Control of Allium white rot (Sclerotium cepivorum) with composted onion waste

We investigated the possibility of composting onion waste to destroy any sclerotia present and then applying the composted waste to soil to stimulate sclerotial germination to disinfest the soil of the white rot fungus. Three mixtures of dry (shale-skins or onion tops) and wet (peelings or chopped whole bulbs) onion waste were incubated for 7 d at 50 °C with aeration to simulate a large-scale composting process. Under these conditions, a mixture of 10:1 (w/w) wet/dry (80% moisture content) produced the desired characteristics of minimal run-off and no unpleasant odour. This ratio of onion wastes was then inoculated with sclerotia of S. cepivorum and incubated for 3 or 7 d at a range of temperatures (18–60 °C). The pathogen was destroyed after 3 d at temperatures of 48 °C and above. Gas chromatography–mass spectrometry (GC–MS) analysis of raw onion waste and waste incubated for 3 or 7 d at 42 or 54 °C (referred to as ‘composted waste’) revealed the presence of the sclerotia germination stimulant, di-n-propyl disulphide (DPDS). Concentrations of DPDS decreased during incubation. A pot bioassay, with sclerotia buried in sandy loam soil mixed with different rates (1, 10 and 50% w/w) of raw onion waste and waste which had been composted, indicated the composted waste was more effective (suppressive) than the raw waste in reducing the viability of sclerotia. Onion waste which had been incubated at the higher temperature (54 °C) was most active despite a reduction in DPDS content compared with the raw waste. The 50% incorporation rate of this waste was most effective with a reduction in viability of sclerotia recorded after 1 month exposure. Both 1 and 10% rates reduced viability of sclerotia but after a longer exposure than for the 50% rate. The results indicate that other factors are more important in the suppressive effect of composted onion waste on S. cepivorum sclerotia than germination stimulant activity. The results demonstrate the potential use of composted onion waste as a method for white rot control, and as a means of disposing of packhouse onion wastes.