Green manure crops for integrated pest management in upland taro fields in Hawaii

Taro (Colocasia esculenta) is a non-graminaceous monocot consumed primarily for its starchy corm. To determine the best green manure crops to rotate with upland-grown taro, 10 genotypes of seven green manure species were evaluated in a greenhouse study to determine their ability to suppress root-knot nematode Meloidogyne javanica. These species were: black hollyhock (Alcea rosea L.); canola (Brassica napus L.); cabbage (B. oleracea L.); French marigold (Tagetes patula L.), sorghum-sudangrass [Sorghum bicolor (L.) Moench nothosubsp. drummondii (Steud.) de Wet ex Davidse]; sunn hemp (Crotalaria juncea L.); and yellow mustard (Sinapis alba L.). Plants were inoculated with eggs of M. javanica and after 6 weeks, nematode eggs, and reproduction factor (Rf) of M. javanica were determined. Marigolds were non-hosts to M. javanica; other crop species that were poor hosts to M. javanica included canola 'Dwarf Essex', sorghum-sudangrass cvs. Piper and Sordan 79, black hollyhock 'Nigra', and sunn hemp. Based on results of this greenhouse study, eight green manure treatments (yellow mustard cultivar 'Ida Gold', French marigolds cultivars 'Nema-gone' and 'Golden Guardian', sorghum-sudangrass cultivar 'Sordan 79' and 'Tastemaker', sunn hemp, an unplanted plot, and a clean fallow control plot) were selected and grown for 3 months in a field trial in Pepeekeo, Hawaii. Each treatment was blocked four times in a randomized complete block design. Based on this field trial, marigold cultivars, sorghum-sudangrass hybrids, and sunn hemp appeared to be the best green manure crops for rotation in an upland taro cropping system.