Plant Health: Downy mildew

The most common symptoms of downy mildew are yellow or tan, angular or blotchy spots on the upper leaf surface.

Downy mildew used to be a winter/spring disease that was most serious in the West coast. In the past 10 years, this has changed. The most recent development was widespread devastating downy mildew on impatiens last fall.

What does downy mildew look like?
Downy mildew looks very different depending on the specific plant, the weather, fungicide use and sometimes the species of fungus involved. The most common symptoms are yellow or tan, angular or blotchy spots that appear on the upper leaf surface. Leaf distortion also occurs giving them a blistered, pale look (common on pansy). On the undersides of the leaves, the white, lavender or purple spores of the pathogen form a “downy” patch between the leaf veins. Occasionally, the spores form all over the leaves. This happens frequently with Alyssum, giving the plants the appearance of being sprinkled with salt or sugar. In some cases, like snapdragons, a systemic infection can occur with severe stunting and distortion of the final appearance. Rose downy mildew is remarkably variable. The most common symptom has been angular, tan spots with a very small amount of white (and sometimes purplish) sporulation on leaf undersides. We found collapsing buds and cankers on newly emerging rose canes, and leaf drop is also common.

What plants get downy mildew?
The list of plants that get downy mildew increases almost as fast as the list of ornamentals we grow. Some of the perennial crops include osteospermum, statice, foxglove, coreopsis, sunflower, geum, felicia, geranium, sunflower, strawflower and scabiosa. Bedding plants with downy mildew include snapdragon, pansy, salvia, alyssum, basil, impatiens, coleus, lamium and many others. Woody plants that I have seen downy mildew on include rose, buddleia and hebe.

What conditions promote downy mildew?
Downy mildew weather is cool to warm, humid and rainy — not hot and dry. Wet leaves and high humidity will trigger sporulation overnight. When the sun comes up, leaves start to dry and spores are released. Most spores spread by fans or wind and infect new leaves before noon. Six hours of constant leaf wetness is enough for spores of many downy mildew fungi to germinate and infect leaves. Even though fans might move spores, you should use them and venting to reduce humidity and leaf wetness. Try to water early in the day or whenever leaves will dry quickly to ensure dry foliage at night. Check all new plants for symptoms upon receipt. Early detection is the only way to control downy mildew. Scout all plants for symptoms at least once a week and preferably every two to three days. Remember that hungry plants are more susceptible to downy mildew. Always maintain a balanced fertility program to protect your crops from this disease. Start early with a rotation of chemical prevention. Weekly spraying to prevent downy mildew on sensitive crops is common. Spraying more than twice a week is counter-productive.

What fungicides work?
Research has shown that the most effective fungicides are:

• Adorn (MOA 43 – tank-mix with another fungicide for downy mildew according to label restrictions)
• Aliette (MOA 33 – phosphonates)
• FenStop (MOA 11 – do not rotate with strobilurins – they are in the same MOA)
• Micora (MOA 40)
• Segway (MOA 21)
• Stature SC (MOA 40)
• Strobilurins (MOA 11 – like Compass O, Disarm O, Heritage, Insignia and Pageant)
• Subdue MAXX (MOA 4 – tank-mix with another fungicide for downy mildew according to label restrictions)

Six hours of constant leaf wetness is enough for many downy mildew fungi spores to germinate and infect leaves.

Can you stop downy mildew with a drench?
A few trials have been performed showing the effects of some of the systemic products when used as drenches. Subdue MAXX has been tested as a drench for a variety of downy mildew diseases including rose, stock (Matthiola), pansy and coleus. Results differed by plant type unfortunately. However, the following products were somewhat to highly effective when used as a drench in at least one trial: Adorn, FenStop, Stature and Subdue MAXX. You should know that whenever a foliar spray of the same product was compared to a drench, it worked better as a foliar spray. Another consideration is that the use rates for drenches may not be the same as those for a spray as a fungicide. One case in point is Heritage, which has a drench rate of 0.45 to 0.9 ounces/100 gallon while the foliar spray rate 1 to 4 ounces/100 gallon. Depending on how you decide to apply a “drench” the total volume of product may be quite a bit higher than that used as a spray, and labor may be far more costly as well.

Be sure to rotate at least two products in different chemical classes. Downy mildew fungi are very adept at becoming resistant to a fungicide when it is used repetitively. It is also important to remember that the only effective rotation program is the one you can implement. If it is too complicated or difficult to deploy then it will not happen. Intending to rotate is not the same as doing it. You might try to start with two products only alternating between them on a 7- to 14-day interval. Be sure to read the product labels – they are still the law.