What Is Impatiens Downy Mildew?

Many people may have already heard about the disease that is attacking one of New England’s favorite plants, the impatiens. Symptoms usually begin with slight discoloration, mainly yellowing or whitening of normally green plant tissue. This will become completely yellow over time. Some varieties will have subtle gray markings on the upper leaf surface as well. A white “downy-like” growth may be present on the underside of primarily yellow leaves, these are known as airborne spores, and can also be found on the underside of green leaves.

As the disease progresses, the leaves will drop prematurely leaving the plants bare with leafless stems. Research has shown no evidence of seed-borne transmission at this time

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There are two types of spores that can be linked to transmission of the disease from one plant to another. One type of spore can be found on the undersides of infected leaves and will spread the disease by blowing in the wind or by water splash. The other type of spore is produced inside the infected stems. These spores are released into the soil from infected plant debris where they can survive and potentially initiate new infections on impatiens planted in the same garden beds for many years. It is not known how long these spores will survive in the soil.

This disease affects garden impatiens of all varieties, except the New Guinea impatiens. Development and expression of impatiens downy mildew is highly influenced by the weather. Wet foliage, cool temperatures (especially at night) and moist air are ideal for the disease to develop. Disease tends to be worse in locations where leaves stay wet for extended periods of time, in very dense beds, and beds receiving overhead sprinkler irrigation because the foliage does not dry quickly. Impatiens with downy mildew can be spread short distances by water splashing from infected plants and greater distances by wind-borne spores from infected plants. 

Once you know you have infected soil your best option for now may be to cycle in different plant alternatives. If you have any questions, please let us know!