We’ve recently seen impatiens downy mildew on crops in the Northeast. Roberto Lopez, Mary Hausbeck, and Blair Harlan wrote a timely e-Gro Alert, check it out here: http://www.e-gro.org/pdf/2020_921.pdf.

 

Given how challenging (or impossible) it is to manage this disease if it gets a foothold in your greenhouse, preventative treatments are a good idea. Know that there have been some reports of impatiens downy mildew strains that are Subdue (mefenoxam)-resistant, and it is unwise to rely solely on this product for management. Resistance has also been seen, though less frequently, with Pageant (pyraclostrobin + boscalid) and Adorn (fluopicolide). The active ingredients oxathiapiprolin (Segovis), dimethomorph (Stature), mandipropamid (Micora), cyazofamid (Segway), and ametoctradin + dimethomorph (Orvego) have a particularly good track record against impatiens downy mildew. 

 

Be sure to keep an eye on and treat all of your Impatiens walleriana, the standard cultivars as well as the resistant cultivars. Remember that, while the resistant cultivars are highly resistant compared to the standard types, they are not immune—they are <100% resistant. Also, don’t forget about your double impatiens or the hybrid types that have I. walleriana in the mix. They also need to be protected from downy mildew.

Thanks to Margery Daughtrey, Cornell University, for comments and input.