Sunn hemp and its allelopathic compounds for vegetable production in Hawaii and beyond

Final Report for SW08-037

Project Type: Research and Education
Funds awarded in 2008: $156,105.00
Projected End Date: 12/31/2012
Region: Western
State: Hawaii
Principal Investigator:
Dr. Inga Zasada
USDA-ARS Horticultural Crops Research Lab
Co-Investigators:
Dr. Koon-Hui Wang
University of Hawaii
Dr. Cerruti R. R. Hooks
University of Maryland
Dr. Ming Li Wang
USDA-ARS, PGRCU
Jari Sugano
University of Hawaii, TPSS
Dr. Mark Wright
University of Hawaii
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Project Information

Abstract:

The overall goal of this project is to improve the use of sunn hemp as a cover crop for soil health, plant-parasitic nematodes and weed management. We completed 10 field trials in Hawaii during this project period to evaluate the benefits of integrating sunn hemp cover cropping (SH) with soil solarization (Sol). A solarization temperature schematic scheme for Hawaii over a one-year period was generated, providing farmers a guide on when to use solarization (April – October) in Hawaii to achieve lethal heat units for nematodes and weeds control. Biomass of sunn hemp varied from 0.73 to 6.67 Mg/ha (1.63 to 14.94 tons/acre) of dry sunn hemp residues over approximately two months of growing period. This variation could be seasonal or due to soil pH. Biomass of five tons/acre (equivalent to 1% w/w) can commonly be achieved if planted between May and August in soil with normal pH. Integration of SH+Sol suppressed weed densities better than SH or Sol alone in the pineapple trial but not in the eggplant and cowpea trials. Weed suppression lasted until one-two months after crop planting. SH suppressed plant-parasitic nematodes most efficiently and consistently. Integrating SH+Sol did not increase the nematode suppressive effect on plant-parasitic nematodes as compared to SH alone unless when the solarization was conducted during late fall to winter. It is possible that high heat from solarization deactivates the SH allelopathic compounds. Based on nematode community analysis, solarization in the summer consistently disturbed soil health. When solarization was conducted during late fall to winter, SH+Sol improved soil health condition better than SH alone. While combination of SH+Sol during the summer is beneficial for weeds suppression, it is not beneficial for nematode and soil health management as compared to SH alone. A series of laboratory assay for allelopathic effect of SH was conducted to resolve some of the questions encountered in field experiments. For best suppression of root-knot nematodes, SH should be planted two to three months to achieve a biomass of at least five tons dry biomass/acre (1% w/w). Leachate from SH materials that are too old (≥ 4-month old) did not suppress root-knot nematodes. Leaf tissue is among the most suppressive component of sunn hemp against root-knot nematodes. Thus, incorporating sunn hemp at vegetative stage will be more effective in nematode suppression than at its mature stage. Sunn hemp residues that were solarized did slightly increase suppressive effect against root-knot nematodes, but not much. Sunn hemp does suppress root-knot nematodes more efficiently than its weed relatives, C. spectabilis and C. retusa. Unlike what had been reported in the literature, current study indicates that SH and C. retusa did not contain monocrotaline but do contain an HPLC product similar to an unusual dehydropyrrolizidine alkaloid. This most probably is responsible for nematode suppressive effect. This project provides growers specific information on when to conduct solarization, when to plant sunn hemp, when to terminate sunn hemp and how to grow sunn hemp in a strip-till cover cropping system followed by mulching which can improve soil health conditions in less than two years. Five scientific publications, five extension newsletters, one extension article, one post card, one video, nine conference presentations and nine field days/workshop events were produced from this project. All participating farmers adopedt sunn hemp cover cropping after the demonstration trial. Two seed producers in Hawaii are selling sunn hemp seeds in 2012 and indicating that the demand is higher than their supply.

Project Objectives:

Our approach was to use sunn hemp (SH) as an interplanted cover crop and green manure, solarization, and combine SH with solarization within the same field for nematode, weed and soil health management. Specific objectives of the proposed research were to: 1. Evaluate the impact of using SH as an organic mulch and green manure, solarization, and SH + solarization on nematode, insect and weed pests during two cropping cycles; 2. Examine how SH and solarization impact soil health, pest and beneficial organisms; 3. Identify compounds in sunn hemp that are toxic to nematodes; and 4. Determine the lethal dosage of SH residue required to suppress nematodes and whether solar heat can enhance its effectiveness.

