Resistance to Downy Mildew Caused By Plasmopara obducens in Impatiens at Different Growth Stages

Impatiens downy mildew (DM) caused by Plasmopara obducens is a devastating disease of garden impatiens (Impatiens walleriana) and has led to severe economic losses to the floriculture industries in the United States and numerous other countries in the world. New Guinea impatiens (NGI) (Impatiens hawkeri), by contrast, is reported to be highly resistant to DM. In this study, the resistance to DM in sixteen cultivars of impatiens and sixteen cultivars of NGI was evaluated at the cotyledon, first and second true leaf stages. Results from in-vivo inoculation assays showed that all I. walleriana cultivars were highly susceptible to DM, regardless of their growth stages, and all NGI cultivars were resistant to DM beginning at the first true leaf stage. During the cotyledon stage, the great majority of NGI cultivars showed some sporulation after extended incubation of artificially inoculated cotyledons, and I. hawkeri cultivars Divine Orange Bronze Leaf and Divine Burgundy showed significantly higher DM incidences than other NGI cultivars tested (P < 0.01). Leaf disc inoculation assays showed that the sporangia density on cotyledons of ‘Divine Orange Bronze Leaf’ was still two fold lower than that on the cotyledons and true leaves of I. walleriana cultivars at 10 days post inoculation (dpi). Growth and development of Plasmopara obducens in the inoculated impatiens leaves were examined by trypan blue staining and microscopic observation. Plasmopara obducens grew and developed rapidly on and in the leaves of I. walleriana cultivars, but did not start its lifecycle on and in the leaves of DM-resistant cultivars, with no fungal vesicle, hypha or haustoriums observed. Our results have shown complete resistance or immunity in many NGI cultivars beginning at the first true leaf stage and the possibility of screening for DM-resistant breeding lines and cultivars at this early stage.