The new disease is downy mildew, and it is destructive on impatiens. Downy mildew is most known on some of our vegetable crops, mainly cucurbits (cucumbers, squash, etc.), but like many diseases, it is host-specific and does not spread from one type of plant to the other.
Last week, my office got a phone call from a woman describing symptoms on her impatiens as yellowing and looking spindly. This week we had some plants brought into the office that looked like they had downy mildew on the leaves.
One set of plants affected in Georgia last week were ones that had reseeded from the previous year, which might indicate that the pathogen is overwintering. Downy mildew reproduces and spreads in mild, wet conditions.
All seed or vegetatively propagated Impatiens walleriana, which includes our regular double and mini-impatiens, are considered susceptible. The downy mildew pathogen can be moved to new areas on infected bedding plants, but once in the landscape, the spores can spread long distances to new plantings.
Early symptoms of the disease can be subtle with a slight yellowing, speckling, and curling of the leaves. Sometimes the yellowing is not visible before leaf curling begins.
As the disease progresses, whitish downy-looking growth will be visible on the underside of leaves. The plants will then appear stunted and drop their leaves, leaving mostly stems. If you suspect that your impatiens have this disease, please contact me or your local Extension office.
When it comes to management of this problem, you should focus on early identification and prevention.
Purchase disease-free plants and place plants in areas of the landscape with good air circulation.
High humidity and more than four hours of leaf wetness will encourage the disease, so water management is important.
If you find any infection on the plants, remove them and surrounding apparently healthy plants. Unfortunately, fungicide treatments will not eliminate all the disease in the infected beds, so they are basically a waste of time and money.