Is Powdery Mildew What’s Really Plaguing Your Perennials?

Powdery mildew is one of the easiest plant diseases to recognize. Nevertheless, the symptoms of this unsightly disease can easily be mistaken for leaf spot disease, spray residue or injury, and nutrient deficiencies. Can you tell the difference?

I asked Cornell University Plant Pathologist Margery Daughtrey to weigh in on how to differentiate between powdery mildew on perennials and other unrelated issues. She shared her pro advice and followed up with some of the newest developments for treating this disease effectively.

Advertisement

Which perennials are most susceptible to powdery mildew, and why are they susceptible?

Daughtrey: The crops most notorious for powdery mildew susceptibility are phlox and monarda, and these are closely followed by peony, sedum, and many of the composites. The powdery mildews are a vast and varied group of fungi. They look pretty much alike to us, but are very specialized regarding which hosts they can infect.

The why of susceptibility is a complex question. I can tell you they are not susceptible because of any predisposing stress factors: plants are susceptible to powdery mildew simply according to their genetics. They don’t need a drought or a nutrient deficiency to be susceptible. You don’t even need overhead irrigation for powdery mildew to strike. High humidity favors powdery mildew diseases — not drippy, wet plant surfaces. High levels of moisture favor the parasites of powdery mildew — fungi such as Ampelomyces quisqualis that prey on the mycelium of powdery mildew. (Yes, even parasites of plants have their own parasites.)

Top Articles
New Survey on Sustainability in U.S. Floriculture Industry

What differentiates powdery mildew from other diseases?

Daughtrey: Powdery mildew is different from other diseases because the fungi that cause it are largely external to the plant — and thus often easy for us to see — with only the nutrient-absorbing structures called haustoria hidden within the cells of the epidermis.

Powdery mildew is also different from many of the other diseases because it almost never kills the plant that it feeds upon: powdery mildew will weaken its host, but it is an obligate parasite, totally dependent upon its host, and thus it has a vested interest in the host surviving. There are other groups of obligate parasites as well, such as rusts and downy mildews, but many leaf spotting and root rotting pathogens are able to switch away from being parasites into a saprophytic existence (feeding on decaying organic matter) in between opportunities to attack a living plant. Powdery mildews, however, are nothing without their hosts.

Powdery mildew symptoms, such as leaf discoloration, can sometimes resemble nutrient disorders or spray residue. How can a grower tell for sure if he/she is dealing with powdery mildew on their perennials? Where on the plant should a grower look for the first signs of the disease?

Daughtrey: Part of the answer is also the when of powdery mildew. Powdery mildew season traditionally begins in late summer and runs through the fall, but you might rarely see plants affected by it earlier in some years. At first the colonies of the powdery mildew may hide out on the undersurfaces of the leaves, where it is cooler. If you want an early warning that there is powdery mildew in a crop you know is prone to it, then when you are scouting for trouble, turn over leaves and look at the undersurface of lower leaves to check for powdery mildew colonies (This is not where you would find powdery mildew on a rose plant, however, as then the powdery mildew goes for the young tender growth). Sometimes there are yellow spots on the top of the leaf opposite white powdery mildew colonies on the undersurface of the leaf that give you a hint of which leaves to turn over.

Margery Daughtrey

Margery Daughtrey

It is actually sometimes hard to tell powdery mildew from spray residue, but think about what you have been doing recently: if you just sprayed the crop with a bicarbonate and you see white crystals uniformly over your crop, it’s the spray residue. A close look with a high-powered hand lens or a dissecting microscope will also allow you to tell the difference: the powdery mildew grows in strands across the surface of the leaf as well as making the spores that give it the sugar-coated look, and you can see the fungus colonies with magnification. Spray droplets have a smooth edge, whereas powdery mildew has more of a fringed edge. Both can be round and white, so look closely.

Sometimes foliage turns yellow or purple when infected by powdery mildew, making you think you have a nutrient deficiency, and this means you should look really closely before you haul out the fertilizer for a crop that you know is prone to powdery mildew. Looking closely at the leaf undersurface is also a good idea because it will help you find the cast skins, eggs, and bodies of spider mites, whose feeding can cause yellowing that you might mistake for powdery mildew effects.

What are the top three things perennials growers should do to manage powdery mildew and prevent transmission of the disease throughout the greenhouse?

Daughtrey: Grow cultivars that you know are less prone to powdery mildew (particularly important for phlox and monarda). Keep plants spaced on the bench to allow air movement around them. Pay attention to each crop: visual inspection will allow you to use fungicides at the beginning of an epidemic before the disease has become disfiguring.

If a grower plans to use a fungicide program to treat powdery mildew on his/her perennials, what active ingredients/products are most effective? What factors should growers consider when using fungicides to combat this problem?

It’s important to know which active ingredients to use against powdery mildew fungi. The strobilurins trifloxystrobin and pyraclostrobin (alone or in combination with FRAC Group 7 or 3 materials) are often very effective against powdery mildew, while azoxystrobin is less valuable against this disease. The strobilurin fungicides must be used strategically, as they cannot be repeated all season long: use them in a rotational scheme and expect them to give you 14 days of control. Potassium bicarbonate is an effective contact-action material: expect it to last seven days. Other materials with some effectiveness against powdery mildew that can be used in a rotational program include triflumizole (FRAC Group 3), thiophanate-methyl (FRAC Group 1), coppers (FRAC Group M1), oils, biologicals with Streptomyces or Bacillus active ingredients, and botanicals such as BLAD (lupine extract) and Reynoutria sachalinensis extract.

What is current research telling us about managing powdery mildew on perennials? Any new developments?

Daughtrey: The newest development is the availability of new combination products with 11 + 7 or 11+ 3 FRAC groups included in their formulation, which are very effective treatments. These combinations allow the grower to get good control of powdery mildew at the same time they are getting good control of leaf spots and Botrytis. Because of how powdery mildews develop into ongoing epidemics, it is important to include other support treatments in between applications of 11+7 or 11+3 materials, and the other fungicides listed are good for keeping powdery mildew in check. Some of the most recent active ingredients on the market are BLAD (Banda de Lupinus albus dolce), which is a protein from lupine that is the active ingredient in Fame, and laminarin, a plant defense stimulant (SAR inducer) found in Vacciplant.

4