The impact of organic amendments, mulching and tillage on plant nutrition, Pythium root rot, root-knot nematode and other pests and diseases of capsicum in a subtropical environment, and implications for the development of more sustainable vegetable farming systems

Abstract

Modifications to the capsicum farming system (adding an organic amendment and replacing the standard plastic mulch with a layer of plant residue) were assessed for their potential to reduce losses from soilborne diseases. An amendment of sugarcane residue (12.5 t dry matter/ha) plus ammonium nitrate (100 kg N/ha) incorporated 4 months before planting capsicum enhanced microbial activity, increased numbers of free-living nematodes, decreased populations of rootknot nematode (Meloidogyne incognita) and reduced the severity of galling caused by the nematode. The amendment also reduced the severity of Pythium root rot caused by Pythium aphanidermatum, while a similar effect was obtained in a pot experiment with Pythium myriotylum. Damage from Pythium root rot was more severe in plastic-covered beds than in beds mulched with plant residue, probably because the organic mulch reduced soil temperatures by as much as 12°C. Mulching with plant residue also changed the pest and disease spectrum because it increased losses from cutworms (larvae of Agrostis sp.) and the severity of a leaf spot disease caused by Xanthomonas campestris pv. vesicatoria. The organic mulch also reduced fruit yield, largely because heavy rainfall leached nutrients (particularly N, K, Ca and Mg) from beds that were not covered with plastic. These results indicate that organic amendments and mulches have the potential to reduce losses from soilborne diseases in vegetable crops, provided plant nutrition can be managed satisfactorily when beds are mulched with organic matter.