Sunnhemp Superhero 2 movie

Introduction:

Soil solarization and cover cropping are some of the viable non-chemical approaches for managing soil-borne plant-parasitic nematodes. Previously, it was found that sunn hemp (Crotalaria juncea) suppresses reniform nematodes by multiple mechanisms (Wang et al., 2001). These mechanisms include being a poor host of reniform nematodes (Rotylenchulus reniformis), delaying the development of female reniform nematodes, producing an allelopathic compound that is toxic to the nematode, and enhancing nematode-trapping fungi that could prey on the nematode (Wang et al., 2001). It was thought that when incorporated into soil, sunn hemp residues produce monocrotaline (Crout, 1968), which might be toxic to plant-parasitic nematodes (Rodriguez-Kabana et al., 1992; Rich and Rahi, 1995; Wang et al., 2001). We hypothesized that since R. reniformis can penetrate sunn hemp roots, planting sunn hemp could prevents R. reniformis from entering into its survival stage (anhydrobiotic state). Integration of sunn hemp cover cropping with soil solarization would allow the solarization heat to target the active vermiform stage of the nematode, and thus should be a more efficient nematode management strategy as compared to either method employed alone. In addition, Wang et al. (2006) had demonstrated that soil solarization alone could exert a negative impact on beneficial free-living nematodes, and thus affect soil health. However, sunn hemp cover cropping is well known to enhance soil health (Wang et al., 2011). Thus, the project was to evaluate the effect of sunn hemp cover cropping and the potential of integration of sunn hemp and soil solarization for nematode and soil health management. In addition, previous research by the PI also documented that suppressive effects of sunn hemp cover crops against reniform nematodes are not consistent (Wang et al., 2002). Thus, an additional objective of this project was to understand factors that might affect the allelopathic effect of sunn hemp.

Cooperators

Click linked name(s) to expand
  • Alton Arakaki
  • Steve Fukuda
  • Naresh Chand Pradhan

Research

Materials and methods:
Objectives 1 and 2

Throughout the project period, four field experimental trials were conducted. Additional six demonstration trials were conducted at commercial farms to promote the use of sunn hemp (SH) or solarization (Sol) for nematode and soil health management. Experiment I was conducted at Poamoho experiment station where SH was planted, strip-tilled and served as living mulch in a cucumber agroecosystem. Additional SH living mulch were clipped and used as surface mulch. Plant-parasitic and free-living nematodes were monitored and compared to a bare ground (BG) system. Experiment was repeated in second year. Details of the study were published by Wang et al. (2011). Experiment II was conducted at Khamphoute Farm, Kunia where a formal cover crop trial (SH vs BG) was split into Sol vs non-Sol plots. Soil was solarized for six weeks and eggplants were transplanted. Plant-parasitic, free-living nematodes and weed data were collected. Experiment III was conducted at Whitmore Experiment Station in a pineapple agroecosystem. Nematodes and weed data from four treatments (SH, Sol, SH+Sol, BG) were monitored and compared to that from a neighboring commercial pineapple field (Dole Plantation) throughout one cropping cycle of pineapple (22 months). A detailed description of the experiment is published by Wang et al., (2010). Experiment IV was similar to that in Experiment III, except that cowpea was used as the bioassay host for reniform nematodes instead of pineapple. A detailed description of this experiment is published by a graduate student (Marrahatta et al., 2012).

