Poor David's Almanac: A Few Clues About Powdery Mildew

by David Ingram, Consulting Rosarian

Part 1: What It Is

As gardeners, we’ve all seen it: one day (or so it seems!), our plants are suddenly covered with a powdery white growth. This is powdery mildew, a fungal disease that can affect vegetables, ornamentals, trees and many shrubs, including roses. Powdery mildew has been around since ancient times; and here in the Rocky Mountain region, is probably the most common fungal disease rose growers face. In this three-part article, we’ll take a look at: 1. What It Is, and 2. What To Do About It, using both culture techniques, as well as fungicidal sprays.

Since it was first described in the literature in 1819, powdery mildew of roses has had several names (I’m talking about scientific names, folks, not what we call it when it shows up on our roses!). According to Compendium of Rose Diseases and Pests, the current handle is Podosphaera pannosa. The pathogen is specific to roses, although some almond, apricot and peach cultivars in the Prunus family can also be affected. P. pannosa is only one of many different species of mildew that specialize on certain groups of plants. Lots of names, but they will all infect your garden under similar circumstances. So the good news is that the mildew that plagues other plants will not move to our roses; the bad news is that when conditions are right, many species of mildew can plague your garden in a hurry!

Rose powdery mildew spores (called conida) need certain conditions in order to germinate. The optimal temperature range is around 60° to 68° F, with relative humidity of 90 to 99 %. As a beautiful warm-to-hot day cools off in the evening, humidity levels within the canopy of a rose plant can easily rise to the levels needed for the spores to “hatch.” If airflow between plants and within plant canopies is poor, your garden may abruptly become a nursery for a problem you do not want to get started. (More on this in Part 2.)

Once the conida germinates (usually on young, lush growth), it sends a tube along the leaf surface that becomes known as a mycelium. This growth quickly generates a haustoria tube that penetrates the leaf surface, anchors it, and begins to drain the leaf cell of nutrients. The mycelium branches and generates more haustoria within the first 24 hours or so. This process can quickly cause young leaves to curl and distort. Within 48 hours, the first spore producing structures appear (each spore is only 1/1,000 of an inch long!). As the mycelium grows radially into a mass and undergoes changes over the next few days, long chains of conida spores grow upward, and the characteristic white powdery coloring appears that we gardeners recognize . . . and do not love. Warm days (80° F) with lower humidity (40 to 50 %) allow the spore chains to dry and disintegrate; the individual spores break free and spread by wind, insects and gardeners to new locations on your roses. Powdery mildew needs a repeated pattern of warm days and cool nights to germinate, grow and spread. This is how infections begin.

The first signs of trouble usually appear on fast-growing young stems, leaves and buds. You may notice reddish, raised, blister-like places on new leaves and other succulent growth, such as the base of developing buds and along maturing prickles. Entire young stems can quickly become infected, leading to curling leaves, and distorted buds that have trouble opening. At first, it may not be obvious that your rose has mildew, since it can take 3 to 7 days for the heavy growth of spore chains to produce the distinctive white powdery appearance. Learning to identify the infection in the first few days, before it “whitens up,” and starts to spread (and forces an otherwise normal rose gardener to achieve freak-out status), offers a great chance to begin treatment.

The odd thing about powdery mildew is that although it needs high humidity to germinate, the presence of water film on the leaves will prevent the spores from germinating. But once the leaf surfaces dry, and if the evaporated water (humidity) is held within the plant canopy, then trouble can begin. As we’ll discuss next month, your watering practices can play an important role in mildew management.

In our area, mildew has two means of overwintering. Fungal mycelia don’t live through our winters, but can survive within the inner bud scales of new bud eyes, then produce infections in the new year. And to share more bad news (yeah, yeah, I know, Bad News Poor David), P. pannosa also produces a different spore-producing growth called a cleistothecia, which can survive on leaves or stems, and produce new spores the following year. This is why we will talk next month about the importance of garden clean-up, removal of infected leaves from plants, and pruning of badly diseased stems.

While powdery mildew will not kill your rose plant, it can cause gross disfigurement of leaves and flowers, and affect the overall health of the plant. Once an infection breaks loose, our precious roses can get uglified in a hurry! A heavy infestation may even produce enough spores to cause respiratory distress for sensitive individuals. So. We understand how mildew works, right? And we can’t just ignore it. So - let’s fight back!

Next month, Part 2, will be about treatment options. We’ll discuss how good culture, prevention techniques, and treating a new infection early can spell the difference between a garden full of beautiful blooms, and the worst kind of late-summer white-out!

