The Secret of the Allelopathic Effect of Sunn Hemp for Suppressing Plant-parasitic Nematodes Koon-Hui Wang 1 , Inga A. Zasada 2 , and Brent S. Sipes 1 1 Dept. Plant and Environmental Protection Sciences, University of Hawaii at Manoa, Honolulu, HI; 2 USDA-ARS, Corvallis, OR. Introduction Biofumigation refers to the suppression of soilborne pests by biocidal compounds released into the soil during plant decomposition (Kirkegaard et al., 1993). Brassica plants have re- ceived the most attention in this area of research, but the possibility of using other plants as biofumigants warrants attention. Sunn hemp (Crotalaria juncea) is a good candidate for biofu- migation research as it is well known to be a leguminous cover crop that releases nematostatic compounds (compounds that can paralyze nematodes) against many plant-parasitic nema- todes (Wang et al., 2002). An article in the Summer 2011 issue of the HānaiʻAi Newsletter summarized the limitations and provided suggestions on ways to improve the use of sunn hemp as a cover crop for soil health and nematode management. This article summarizes fac- tors that might affect the allelopathic effect of sunn hemp for suppressing plant-parasitic nema- todes. A series of experiments was conducted to answer some questions frequently asked by farmers and gardeners. Specific objectives of this project were to examine the allelopathic ef- fects of sunn hemp on root-knot nematode (Meloidogyne incognita) as affected by crop age, plant tissue, amount of sunn hemp biomass, Crotalaria species, and soil solarization. Materials and Methods Experiment I: Examine the effect of crop age, plant part, and amount of biomass Sunn hemp tissues were collected at 1, 2, 3, and 4 months after planting and samples col- lected were separated to leaf, stem, flower, roots, or remained as whole plant. All tissues were oven dried (70°C for 2 days) and ground into powdered form (Fig. 1) prior to assay for allelo- pathic effect against root-knot nematodes. Ground powder was made into leachate by soaking in water, filtered, and each tissue type diluted into 0, 0.1, 0.5, 1 and 2.5 dilutions. Juveniles of Hānai‘Ai / The Food Provider June-July-August 2012 1 Fig. 1. Sunn hemp flower, leaf and stem tissue ground up into powdered form for assay.