The potential of Brassica green manure crops for controlling root-knot nematode (Meloidogyne javanica) on horticultural crops in a subtropical environment

Abstract

In glasshouse experiments, Meloidogyne javanica reproduced on Brassica juncea, B. napus and BQMulch, a commercial biofumigant crop consisting of both B. napus and B. campestris. Although nematode multiplication rates were only 3–23% of the rate on a highly susceptible crop (tomato), they were high enough to suggest that Brassica rotation crops will increase root-knot nematode populations to potentially damaging levels in situations where temperatures do not limit nematode reproduction. A field study showed that this problem could be prevented in subtropical climates by restricting the growth of Brassica crops to June, July and August, when low temperatures limit nematode development and eggs are not produced until about 10 weeks after planting. Field and pot experiments in which M. javanica was introduced into soil following incorporation of Brassica residues failed to demonstrate a 'biofumigation' effect, possibly because the soil was too dry or the degree of tissue disruption was insufficient to maximise isothiocyanate release. Numbers of root-knot nematodes were reduced when Brassica roots, leaves and stems were incorporated into soil at 17 t DM/ha, but it is unlikely that this was due to the production of nematoxic chemicals, as large numbers of free-living nematodes multiplied during the decomposition process. From a practical perspective, these results suggest that green-manured Brassica rotation crops are unlikely to be useful for controlling root-knot nematodes on some subtropical horticultural crops that are currently fumigated for nematode control. The susceptibility of brassicas to M. javanica and the need to grow them during winter limits their potential in the vegetable industry, while difficulties in obtaining a biofumigation effect in dry soil will reduce effectiveness in non-irrigated pineapple soils. Brassicas are likely to be most useful in the ginger industry, as they can be grown during the winter break between ginger crops and fields can be irrigated before the Brassica crop is incorporated into soil.