Impatiens downy mildew is a fast-moving disease that can go from bad to worse in as little as one month if conditions are right, says Agricultural Consulting’s Ann Chase, who covered the latest impatiens downy mildew developments during her Cultivate’14 presentation. The rapid spread is one of the challenges with controlling downy mildew, one minute it doesn’t look like it is going to be a big problem, then all of a sudden it explodes by the end of the season.
During her presentation, Chase said growers should start treating for prevention of impatiens downy mildew at the beginning of plug production for the best chances of effective control throughout the season.
Downy mildew symptoms include curling leaves, yellowing or speckled leaves and leaves that have white undersides. Curling leaves can make it difficult to get spray coverage on the undersides of the leaf surfaces, which is where systemic products come into play.
Although researchers don’t believe there is any evidence that impatiens downy mildew is in the seed, field trials have shown that it can linger in the soil for up to 10 years, which complicates the problem, Chase said.
She cited Dr. Aaron Palmateer’s (University of Florida) research, which evaluated the effectiveness of fungicide rotations for downy mildew control in the landscape. The research showed that phosphonates are a good long-term treatment for the landscape, but growers should keep in mind that impatiens downy mildew cannot be eradicated with fungicides.
Fungicide resistance does occur, often due to using a product repeatedly without rotation or tank-mixing and using products at lower than labeled rates. Recently, mefenoxam resistance has been reported in Florida. Chase said even if growers use half the rates of two chemicals to cut costs, they would still get resistance. It is more effective to use the products at full rates in rotation, rather than tank-mixing, unless the label says to tank mix.
Chase also talked about the use of drenches versus sprays for effective control of downy mildew. She referred to trials on the longevity of drench published by Dr. Colleen Warfield of Ball Horticultural Co., which have shown that drenches last better than sprays on impatiens. Warfield found that drenches of Adorn and/or Subdue MAXX exhibited the longest residual efficacy (21 to 28 days) of all the fungicides tested in a limited number.