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

• 2009 annual report
• 2010 annual report
• 2011 annual report

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.

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.

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