Objectives 3 and 4

Allelopathic assay: A series of laboratory studies were conducted to understand if the allelopathic effect of sunn hemp against root-knot nematodes, Meloidogyne incognita, was affected by SH stage of growth, tissue components or leachate concentration, and if the sunn hemp residues were solarized or not. Tissues of SH were collected at one, two, three and four months after planting and partitioned into stem, leaves, flowers and roots or remained as whole plant tissues. SH tissues were oven dried, ground into powder and prepared into leachates of 0.1, 0.5, 1.0, and 2.5% (w/w) concentration. Juveniles (J2) of M. incognita were imbibed into designated leachate for 48 hours and transferred to water for another 24 hours to test for number of nematodes revived in water (if the nematode revived, then the suppressive effect is only nematostatic and not nematicidal). Eggs were imbibed in designated leachates for one week and counted for % hatched. In addition, allelopathic effect of SH were compared to its weedy relatives, C. spectabilis (Cs) and C. retusa (Cr). To address the question on whether Sol affected the allelopathic effect of SH, another laboratory experiment was set up to compare allelopathic effect of SH tissues bagged in liter bags buried in solarized or not solarized soil for six weeks in the field. A detailed description of this study was published by Wang et al. (2012) at http://www.ctahr.hawaii.edu/sustainag/news/articles /V12-Wang-allelopathic.pdf. HPLC: To determine if monocrotaline is responsible for nematode suppression in SH tissues, all the SH tissues prepared in the allelopathic assay were subsampled for HPLC analysis. Subsamples of 0.5 g of each of the tissue listed above was imbibed in 20 ml MeOH under 60oC for one hour to elute the tissue content, then vacuumed with rotary evaporator (Sigma). Standard monocrotaline obtained from Sigma was included. Dried pellet was re-suspended using 0.2M HCl and pH was adjusted to 8-10 using ammonia water. The final dried pellet after three times extraction was re-suspended with 20 ml CHCl3 then vacuumed dried. For each sample tube, 10 ml of HLPC mobile phase solution was added to re-suspend the dry pellet and run through a HPLC System LaChrom Elite® (Hitachi) on a Diamonsil C18 (200mm x 4.6 mm, 5 μm) column. The mobile phase solution composed of acetonitrile-0.01 mol-L-1 potassium dihydrogen phosphate-triethylamine (5:95:0.1, adjusted pH to 3.0 with phosphoric acid). HPLC analysis was run at a flow rate of 1.0 ml-L-1 at column temperature of 25 oC, and the detection wavelength for monocrotaline was at 205 nm.

Research results and discussion:
Objective 1 and 2

A detailed result of Experiment I is published by Wang et al. (2011b). In Experiment I, sunn hemp planted in a strip-till cover cropping system followed by clipping as a surface mulch practice (STCC+SM) (Fig. 1) suppressed plant-parasitic nematodes towards the end of Trial I and up to mid-term of Trial II. SH consistently increased (P < 0.05) the abundance of bacterivorous and fungivorous nematodes in both trials, indicating more nutrient decomposition. Although enhancement of omnivorous nematodes were not significant in both years, STCC+SM of SH eventually resulted in significantly (P < 0.05) higher Structure Index in Trial II, indicating a more structured soil food web condition than the BG treatment. SH plots were also associated with higher (P < 0.05) abundance of collembolan and predatory mites. This result is encouraging, as literature revealed that soil health conditions (nutrient enrichment and community structure) usually were not improved until > six years of no-till practice. Although SH did not increase crop yield in Trial I, it significantly increased crop yield in Trial II. The enhancement of soil health within two years of STCC+SM practice might have been attributed to reduction in soil disturbance and the continuous supply of surface organic mulch to sustain higher hierarchy soil fauna. This led to a more structured soil food web and resulted in improved crop growth and yield in the second year of STCC+SM practice. A poster for this study is posted at http://www.ctahr.hawaii.edu/sustainag/Downloads/2010_APS_khwang2-small.pdf. A detailed result of Experiment II is posted online (http://www.ctahr.hawaii.edu/sustainag/ Downloads/2009UpdatesSunnHempSuperhero.pdf), whereas that of Experiment III and IV are published by Wang et al. (2010) and graduate student Marahatta et al. (2012), respectively. Summarizing all the temperature data collected, we were able to generate a solarization temperature schematic scheme for Hawaii over a one-year period (Fig. 2). Sufficient lethal heat to kill nematodes in the top 4” soil can only be obtained between April and October. By gathering all the sunn hemp biomass generated in all these field trials over the year, we also generated a schematic scheme of sunn hemp biomass production (Fig. 3). Biomass of sunn hemp varied from 0.73 to 6.67 Mg/ha (1.63 to 14.94 tons/acre) of dry sunn hemp residues over approximately two months of growing period. This variation could be seasonal or due to soil pH. Biomass of five tons/acre (equivalent to 1% w/w can commonly be achieved if planted between May and August in soil with normal pH). Although Sol alone suppressed weeds efficiently (Fig. 4), integration of SH+Sol suppressed weed densities better than SH or Sol alone in the pineapple trial (Wang et al., 2010 http://www.ctahr.hawaii.edu/sustainag/Downloads/2009_pineapple_SHSol_project.pdf). However, integration of SH and Sol did not improve weed suppressive effect as compared to Sol alone in the eggplant and cowpea trials (Marahatta et al., 2012). Weed suppression usually only lasted at one or two months after crop planting but no difference among treatments thereafter. SH suppressed plant-parasitic nematodes most efficiently and consistently except at Khamphoute’s eggplant trial, where cover crop treatment was imposed one cropping season ago. Integrating SH+Sol did not increase the nematode suppressive effect on plant-parasitic nematodes as compared to SH alone unless when the solarization was conducted during late fsall to winter. It is possible that high heat from Sol deactivate the SH allelopathic compounds against plant-parasitic nematodes. Based on nematode community analysis, Sol in the summer consistently disturbed soil health conditions. When solarization was conducted during late fall to winter, SH+Sol improved soil health condition better than SH alone. While combination of SH+Sol during the summer is beneficial for weeds suppression, it is not beneficial for nematode and soil health management as compared to SH alone.

