Response of nematode communities to sudangrass and sorghum-sudangrass hybrids grown as green manure crops.

Two cuhivars of sudangrass (Piper and Trudan 8) and three of sorghum-sudangrass (Sordan 79, P855F, and P877F) were grown as green manure crops in 1993 and 1994 and compared with sweet corn for their impact on nematode population dynamics. Nematodes were identified to trophic group, order, and to lower taxa when possible. Population densities were determined after 7 weeks of crop growth and 3 weeks after incorporation of green crop residue. Plant-parasitic nematode genera included Pratylenchus, Longidorus, Xiphinema, and Paratrichodorus. The plant-feeder trophic group increased or was maintained on all crops after 7 weeks, at which time population densities were lowest on corn in 1993 and equivalent among crops in 1994. The total number of nematodes in the plant-feeder trophic group did not differ before and after incorporation in 1993 and increased for Piper sudangrass, Sordan 79 and P855F sorghum-sudangrass, and sweet corn in 1994. After incorporation, numbers of bacterial-feeding nematodes increased for all crops in 1994 and for Piper sudangrass in 1993. There were no consistent crop treatment effects on the fungal- feeding, omnivore, and predator trophic groups after incorporation. Sudangrass (Sorghum vulgare var. sudan- ense) and sorghum-sudangrass (S. bicolor X S. vulgare var. sudanense) grown as cover crops (8) and as green manure crops (13) reduced population densities of some Meloidogyne spp. There is considerable in- terest in using these crops to control root- knot and other nematode populations in vegetable production systems. Presumably, Meloidogyne spp. are inhibited because these crops are poor hosts for nematode reproduction and, in the case of green ma- nures, because the leaves contain chemi- cals that hydrolyze to form nematicidal compounds. In laboratory and microplot studies with methyl bromide-treated soil, Mojtahedi et al. (13) found that leaves of sudangrass and sorghum-sudangrass hybrids chopped and incorporated as green manure sup- pressed M. chitwoodi compared with fallow or wheat green manures. Buried leaves of Trudan 8 sudangrass inhibited migration of M. chitwoodi second-stage juveniles.