The potential of organic and inorganic soil amendments, and a biological control agent (Trichoderma sp.) for the management of Phytophthora root rot of papaw in far northern Queensland

Abstract

Non-chemical options for the management of Phytophthora palmivora on papaw in far northern Queensland were evaluated in pot and field experiments. In pots, sawdust (150 m3/ha) + urea (600 kg/ha) reduced root rot, increased plant growth and reduced Phytophthora inoculum to undetectable levels. The addition of filterpress (400 m3/ha) and mill ash (200 m3/ha), reduced the damage caused by P. palmivora, but amendment with chicken manure (4% v/v), MinPlus(4.5 t/ha) and Trichodry (1.0 kg/m3) plus Trichoflow (5.0 kg/ha) failed to reduce root damage. In a second pot experiment, growing brassica (Bioquire Mulch) as a green manure significantly (P < 0.05) reduced Phytophthora inoculum levels and root rot severity, whereas the incorporation and composting of soybean, brachiaria, banana and sugar-cane failed to reduce the severity of root rot in papaw compared to a bare fallow. In a field crop of papaw grown for 46 weeks on 0.75 m high mounds, the suppressive effects of sawdust (150m3/ha) + urea (600 kg/ha), filterpress (400 kg/ha), molasses (100 L/ha/week for 20 weeks), brassica (Bioquire Mulch, 4.5 kg/ha), soybean (cv. Leichardt, 40 kg/ha) and gypsum (5 t/ha) were compared. Soil amended with sawdust + urea had the lowest incidence of root rot. Soil amended with filterpress or molasses had the highest incidence of root rot and the highest populations of P. palmivora. Cumulative totals of percentage soil moisture were greatest in soil amended with filterpress or molasses, and this was evidence of the influence soil moisture retention has on root rot development.