Research conclusions:
Objective 3 and 4

For best suppression of root-knot nematodes, sunn hemp should be planted for two to three months to achieve a biomass of at least five tons dry biomass/acre (equivalent to 1% w/w concentration) (Fig. 5A ) and incorporated into soil. Incorporating sunn hemp materials that are too old (≥ four-month old) will not suppress root-knot nematodes effectively. Leaf tissue is among the most suppressive component of sunn hemp against root-knot nematodes (Fig. 5B ). Thus, incorporating sunn hemp at vegetative stage will be more effective in nematode suppression than at its mature stage. Sunn hemp does suppress root-knot nematodes more efficiently than its weed relatives, C. spectabilis and C. retusa (Fig. 5C) . Although, C. spectabilis is a common weed and has nematode suppressive effects, it contains monocrotaline (Fig. 6), which is why it is considered a noxious weed that can harm livestock. Unlike what had been reported in the literature, current study indicates that SH and C. retusa do not contain monocrotaline but contain an unusual dehydropyrrolizidine alkaloid (Fig. 6), much similar to the diastereoisomers of isohemijunceine, a monoester of retronecine with an unusual necic acid.recently reported by Colegate et al. (2012). Our allelopathic assay results are consistent with our field experiments (Objective 1), showing that it is not necessary to integrate soil solarization with sunn hemp cover cropping for nematode suppression, as long as the cover crop could produce > five tons dry biomass/acre for soil amendment (Fig. 5 D). However, soil solarization could reduce initial population densities of weed pressure (Wang et al., 2011), and thus, might worth the effort to integrate SH with Sol.

Literature Cited

1. Colegate, S.M., Gardner, D.R., Joy, R.J., Betz, J.M., Panter, K.E. 2012. Dehydropyrrolizidine alkaloids, including monoesters with an unusual esterifying acid, from cultivated Crotalaria juncea (sunn hemp cv. Tropic Sun). J. Agric. Food Chem. 60: 3541-50. 2. Crout, D. H. G. 1968. Pyrrolizidine alkaloids. The co-occurance of monocrotaline and trichodesmine in Crotalaria recta Steud. ex A. Rich. Phytochemistry 7:1425-1427. 3. Marahatta, S. P., K.-H. Wang, B.S. Sipes, and C.R.R. Hooks. 2012. Effects of the integration of sunn hemp and soil solarization on plant-parasitic and free-living nematodes. Journal of Nematology 44: 72-79. 4. Marahatta, S. P., K.-H. Wang, B.S. Sipes, and C.R.R. Hooks. 2012. Effects of Crotalaria juncea on anhydrobiotic state of Rotylenchulus reniformis. Nematropica (in press). 5. Rodriguez-Kabana, R., J. Pinochet, D. G. Robertson, C. F. Weaver, and P. S. King. 1992. Horsbean (Canavalia ensiformis) and Crotalaria (Crotalaria spectabilis) for the management of Meloidogyne spp. Nematropica 22:29-35. 6. Rich, J. R., and G. S. Rahi. 1995. Suppression of Meloidogyne javanica and M. incognita on tomato with ground seed of caster, crotalaria, hairy indigo and wheat. Nematropica 25:159-164. 7. Wang, K.-H., R. McSorley, N. Kokalis-Burelle. 2006. Effects of cover cropping, solarization, and soil fumigation on nematode communities. Plant and Soil 286: 229-243. 8. Wang, K.-H., B. S. Sipes, and D. P. Schmitt. 2001. Suppression of Rotylenchulus reniformis by Crotalaria juncea, Brassica napus, and Tagetes erecta. Nematropica 31(2): 235-249. 9. Wang, K.-H., B. S. Sipes, and D. P. Schmitt. 2002. Crotalaria juncea as a cover crop for nematode management: A review. Nematropica 32: 35-57. 10. Wang, K.-H., B.S. Sipes, and C.R.R. Hooks. 2011a. Sunn hemp cover cropping and solarization as alternatives to soil fumigants for pineapple production. Acta Hort (ISHS) 902: 221-232. http://www.actahort.org/books/902/902_22.htm 11. Wang, K.-H., C.R.R. Hooks, and S.P. Marahatta. 2011b. Can using a strip-tilled living mulch system enhance organisms higher up in the soil food web hierarchy? Applied Soil Ecology 49: 107-117( doi:10.1016/j.apsoil.2011.06.008).

