Composting of Rhododendron and Bilberry Wastes To Contain Spread of Exotic Plant Pathogens Phytophthora kernoviae and Phytophthora ramorum

Abstract

Plant material infected with the exotic pathogens Phytophthora kernoviae and Phytophthora ramorum, particularly of the invasive and highly susceptible Rhododendron ponticum, can pose a risk to indigenous host flora in Britain. Areas of infected bilberry (Vaccinium myrtillus) can also threaten surrounding non-infected heathland. Composting was examined as a more environmentally acceptable method of disposal of infected plant material than burning. Three types of low cost composting systems were developed and tested on shredded rhododendron and chopped bilberry wastes: permanent and mobile insulated bays, and insulated cargo containers, located at six different sites. In addition to temperature-time profiles of the composting wastes, the discoloration of the waxy leaves of R. ponticum and Portugal laurel (Prunus lusitanica) was developed as a potential indicator of the sanitising effect of the composting process. The relationship between the mean compost temperature and the percentage of green area of leaves positioned in the compost enabled the sanitising effect of a composting process to be immediately assessed. Mean compost temperatures and exposure times achieved in shredded rhododendron or chopped bilberry wastes in the majority of the compost in the insulated composting systems were above those needed to reduce P. ramorum and P. kernoviae to below detectable limits, and to eliminate any green colour in the indicator leaves. The exception was in the corners of the systems that contained >4 m³ waste, and in the outer surfaces at one site where the volume of waste was only 2.9 m³. Temperature-time profiles of the composts and positioned indicator leaves demonstrated that the main pathogen survival risk was in the corners of the insulated composting systems; pathogen survival risk could be minimised by positioning the corner material into the centre of the composting system during sequential refilling.