During this growing season, many of you will hear, and some of you have already heard, about Impatiens Downy Mildew. This disease has recently broken out throughout the United States, but is not a new one. Impatiens downy mildew dates back to the late 1800s. A few years back, it began to pop up again in the South, then moved to the East coast and has recently hit the Midwest. This disease spreads to your typical bedding impatiens through the air, water and in previously infected soil. The fungus produces two different kinds of spores, which contribute to how easily it is spreading and the difficulty to fight it.
The first type of spore is Zoospores, which travel through water and wind. Oospores are the second type, which form in plant tissue. These spores make the disease particularly hard to fight because they will remain in the plant bed from year to year. Some studies have shown it may be killed if temperatures reach -5*F during the winter, but a very deep, long freeze is needed. With the mild winters we have been having, the disease has stuck around and spread further.
Impatiens downy mildew was recorded in 33 states last summer. The good news is that we had a long growing season last year, starting with 80 degree temperatures in March. With such a late start this year, we are hoping that the disease will not be as bad as last year or not be able to appear until the very end of summer.
Last year, the infection began to turn up in the Midwest during the later summer months, especially August. It is during this time that we have the high humidity that this disease thrives on. The high humidity, along with moisture on the leaves, are two of the main causes of this disease. Meaning, when you water your garden in the evening when you get home from work, you are increasing the chance of downy mildew as well as other diseases like root rot. The plant does not have a chance to dry off enough before the sun goes down, so they sit wet all night. This is not good for impatiens, or any other plants.
So what should you look for on your impatiens? The first sign is sporulation on the underside of the leaves. This means you will start to see white or gray fuzz on the underside of the leaves. From the top, the plants will look healthy, but underneath the mildew is already starting to take hold of the plants. Other symptoms can be mistaken for nutritional imbalances or other problems such as spider mites. Leaves will begin to turn a dull green and paler in color as time goes on, with yellow speckling beginning to appear on the foliage. The plants appear stunted in growth, with new growth being distorted. Foliage will begin to curl downward and the plant will appear wilted. If the disease gets far enough, the plant defoliates leaving you with just bare stems and tiny yellow leaves.
As discussed earlier, Impatiens Downy Mildew spreads quickly and can be hard to treat. The main treatment is prevention. This starts here, at the greenhouse. Our impatiens have been grown from seeds, meaning that we don’t use cuttings from growers in the south or anywhere else. By starting these from seed, there is no chance for the plants to be infected when we start our transplanting. We also plant these seeds, and all our plants, in sterilized soil. By sterilizing, or cooking, the soil, we kill any weed seed or fungus that may have existed in the material. So when you come to shop at the greenhouse, you can be assured we are sending you home with disease-free plants. In fact, last year was one of our best years for our impatiens. They were full of colorful blooms and healthy growth throughout the summer.
Your prevention can start before you even bring your plants home. Be sure to remove as much plant debris as possible at the end of each season, including dead leaves, stems and roots. When you plant your impatiens, as well as any plant, be sure not to overcrowd them. Give them enough space in between to allow proper air movement. Also, be sure that where you’re planting your flowers has proper drainage. As far as watering goes, eliminate night-time watering. The best time is to water is in morning, or at least five hours before sundown. Avoid getting water on leaves while you are watering. This can be done by using drip hoses through your garden or watering at the base of plants with a wand. Avoid planting your impatiens where they will be hit with your sprinkler every day. This will make it harder to control whether or not water is getting on the leaves, especially if your sprinklers go on in the evening.
A few weeks after you have planted your impatiens, be sure to inspect your leaves, top and bottom, on a regular basis. If you suspect a plant may have downy mildew, dispose of this plant immediately. Place roots, soil and all of the infected plants into a plastic bag and discard in the trash. Check the plants within a three foot radius of this infected plant as well. By properly discarding the infected plants, you will help prevent this disease from spreading any further throughout your yard or your neighborhood.
Unfortunately, there is no spray available to use once your plants have downy mildew. On a positive note, however, this only affects your bedding impatiens. Other flowers and even New Guinea impatiens and Sunpatiens are not affected by this strain. If your yard does get hit with downy mildew, do not continue to plant impatiens in your garden. Use the following alternatives for at least two to three years to be sure that the disease is completely out of your soil. If in a container, completely empty and throw away the soil in that container. Do not re-plant impatiens there the same year.
Some other shade loving annuals that you can use are coleus, begonias, New Guinea impatiens (not affected like bedding impatiens), caladiums and torenia. If your yard gets part shade, you have a few more options to plant. Flowers like nicotiana, ivy geraniums, diamond frost, lobelia, dusty miller and alyssum.
If you had downy mildew last year, or suspect that you may have had it, we recommend using an alternative plant this year. If you didn’t, there is no guarantee of whether you will or will not get it this summer. By using the steps we have listed, you can help prevent it in your yard. In the end it is up to you on whether you want to take the risk. I know I will still be planting my bed up as usual with my colorful mixed impatiens.