Participation Summary

Educational & Outreach Activities

Participation Summary:

Education/outreach description:
Outreach (Workshops or field day events)

1. Wang, K.-H., S. Fukuda, and J. Sugano. 5 Aug, 2010. Cover crop research update. University of Hawaii, Poamoho Experiment Station, Poamoho, HI. 2. Wang, K.-H. 2010. Nematode research program. 15 June 2010. CTAHR Road Show. Windward Community College, Kaneohe, HI. 3. Wang, K.-H. 2010. Non-chemical approaches for nematode management. 7-8 June, 2010 Integrated Crop and Livestock Management Workshop, Komohana Extension and Research Center, Hilo, HI. 4. Wang, K.-H. and C.R.R. Hooks. 2010. Is your cover cropping practice benefiting? A soil ecologist’s point of view. 20 February 2010. 19th Annual MOFFA (Maryland Organic Farmers Association) Winter Meeting, Annapolis, MD. 5. Wang, K.-H. and C.R.R. Hooks. 2010. Does reduce pesticide use benefit soil health: A nematologist’s point of view. 25 February 2010. Harford County Mid-Winter Educational Meeting, Deer Creek Overlook, 4-H Building, Street, MD. 6. Wang, K.-H. and C.R.R. Hooks. 2010. Use of nematodes and soil microarthropods as soil health bioindicators: a visit of Hawaii ecological based pest management projects. 26 February, 2010. Entomology Colloquium, Department of Entomology, University of Maryland, College Park, MD. 7. Wang, K.-H. Emerging strategies for controlling plant-parasitic nematodes organically. HOFA Annual Conference. 21 October, 2011. Attendance: 75. http://www.ctahr.hawaii.edu/sustainag/workshop/HOFA-Oct2011.html 8. Wang, K.-H., L. Kaufman, T. Radovich, and J. Sugano. Strip-till cover cropping and vermicompost tea workshop. Twin Bridges Farm, Waialua, HI. 19 May, 2011. Attendance:30 http://www.ctahr.hawaii.edu/sustainag/workshop/downloads/KH-striptill-cc.pdf 9. Wang, K.-H. Field trip to Twin Bridges Farm: Strip-till cover cropping and vermicompost tea treatment. Guess Lecture for Organic Food Production. Department of Tropical Plant and Soil Sciences, University of Hawaii, HI. October, 2011.Attendance: 20 students. 10. Post card

Peer reviewedpublications

1. McSorley, R., K.-H. Wang, J.J. Frederick. 2008. Integrated effects of solarization, sunn hemp cover crop, and amendment on nematodes, weeds, and pepper yields. Nematropica 38: 115-125. 2. Wang, K.-H., B.S. Sipes, and C.R.R. Hooks. 2011. Sunn hemp cover cropping and solarization as alternatives to soil fumigants for pineapple production. Acta Hort (ISHS) 902: 221-232. http://www.actahort.org/books/902/902_22.htm 3. Wang, K.-H., C.R.R. Hooks, and S.P. Marahatta. 2011. Can using a strip-tilled living mulch system enhance organisms higher up in the soil food web hierarchy? Applied Soil Ecology 49: 107-117( doi:10.1016/j.apsoil.2011.06.008). 4. Marahatta, S. P., K.-H. Wang,B.S. Sipes, and C.R.R. Hooks. 2012. Effects of the integration of sunn hemp and soil solarization on plant-parasitic and free-living nematodes. Journal of Nematology 44: 72-79. 5. Marahatta, S. P., K.-H. Wang,B.S. Sipes, and C.R.R. Hooks. 2012. Effects of Crotalaria juncea on anhydrobiotic state of Rotylenchulus reniformis. Nematropica (in press).

