Epidemiology and management of impatiens downy mildew in the United States
2017
Harlan, B. R. | Granke, L. | Hausbeck, M. K.
Impatiens (Impatiens walleriana), a flowering annual bedding plant, is a highly popular shade plant that is revered for its season-long flowering and bright colors. Downy mildew, caused by the oomycete Plasmopara obducens, can rapidly defoliate and kill impatiens. Epidemics have been observed throughout the United States (U.S.) in greenhouses and landscapes and control strategies are needed. The influence of temperature and leaf wetness duration on germination of sporangia and infection of impatiens by P. obducens was examined in controlled environmental studies. Impatiens plants were inoculated with P. obducens sporangia and immediately placed into a growth chamber set at 15, 20, 25, or 30°C. A greater proportion of leaves hosted sporangia following incubation at 20°C than the other temperatures studied. Inoculated plants incubated at 15 and 25°C yielded a similar proportion of infected leaves. No pathogen sporulation was observed on plants incubated at 30°C. A leaf wetness period ≥6 h yielded a greater proportion of leaves hosting sporangia than a leaf wetness period of 3 h. Conventional and reduced-risk fungicides were evaluated for efficacy. Reduced-risk products cyazofamid, mandipropamid and mefenoxam significantly reduced infection compared to the untreated inoculated plants. An experimental fungicide, oxathiapiprolin, was especially efficacious and provided protection from P. obducens in the landscape for >12 weeks after the final greenhouse application. Environmental manipulation coupled with effective reduced-risk fungicides can aid growers and ensure the continued planting of impatiens in U.S. landscapes.
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