Disease suppression and phytosanitary aspects of compost

Western Europe, approximately 25% of the 200 million tons of municipal solid waste that is generated each year is of organic origin and therefore compostable. Presently 35% of this organic waste is composted, resulting in 9 million tons of compost, and used mainly in agriculture, horticulture or hobby gardens.Increasing the opportunities to use compost in horticulture and agriculture as a (potting) soil amendment and nutrient source for plants and the soil microbial community would contribute to the recycling of waste and reduce the use of non-renewable peat and artificial fertilizers.This thesis is concerned with disease suppressive properties and phytosanitary aspects of compost.A major result of this thesis is that a compost cannot be simply assigned the label 'disease suppressive'. Its disease suppressive properties depend on both the plant species and pathogen species involved. Some composts were found to be highly disease suppressive for one or a few pathosystems, but not for others. The same was the case if disease suppression was tested for one pathogen, Pythium ultimum , on multiple hosts. Although variation in disease suppression was found as function of compost, host species and pathogen species, stimulation of disease as an effect of compost application was the exception (3% of the cases), and significant disease suppression (54%) or no effect (43%) the rule in a large experiment where 18 composts were tested with 7 pathosystems. Indications were obtained that the microbial change as affected by mixing compost with peat, both in the bulk mix as well as in the rhizosphere of the plants, is a parameter that correlates positively with disease suppression. Prediction of disease based on biotic and abiotic characteristics of compost was quite weak as compared to that based on the peat/compost mixes. The results indicate that there is scope for development of specialty products, designed for certain pathosystems. Different compost batches sampled at different times at the same composting facility showed rather similar levels of disease suppression.For assessing the phytosanitary risks associated with the application of compost it is necessary to include all risk factors and not only those occurring during composting. It seems safe to use composts produced from Vegetable, Fruit and Garden (VFG) compost although the conditions required for inactivation of some pathogens are still unknown. With respect to the assessment of the phytosanitary risks associated with composting itself, the determination of the degree of inactivation as function of temperature likely overestimates the temperature needed during composting, since other factors such as temporary anoxia, which brings about the formation of a range of toxic compounds, also contributes to inactivation of pathogens, as was shown here for Polymyxa betae , a pathogen of sugar beet