Conference presentations

1. Marahatta, S. P., K.-H. Wang, and B.S. Sipes. Effects of a strip-till cover cropping system on nematode communities. 2009 Society of Nematologists and Soil Ecology Society Conference. Burlington, VM. July 2009. 2. Wang, K.-H., B. S. Sipes, and C.R.R. Hooks. 2010. Sunn hemp cover cropping and solarization as alternatives to soil fumigants for pineapple production. 2010 International Pineapple Symposium, Persada, Johor Bharu, Malaysia. July 13-15, 2010. http://www.ishs-horticulture.org/workinggroups/pineapple/ 3. Wang, K.-H., C.R.R. Hooks, S. P. Marahatta, R. Manandhar. Use of a strip-till cover crop system to manipulate above and below ground organisms in cucurbit plantings. 2010 Meeting of the American Phytopathological Society (APS) at Charlette, NC. Aug 2010. 4. Wang, K.-H. and I. Zasada. Allelopathic effects of crotalaria spp. against M. incognita as affected by crop age, biomass, heat and species. 2011 Society of Nematologists Conference, Corvallis, OR. July 2011. 5. Marahatta, S.P., Wang, K.-H., and Sipes, B.S. 2011. Effects of Crotalaria juncea on the anhydrobiotic stage of Rotylenchulus reniformis. Oral presentation on fiftieth annual meeting of the Society of Nematologists, Corvallis, Oregon. 6. Marahatta, S.P., Wang, K.-H., and Sipes, B.S. 2011. Integration of sunn hemp cover cropping and soil solarization for reniform nematode, Rotylenchulus reniformis, management. Phytopathology 101:S113 (Abstr). 7. Marahatta, S.P., Wang, K.-H., and Sipes, B.S. 2011. Improvement in sunn hemp cover cropping for reniform nematode, Rotylenchulus reniformis, management. Oral presentation at 23nd Annual CTAHR Student Research Symposium, Honolulu, Hawaii. 8. Wang, K.-H. Use of nematode community ecology to develop emerging strategies for controling plant-parasitic nematodes: avoiding the biological vacuum. 2011. Society of Nematologists Conference, Corvallis, OR. July 2011. 9. Wang, K.-H. Using nematodes as a soil health bioindicator to develop sustainable pest management. Kaoh-Siung Normal University, Kaoh-Siung, Taiwan, R.O.C. Jun 10, 2011.

Extension articles

1. Wang. K.-H. and B.S. Sipes. 2009. Solarization and Cover Cropping as Alternatives to Soil Fumigants for Nematode Management in Hawai‘i’s Pineapple Fields. CTAHR Cooperative Extension Service SCM-29. 4 pp. http://www.ctahr.hawaii.edu/oc/freepubs/pdf/SCM-29.pdf. 2. A peer reviewed article and a newsletter article (http://www.ctahr.hawaii.edu/sustainag/news/articles/V1-wang-solarization.pdf) related to this project were published. 3. Wang, K.-H., B.S. Sipes, and C.R.R. Hooks. 2010. Environmental friendly approaches for managing nematodes and weeds on pineapple. Pineapple News 17: 27-32. May 2010. http://www.ishs-horticulture.org/workinggroups/pineapple/PineNews17.pdf. 4. Wang, K.-H. Soil solarization and cover cropping as alternatives to soil fumigation for pineapple growers in Hawaii. H?nai?Ai Newsletter Spring 2010. http://www.ctahr.hawaii.edu /sustainag/news/articles/V3-Wang-SHPineapple.pdf. 5. Wang, K.-H., B.S. Sipes, C.R.R. Hooks, and J. Leary. Improving the status of sunn hemp as a cover crop for soil health and pests management. H?nai‘Ai Newsletter Summer 2011. http://www.ctahr.hawaii.edu/sustainag/news/articles/V8-Wang-sunnhemp.pdf. 6. Wang, K.-H., I. Zasada, and B.S. Sipes. 2012. The Secret of the allelopathic effect of sunn hemp for suppressing plant-parasitic nematodes. H?nai‘Ai Newsletter Summer 2011. http://www.ctahr.hawaii.edu/sustainag/news/articles/V12-Wang-allelopathic.pdf.

