To minimise the risk posed by the resistant strain, proactive action was taken to restrict impatiens production using cutting-raised plants. This approach appears to have been successful because no IDM infections caused by the resistant strain were detected during monitoring in 2012 - and in 2013 no infections of impatiens were reported.
AHDB project PO 011b "Monitoring metalaxyl-M sensitivity of downy mildew infection of impatiens" aims to continue monitoring the work undertaken to provide growers with an early warning system for identifying the presence of metalaxyl-M resistance, in order to assist with decisions on spray programmes and offer guidance as to the prevalence and geographical distribution of metalaxyl-M resistance compared to metalaxyl-M sensitivity in the wider environment.
During 2014 and 2015 seven samples were received, mainly from private gardens but also from one nursery. Metalaxyl-M-resistant isolates of P. obducens were present on samples sent in only during 2014 from two of the sites. In both years, disease detection was late in the season, and a lack of samples from nurseries suggested that the disease had not established from plant material but from other inoculum sources. This validates the industry decision to limit production to seed-raised impatiens only.
"Although there is still a risk growing impatiens walleriana, the practice of growing them from seed instead of from vegetative cuttings is continuing to be successful in reducing the incidence of impatiens downy mildew," says Mike Smith of WD Smith & Son.
For details on all AHDB Horticulture activity, see horticulture.ahdb.org.uk