 Sept. 2012. Solarized plots were not tilled.
Cucumber (Cucumis sativus L.) seeds of Taiwanese origin provided by the grower were seeded in polyethylene trays at the Organic Country Farm on 19 Sept. 2012. Seedlings were transplanted in a single row with a between plant spacing of 3 ft on 3 Oct. on white-on-black polyethylene mulch. Cucumber plants were thinned and dead transplants were replaced on 10 Oct. 2012. MicroSTART60 (Perdue Agrirecycle LLC, Seaford, DE) 3-2-3 fertilizer was applied preplant at a rate of 62, 41, and 62 lbs N-P-K acre-1 respectively. The crop was also fertigated for 1 h daily with a 2.8-1.5-1.0 N-P-K solution (Converted Organics Inc., Boston, MA), which was increased to 2 h daily during fruiting using two drip tapes per bed.
Winter fallow treatments (Table 1) were initiated on 15 Jan. 2013. They were the same as summer treatments with the exception that the soil solarization treatment was replaced with sunn hemp (Crotalaria juncea L.) (Kauffman Seeds Inc., Haven, KS) seeded at 50 lbs acre -1; and US-1137+SS replaced with buckwheat (Fagopyrum esculentum Moench) (Pennington Seed Inc., Madison, GA) seeded at 90 lbs acre -1. Winter cowpea treatments were seeded at a density of 50 seeds m-2. All winter and summer applications of legume cover crops were inoculated with cowpea type Rhizobium sp. (INTX Microbials, LLC, Kentland, IN) before seeding and were watered using overhead irrigation. The winter fallow was terminated using a disk harrow at 6 weeks after planting.
‘Coastal Star’ romaine lettuce (Lactuca sativa L.) (Johnny’s Selected Seeds Co., Fairfield, ME) was seeded in 128-cell trays (Speedling Inc., Ruskin, FL) on 17 Jan. 2013 under greenhouse conditions using Fafard Organic Potting Mix (Conrad Fafard Inc, Agawam, MA). The seedlings were fertilized with Neptune’s Harvest Fish/Seaweed Fertilizer 2-3-1 (Ocean Crest Seafoods Inc., Gloucester, MA). The preplant fertilizer in the high tunnel at St. Cloud was McGeary’s 6-0-4 (Lancaster, PA) at a rate of 200 lbs N acre-1. Lettuce plants were transplanted on 1 Mar. 2013 onto raised beds on bare soil in double rows spaced 18-in apart and an intra-row spacing of 10 in. Plants were drip irrigated with two drip tapes per bed. The crop was harvested 6 weeks after transplanting and number and weight of heads were determined.
Just prior to termination, weed and cover crop biomass were sampled on 12 Sept. within a 0.5 m x 0.5 m quadrat that was randomly positioned within each plot and dried in a forced air oven at 65 °C. Weeds were separated into the following categories: broadleaf, grass, sedge, and Commelina benghalensis. Total and marketable number and fresh weights of cucumber were recorded after each of 14 harvests conducted between 13 Nov. and 3 Dec. 2012.
Baseline nematode populations were determined prior to initiating off-season fallow treatments by taking five soil cores per plot on 18 July 2012. Plots were again sampled at the end of the cucumber crop on 12 Dec. from the root zones of four cucumber plants within each plot. Root-knot nematode assessment was performed by Waters Agricultural Laboratories, Inc. (Camilla, GA). The roots of one plant cucumber plant per plot were rated for nematode galling (Bridge and Page, 1980) on 12 to 13 Dec. 2012. For the lettuce crop, soil for root-knot nematode counts was aggregated from 4 soil cores taken within the root zone on 10 Apr. 2013. Two lettuce plants per plot were rated for nematode galling on 11 Apr. 2013.
Analysis of variance was conducted using the GLM procedure of SAS version 9.2 (SAS Institute, Inc., Cary, NC). Planned comparisons were performed using orthogonal contrasts with a 5% level of significance.
