Development of a system simulating commercial production conditions for assessing the potential spread of Phytophthora cryptogea root rot of hardy nursery stock in recirculating irrigation water

A system was developed for monitoring the potential spread of Phytophthora cryptogea root rot of the hardy nursery stock (HNS) species Chamaecyparis lawsoniana, Erica x darlyensis and Calluna vulgaris in recirculating irrigation water on semi-commercial scale production beds. The spread of disease was monitored by measurements of symptoms and by baiting bioassays. A range of baits was tested and the most sensitive was found to be shoot segments of Lupinus angustifolius. The pathogen spread rapidly throughout the system from infected plants in the recirculating irrigation water. Although the percentage of plants infected was high (in Chamaecyparis 73% in 1994 and 93% in 1995), the percentage of infections actively sporulating at any one sample date was much less (47% in 1994 and 63% in 1995) and shoot symptoms were even less evident (in Chamaecyparis 22% in 1994 and 38% in 1995). In Chamaecyparis the incidence of shoot symptoms was less than expected from results of inoculations of similar plants not irrigated with recirculated water (80% infected, 60% with symptoms). The reason for this difference is not known but it poses a serious threat to the quality of HNS produced using recirculated irrigation water and underlines the need for reliable pathogen control methods.