Impatiens downy mildew, caused by Plamopara obducens, is a foliar disease specific to impatiens. A metalaxyl-M resistant strain of P. obducens was introduced into commercial production in 2011, resulting in widespread downy mildew infections that were difficult to control. To minimise the risk posed by the resistant strain, steps were taken to restrict impatiens production using cutting-raised plants. This appears to have been successful as no infections caused by the resistant strain were detected during monitoring in 2012 (PO 011) or 2013 (PO 011a), but the risk remains.
Monitoring of metalaxyl-M resistance is ongoing in this six-year project, by inoculating four-week-old impatiens plants (half of which were given a Subdue drench treatment) with samples sent in to the Food & Environment Research Agency. If treated and untreated plants both become infected, then it is the resistant strain. Early warning of resistance will help growers decide on the most suitable spray programmes. Monitoring will also provide information on the prevalence, persistence and geographical location of the metalaxyl-M resistant strain.
Again there was a lack of disease samples from nurseries, reflecting the continued success of only using seed-raised impatiens. Disease in the wider environment has generally appeared late season, once impatiens displays are beyond their best. Late-season expression in the wider environment would not pose a risk to nursery production. However, early-season transport of infected impatiens material from continental Europe still could.
Other control measures include applying a protectant fungicide programme during the production phase, monitoring crops for signs of the disease, providing good ventilation, avoiding watering at night and disposing of infected plant material in sealed bags or bins.
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