Suppression of Pythium aphanidermatum in composts prepared from sugarcane factory residues

Abstract

Composts prepared from mixtures of bagasse + filter mud (BF) and bagasse + vinasses + filter mud (BVF) were evaluated for suppressiveness to Pythium aphanidermatum in climatic chamber experiments. Twenty five-g samples of BF and BVF composts in plastic pots (130 mL) were infested with 1,000 oospores of P. aphanidermatum produced on oat meal agar. After 1, 15, 30 and 45 days, survival of the fungus was estimated by measuring inoculum density. Disease incidence was appraised on cucumber (Cucumis sativus) “Vert Long Anglais” seedlings raised on the composts. Propagules of P. aphanidermatum surviving in the compost after 24 hr was estimated at 22 and 18 cfu g⁻¹ dry wt. potting mix, for BF and BVF, respectively. This population decreased significantly to 6–7 cfu g⁻¹ of compost for the 15–45-d incubation treatment. Seedling mortality was not observed in uninfested controls. In uninfested treatments, 40 and 67% of seedlings died for the 1-d incubation treatment in BVF and BF, respectively; no mortality was recorded thereafter. Heat treatment of the composts revealed that the suppressive effect was biological in nature. Quantitative reduction of micro-organisms occurred in pasteurized composts (55°C for 2 h), compared to the populations in unheated controls. However the greatest decrease was observed for fungal populations. The main fungal species observed in unheated, suppressive composts were Aspergillus sp., Geotrichum sp. and a non-sporulating Pythium. The last two species disappeared in pasteurized, conducive composts.