As noted in our May 4 update, impatiens downy mildew has been observed recently in some Massachusetts greenhouses, particularly on double impatiens. Double impatiens have double rose-like blossoms, and like seed impatiens, they are susceptible to impatiens downy mildew (Plasmopara obducens). Remember to scout regularly for downy mildew by looking on the underside of leaves. If leaves are chlorotic (yellowing) with white fuzz on the underside, they should be placed in bags and discarded (not composted) immediately. All remaining impatiens should be treated with fungicides.
To this end, Dr. Mary Hausbeck at Michigan State University has recently devised a fungicide approach using systemic fungicides that are absorbed by the plants. According to Dr. Hausbeck, Subdue MAXX (mefenoxam) and Segovis (oxathiapiprolin) are very effective when applied as a drench. She outlines a three-step program for managing downy mildew:
- Step 1 - Apply Subdue MAXX as a drench at the rate of 1 fl oz /100 gallons water soon after plants are received unless the propagator treated the plants before shipment.
- Step 2 - Apply Segovis as a drench at the rate of 3.2 fl oz/ 100 gallons water seven (7) days after planting.
- Step 3 - Apply Subdue MAXX at the rate of 1 fl oz/100 gallons + Segovis 3.2 fl oz /100 gallons as a drench before shipment.
Always read the label for plant safety information and make sure to follow all label recommendations and restrictions.
For more information refer to the following links:
- Geoffrey Njue, Extension Educator, UMass Extension Greenhouse Crops & Floriculture Program