Impatiens downy mildew

Impatiens downy mildew was a major topic of discussion among growers, landscapers and retailers this year. Make sure you’re not caught off guard if the disease makes a comeback next year.

Impatiens downy mildew is caused by Plasmopara obducens. Downy mildew diseases are caused by fungus-like organisms called oomycetes or water molds and are closely related to Pythium and Phytophthora. The downy mildew that has been recently affecting impatiens in our industry is caused by Plasmopara obducens.

All seed- or vegetatively-propagated Impatiens walleriana (including double impatiens and mini-impatiens) and any I. walleriana interspecific hybrids are considered susceptible.
New Guinea impatiens (I. hawkeri) and interspecific hybrids such as SunPatiens appear to be tolerant to this disease.
Identifying the disease as early as possible is critical in achieving management; management should focus on prevention. Vigilantly check incoming shipments of plants and regularly check your crops in the greenhouse.

What to look for:
Early symptoms can be very subtle. Look for a slight stippling or chlorosis of the leaves. These symptoms might appear similar to a nutritional deficiency or spider mite injury. Leaves can exhibit a slight flagging or curling downward, sometimes giving the appearance that the plants need to be watered. Under the right conditions, you will see white-colored sporulation on the undersides of the foliage. Sporulation will not always be present, and won’t likely be observable in warm and dry conditions. In advanced stages, plants will appear stunted, can drop their leaves and flowers, and collapse. Contact your local extension specialist or diagnostic lab for assistance with diagnosis.
Downy mildew can be brought into a greenhouse, or moved around within a greenhouse operation, via infected plants. Downy mildew can also be spread by water splash and by wind-dispersal from infected plants in the greenhouse or nearby landscapes. There is currently no evidence that this downy mildew is seedborne. Oospores have been found in stem tissue from landscape impatiens in various regions. Oopsores are thick-walled spores that are known to help pathogens similar to P. obducens survive winters, however we are not yet certain if and under what conditions P. obducens will be able to survive overwinter.

Downy mildew thrives in moist or humid and cool conditions. New infections will occur under conditions that create long periods of leaf wetness (4 hours or longer).

Some cultural practices can help manage downy mildew:
Focus on prevention and early identification. Be familiar with symptoms and inspect all incoming plants and regularly scout crops.
Manage humidity and leaf wetness.
Aim for a RH of 85% or less and leaf wetness periods of 4 hours or less. Provide good air circulation with vents, fans, and appropriate plant spacing.
Use smart irrigation practices – avoid overhead irrigation where possible, water early in the day so that the foliage dries quickly, and avoid irrigating in cloudy and cool conditions. Try to avoid large swings in temperature that can result in condensation on the foliage, and heat and vent greenhouse air to lower humidity, if necessary.
Step up your sanitation practices. Start with disease-free plants and stock plants. Clean up plant debris between crops and at the end of the season and sanitize. Remove infected plants, as well as nearby Impatiens spp. plants (within ~3ft radius). When removing infected plants or plant debris, immediately bag and remove from the greenhouse to avoid spreading the disease. Do not compost infected plants.
Keep plants separate. Keep impatiens plants from different sources separate, and keep seed-propagated plants separate from cutting-propagated plants.

Source: Nora Catlin, Floriculture Specialist, Cornell Cooperative Extension of Suffolk County