Phytosanitary risk assessment of composts

Assessment of phytosanitary risks associated with application of composts in agriculture generally has focused on the sanitation (self-heating) phase during composting when most plant pathogens are inactivated due to lethal temperatures. However, a few plant pathogens are heat resistant and they may survive a properly monitored and controlled composting process. To assess the phytosanitary risks associated with compost utilization, several additional factors need to be considered which all relate to a tracing-and-tracking principle. It includes the composition of the original waste and several aspects related to compost utilization. The following parameters are considered to be key-factors: 1) the proportion of host biomass relative to the total quantity of biowaste, 2) the proportion of host infected with a pathogen, 3) the density of infected host material, 4) the proportion of propagules of a pathogen that survived the process, and 5) the threshold density of a pathogen in soil above which disease of the host is expected to develop. While the first two parameters may be rather easy to estimate and information on survival of many plant pathogens can be obtained from the literature, little knowledge exists on the density of the pathogens in host materials or on threshold values. This applies particularly to virus diseases. The phytosanitary risk of several types of plant pathogens is discussed in some detail in this paper. Recommendations are given for testing of a composting process for phytohygienic safety.