Video

A video “Sunn hemp for soil health management” was posted on YouTube (http://www.youtube.com/watch?v=AG_CYsVmqN4).

Project Outcomes

Project outcomes:

Sunn hemp cover cropping alone will cost ~$170/acre. Adding solarization mulch will add additional $329/acre. For pineapple production, pre-plant soil management required $525/acre for Telone (1,3-dichloropropene) fumigation. See table attached.

Farmer Adoption

All participating farmers are adopting sunn hemp cover cropping. They include small scale (< 10 acre), medium scale (10 to 100 acres) and large scale (> 1,000 acres). Although only a small portion of their farm site is planted with sunn hemp, these farmers are showing initial adoption of sunn hemp cover cropping. They learn that planting sunn hemp is a good solution to their nematode problem, besides having the additional benefits in building up soil health. Farmers and master gardeners that attended the workshop received sunn hemp seeds for planting. A teacher at McKinley High School learned about sunn hemp through a student working in our lab. She then requested sunn hemp seeds to demonstrate the use of the cover crop in their high school agriculture project. Aloun Farm and Khamphout Farm received additional sunn hemp seeds to be planted in two additional acres of their farmland besides the demonstration plots. Aloun Farm had shown their interest to expand their sunn hemp planting after corn, as continuous planting of corn had shown nutrient deficiencies despite regular fertilization. Alberta deJetley in Lanai is purchasing more sunn hemp seeds from Crop Care Hawaii, LCC for their farm and is participating in soil conservation program in the NRCS EQIP Program. Rick Tamanaha in Molokai is convinced that planting of sunn hemp as ground cover can maintain soil moisture and will be key to increasing his organic papaya production in Molokai; a prime place for papaya production in Hawaii as it is located in a papaya ring spot virus-free zone. Many home gardeners like the idea of planting a crop to protect their vegetable crops or landscape plants from nematode damage. It is always a welcoming suggestion for them to plant sunn hemp in their garden. Introducing solarization techniques to some of these farmers provide another non-chemical management strategy for these farmers to manage nematode and weed pests. Schematic scale of solarization temperature over four seasons in Hawaii will be useful information for farmers that are interested to perform solarization in Hawaii. We had demonstrated the effects of sunn hemp cover cropping on leguminous crops, leafy vegetables, eggplant, tomato, papaya and pineapple. While we failed to observe that integration of sunn hemp and summer solarization, Objective 4 helps farmers to determine how long to grow sunn hemp: two-three month old sunn hemps are most suppressive to plant-parasitic nematodes and five tons dry matter/acre is required for effective kill of nematodes. This would help the farmers to better adopt sunn hemp cover cropping. Since spring 2012, there are now two new producers of ‘Tropic Sun’ sunn hemp seeds in Hawaii: 1) Molokai Seed Company at www.molokaiseedcompany.com; and 2) Oahu RCD at http://www.oahurcd.org/. Oahu RCD says that they could not keep up with the sunn hemp seed demand throughout the state, an indication of high adoption of sunn hemp cover cropping in Hawaii.

Recommendations:

Areas needing additional study

The specific compound responsible for allelopathic effect of sunn hemp cannot be fully determined. Based on HPLC study results conducted here, we know that monocrotaline is not the allelopathic compound responsible for suppression plant-parasitic nematodes as it was not detected in C. juncea and C. retusa. While we speculated that it could be isohemijunceine detected by Colegate et al. (2012), the standard chemical of this compound is not available for testing. Future study could be conducted to determine the relationship of concentration of isohemijunceine and nematode suppression. As the project progressed, we found that sunn hemp cover crop is challenged by a flour beetles which bored inside the stem tissues after continuous planting of sunn hemp in the same field for more than two cropping cycles. The beetles cause sunn hemp to wilt prematurely and results in very low biomass production. Future research should look into alternative tropical leguminous cover crops that have allelopathic effect against plant-parasitic nematodes without being a host to this flour beetles so as to serve as a rotational cover crop for soil health management in the tropics.

Information Products

Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.