Various stages of the off-season management of the high tunnel are presented in Fig. 1. Figures 1c and 1d illustrate the profound differences in weed infestation that occurred with the off-season fallow treatments. The cowpea cover crops IC, US-1136, US-1137, and US-1138 resulted in 131, 115, 153, and 95 g m-2 of dry biomass, respectively. However, no significant cultivar difference was observed. The predominant weed type present in the weedy control by biomass was grasses (Table 2) with fall panicum (Panicum dichotomiflorum Michx.) being the primary species (data not shown). The highest total weed biomass (334 g m-2) was obtained with the weedy control (Table 2). All of the off-season fallow treatments suppressed total weed biomass and grass weed biomass to levels that were significantly lower than that with the weedy control. The four cover crops were equally effective in suppressing weed biomass to less than 70 g m-2. However, complete weed suppression was obtained only with the SS, US-1137+SS, and weed-free treatments.
Although there were also numerous spiny amaranth (Amaranthus spinosus L.) plants (data not shown), broadleaf weed species comprised only a small proportion of the weed biomass because spiny amaranth was effectively suppressed due to incidental biological control by armyworms (Spodoptera sp.). Therefore, the plants were largely reduced to short, leafless stalks. Cowpea cover crop treatments did not suppress Commelina benghalensis L.
No response to summer fallow treatments was obtained for total number of cucumbers and total weight of cucumbers harvested (Table 3). The number of marketable cucumbers obtained with the fallow treatments was on average higher than the number obtained with the weedy control and with the weed-free fallow. However, this difference was apparent for marketable cucumber weight. Cucumber marketable yields with US-1137 were comparable to those obtained with IC. Additionally US-1137 resulted in more marketable cucumbers than US-1136 and more marketable weight than both US-1136 and US-1138. The use of US-1137 in combination with soil solarization did not improve cucumber fruit weight over that obtained with SS solarization alone.
There was no significant effect of off-season fallow treatment on total and marketable numbers of lettuce heads (Table 4). The highest total and marketable lettuce weights were obtained with the weedy and weed-free treatments. However, we are not confident that this difference was in response to off-season treatment since lettuce sizes were smaller in plots where plants were located further away from the drip tape.
The baseline root-knot nematode population in July was 82 per 100 cc, which was higher than the December mean of 5 per 100 cc (P = 0.0008) [data not shown]. The off-season fallow treatments had no effect on the incidence of root-knot nematodes in the soil in December (Table 5). Additionally, galling of the cucumber roots with the off-season fallow treatments were either not significantly different or were higher than the galling observed with the weed-free and the weedy control treatments. In future studies, sampling for changes in nematode populations with off-season management is recommended to occur immediately after the treatments in addition to after the subsequent cash crop. This will allow the distinction between the effects of the off-season treatments and other factors such as the cash crop itself and decreasing soil temperatures on root-knot nematode populations. A likely scenario may be that fall panicum in the weedy control treatments may have effectively suppressed root-knot nematodes in that treatment. Kaur et al. (2007) reported that fall panicum was a nonhost for five Meloidogyne species. Also, cucumber is considered to be resistant to M. hapla and breeder’s seed of cucumber germplasm resistant to other Meloidogyne species has been available since 1996 (Walters and Wehner, 1996). Therefore, other explanations of these results are that the infestation may be primarily M. hapla or the Taiwanese cucumber cultivar may have been bred for Meloidogyne spp. resistance.
Analysis of variance indicated that there was no significant effect of off-season fallow on galling of lettuce roots. Planned comparisons were similarly nonsignificant except that more galls occurred with the US1137+SS/buckwheat treatment than plants within the SS/sunn hemp treatment. This could be due to either to the poor stand of buckwheat that was obtained (data not shown) or to susceptibility of buckwheat to root-knot nematode.
In conclusion, cowpea cover crops and soil solarization were effective measures for suppressing weed growth in an organic high tunnel during the summer off-season. Soil solarization was more effective than cowpea cover crops for weed suppression. Although the US-1137+SS suppressed weeds more effectively than US-1137 by itself, the yield with US-1137 alone was higher than the yield with the combined treatment. Lower root-knot nematode populations were obtained in December and April after the cucumber and lettuce crops than in July prior to the application of the off-season summer treatments that could not be attributed to the fallow treatments. However, root-knot nematode populations and galling on cucumber roots with off-season treatments did not differ from the results obtained with the weedy control.
