Effects of Brassica residues and other organic amendments on abundance and sex ratio of Tylenchulus semipenetrans in soil

Abstract

Summary. The activity of soil-incorporated residues of 9 commercial Brassica cultivars and the weed B. tournefortii, and of 5 organic mulches against Tylenchulus semipenetrans was evaluated in laboratory experiments. Residues of all Brassica spp. reduced soil populations of T. semipenetrans. Humus rape had the highest activity (81% reduction in numbers of second-stage female larvae at 40 g/kg of soil) in an experiment comparing the effectiveness of fresh residues of greenhouse-grown plants, but Rangi, Arran and Hobson rapes and Simax hybrid were also effective at 80 g/kg. Tylenchulus semipenetrans was undetectable in soil amended with freeze-dried residues of Indian mustard meal at rates of 10 g/kg of soil or higher. Residues of B. tournefortii plants grown artificially with fertilisers were more effective against T. semipenetrans than those of naturally grown weeds. Leaf residues of B. tournefortii weed plants were more effective than either whole shoots or roots. Of 5 organic mulches tested, only chicken litter reduced the T. semipenetrans population when incorporated into soil.

Soil amendment with shoots or roots of B. tournefortii weeds, or with low rates of Indian mustard residues, increased the T. semipenetrans population, suggesting that incorporation of poor stands or ineffective Brassica green manure crops could increase the risk from T. semipenetrans to citrus replants.

Soil amendment with B. tournefortii weed residues, low rates of Indian mustard residues or with 3 of the mulches (organic compost, composted green waste or chicken litter) reduced the ratio of males (adults and larvae) to female larvae. Organic matter is postulated to have a significant influence on sex differentiation of T. semipenetrans.