Eradication of plant pathogens and nematodes during composting: a review

The effects of temperature–time combinations and other sanitizing factors during composting on 64 plant pathogenic fungi, plasmodiophoromycetes, oomycetes, bacteria, viruses and nematodes were reviewed. In most cases pathogen survival was determined by bioassays of unknown sensitivity and minimum detection limits of 5% infection or more. For 33 out of 38 fungal and oomycete pathogens, all seven bacterial pathogens and nine nematodes, and three out of nine plant viruses, a peak temperature of 64–70°C and duration of 21 days, were sufficient to reduce numbers to below the detection limits of the tests used. Shorter periods and/or lower temperatures than those quoted in these tests may be satisfactory for eradication, but they were not always examined in detail in composting systems. Plasmodiophora brassicae (clubroot of Brassica spp.), Fusarium oxysporum f.sp. lycopersici (tomato wilt) and Macrophomina phaseolina (dry root rot) were more temperature-tolerant, as they survived a peak compost temperature of at least 62°C (maximum 74°C) and a composting duration of 21 days. Synchytrium endobioticum (potato wart disease) survived in water at 60°C for 2 h, but was not examined in compost. For Tobacco mosaic virus (TMV), peak compost temperatures in excess of 68°C and composting for longer than 20 days were needed to reduce numbers below detection limits. However, TMV and Tomato mosaic virus (TomMV) were inactivated over time in compost, even at temperatures below 50°C. Temperatures in excess of 60°C were achieved in different composting systems, with a wide range of organic feedstocks. The potential survival of plant pathogens in cooler zones of compost, particularly in systems where the compost is not turned, has not been quantified. This may be an important risk factor in composting plant wastes.