- Table 2. Cowpea and weed dry biomass at eight weeks after initiation of the offseason fallow treatments.
- Table 3. Total and marketable cucumber yields from the center four plants within each plot in response to summer off-season treatments.
- Table 4. Total and marketable lettuce yields in response to summer and winter off-season treatments.
- Table 5. Root-knot nematodes in soil and galling on cucumber and lettuce roots at final harvest in response to off-season fallow treatments.
- Fig 1. High tunnel (a) prior to initiation of study, (b) at installation of treatments, (c) with cowpea and soil solarization at 4 weeks, (d) with weedy and weed-free treatments at 4 weeks, and (e) after termination of the summer fallow treatments.
Educational & Outreach Activities
Oral presentation and abstract
Zambon, F.T., C.A.Chase, and X. Zhao. 2013. Weed and Root-Knot Nematode Management in an Organic High Tunnel. Florida State Horticultural Society meeting, Sarasota, FL. (Abstract located at http://fshs.org/wp-content/uploads/2013/01/2013_Abstracts.pdf).
Poster presentation and abstract
Zambon, F.T., C.A.Chase, and X. Zhao. 2013. Cultural and physical weed management for root-knot nematode suppression in organic high tunnels. Weed Science Society of America meeting. Baltimore, MD. (Abstract located at: http://wssaabstracts.com/public/17/abstract-45.html).
Zambon, F.T., C.A.Chase, and X. Zhao. 2013. Weed and root-knot nematode management in an organic high tunnel. Florida State Horticultural Society Proceedings 126 (submitted).
- An undergraduate intern from University of São Paulo, Luiz de Queiroz College of Agriculture, Piracicaba/SP, Brazil was hosted and trained during this project.
Soil solarization was an effective method of suppressing weeds in a high tunnel in central Florida during the summer off-season.
We determined that it was possible to successfully grow cowpea cover crops in a high tunnel during the summer when air temperature and humidity are very high.
Three recently released germplasm lines from the USDA-ARS in Charleston, South Carolina (Harrison et al. 2010b) appeared to be as well adapted as the standard Iron Clay cowpea for use in high tunnels in summer as a cover crop. All three lines are reported to be resistant to southern root-knot nematodes and have the added advantage of being soft-seeded (Harrison et al. 2010a). As a result, the new cowpea germplasm will be less likely to result in volunteers in subsequent crops, which is a limitation of the Iron Clay cultivar.
Root-knot nematode populations with all treatments including the weedy control were an order of magnitude lower with fall cucumber and spring lettuce than in summer before initiation of the study.
The cooperating farmer is already utilizing cowpea as a green manure in his cropping system. This bodes well for the possibility of expansion to off-season practices for weed and root-knot nematode suppression. The presentation of the results at the 2013 Florida State Horticultural Society (FSHS) meeting and publication in the FSHS Proceedings will make the information available to extension faculty. Extension faculty can share with their clientele that cowpea production in the summer in high tunnels is possible despite very high air temperatures and that cover cropping and soil solarization are valid approaches for weed suppression in high tunnels during the summer off-season. However, US-1136, US-1137, and US-1138 are not yet commercially available, so early adopters will have to continue to rely on Iron Clay cowpea.
Areas needing additional study
This type of study will need to be conducted over several years in order to begin seeing the reduction in weeds by depletion of weed seed bank. Soil sampling for nematodes should be conducted at the end of the off-season treatments to allow for distinguishing between the effects of the off-season treatments from environmental effects and the effects of subsequent cash crops. The study was started in July, which resulted in some of the soil solarization occurring during the cooler month of September. The spring high tunnel cash crops are often complete by April or May, so it would be worthwhile to evaluate if soil solarization would be even more effective if initiated in June. Assessing the suitability of additional root-knot nematode resistant cover crops such as sunn hemp in high tunnels is also recommended.
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