Prevention tips for Impatiens Downy Mildew

Discussions of Impatiens Downy Mildew are still burgeoning in the horticulture world. The disease is not new, having been in the US since at least 1897 when it was reported on wild species of impatiens. The disease has been more prevalent in the last ten years and was significant in some landscape plantings in the US in the last two years. However, prudent measures will allow this beautiful plant to continue to be a valuable part of our crop mix.

The causal agent of impatiens downy mildew affecting crops is Plasmopara obducens, a "water mold" or oomycete. This pathogen only infects some species within the genus Impatiens and cannot threaten other flower crops. Impatiens walleriana (common garden impatiens) are affected by impatiens downy mildew but New Guinea impatiens (I. hawkeri) are highly tolerant of this disease.

Symptoms

Impatiens infected with downy mildew can simply appear non-vigorous with yellowish or pale green foliage and mild, inconspicuous mottling. These subtle symptoms can be missed or mistaken for other problems. As the disease progresses, advanced symptoms may include stunting of plant growth, downward curling or distortion of the leaves, loss of leaves, wilting and plant collapse.

The most distinct sign of downy mildew of impatiens is white to light gray downy 'fuzz' on the undersides of the leaves, but because the fungal growth is only visible on the undersides of the leaves, it can be missed. Frequent scouting is essential.

Prevention - Cultural Practices

Prevention of impatiens downy mildew is essential because control of an infection is extremely difficult. Start with propagation material free of the disease. Since the disease is not transmitted by seed, seedling plugs of I. walleriana that have been isolated from any source of the disease will be disease-free. Vegetatively propagated I. walleriana from reputable suppliers should be disease-free. However, since the disease may be initially symptomless, it is prudent to isolate vegetatively propagated I. walleriana from seed-propagated impatiens.

Any impatiens plants left after the main crop has been shipped should be removed from the greenhouse immediately and discarded. No volunteer plants should be allowed to grow in or around the production area, even briefly.

Prevention - Plant Protection

A downy mildew prevention program should include a rotation of effective fungicides with different modes of action. Not only does a complete control program give control of the disease during greenhouse production, but research has shown some products will provide early-season protection against downy mildew in the landscape. We specifically recommend a drench application to finished plants shortly before shipping. While no product will give season-long control of impatiens downy mildew, drench applications prior to shipping have been shown to provide early season protection against the disease and longer, more successful performance in the landscape. Please see the Impatiens Disease Prevention Schedule for a recommended rotational program and visit GreenTrust™ 365 for an ornamental downy mildew multipack offer to establish your disease-free greenhouse.

What to do if you find downy mildew

If impatiens downy mildew is found, all plants with symptoms and any fallen leaves must be immediately placed in sealed bags and removed from the greenhouse. Also remove and discard plants from a buffer area of three feet (one meter) radius or more around infected plants. Do not leave discarded plants in cull piles; remove them from the property. Use an approved greenhouse disinfectant to disinfect greenhouse surfaces contacted by infected plants. Immediately begin a preventive fungicide program on all remaining I. walleriana.

Source: Syngenta