Part 2: Just For You, What To Do

Last month, we learned that powdery mildew of roses is a fungus that needs high humidity in an environment of warm days (80-85° F) and cool nights (60-68° F) to germinate on its favorite surface: lush new rose leaves and stems. In both spring and early fall, we get a whole lotta days like that along the Front Range. What’s a poor, put-upon rosarian supposed to do to keep mildew at bay?

You start with good culture. Grow your roses as well as you know how. Reach out to those around you. Keep learning. None of us knows it all. Fungal diseases love to target stressed foliage; the happier your plants are, the healthier they will usually be.

Here’s some tips that can help:

Space your plants well. There’s an old adage that says roses can be close enough to shake hands but never embrace. This means using air circulation to keep plant leaves dry and humidity build-up below disease thresholds. If the air circulation in your garden is compromised by buildings or high fences, you may need to space your plants even further apart. Climbers attached against walls are at risk; airflow behind as well as through the canes will keep them healthier.

Prune for airflow. Roses are notorious for producing excess, unruly growth while you’re not looking. During summer deadheading, evaluate the entire plant to see if a few extra snips can improve airflow within the plant canopy. Look for places to thin the upper canopy, which will help keep Mr. Mildew at bay. Try to keep the center of the plants open, and remove crossing growths.

Keep the garden clean. Although there is some professional discussion about the exact methods that PM uses to overwinter, you and I both know that this annoying and destructive fungus shows up in our yards on schedule every year. The best strategy to minimize the overwintering of all fungal diseases is to keep your garden clean of fallen leaves and rose debris year round. Remember that mycelia will overwinter inside rudimentary buds. Trim off obviously diseased stems in the winter. Spring-pruned canes may contain disease. Discard this stuff, don’t compost it.

Enough sun. Trying to place roses where they don’t get enough sunlight (5 to 6 hours per day) is a recipe for stressed, vulnerable plants. Should we call it: Growing For Mildew?

Water at the right time. Morning watering allows the warmth of the day to dry leaves and reduce humidity levels before fungal spores can germinate, whether black spot, rust, or mildew. It’s okay to wash off foliage as long as the plant is able to dry before trouble can begin. There’s not much we can do about evening rain showers; but if you’ve spaced your plants well, managed the canopy to foster good airflow, and kept a careful IPM eye on your new growth, you’re on the right track.

Proper fertilizing. In our area, heavy feeding, particularly with nitrogen, can produce the kind of lush new growth that is easily targeted by mildew spores. I see this sometimes in the spring, but most commonly in the fall. All too often, rosarians with a heavy hand on the fertilizers also have to be dedicated fungicide sprayers. I recommend the proper dosage of an organic slow release fertilizer such as Mile-Hi Rose Feed to promote the kind of balanced growth that is best able to stay healthy, and still produce lots of beautiful flowers.

Plant resistant varieties. Is this a real thing? Well actually, yeah. During the 20th century, the marketplace supported hybrid teas and floribundas that displayed the best form and color. Traits like disease resistance and fragrance became lost in the search for sales. But a new generation of hybridizers are making special efforts to emphasize disease resistance, and bring back fragrance (yes, please!!!). It is up to us to support the sales of the healthiest, most fragrant new roses . . . that are also gorgeous and must have!

But there are roses such as species and many OGRs, as well as some of the tough Canadians and other shrubs that seem to shrug off mildew. All of us know varieties that stay healthy in our gardens, even if we’re not sure why. (Of course, it must be the excellent care you take of your plants!) Sharing healthy rose information is a great reason to be part of a rose society. Along with the snacks!

There’s lore that strong water streams can actually knock mildew off your rose leaves. The idea is that mildew forms mycelial masses on leaf surfaces, anchored by haustoria that penetrate the leaf and suck nutrients out of the inner cells. If you can hose off the mycelial mass, the haustoria die. This will work, within limits. In practice, I find that the force of water needed to dislodge enough mildew to make a difference can also destroy open blooms and damage new growth before it can flower. I also have concerns that I’m hosing a bazillion spores to other parts of the plant where they’ll lurk, waiting for the water to evaporate and create the right humid conditions for germination. Try it - carefully - but keep a sharp eye on the results.

Powdery mildew is pernicious, adaptive, and invasive. What do we do if we get a gold star for our garden culture, and PM shows up anyway (How dare it!)? Tune in next month, and we’ll discuss fungicides in general, and a couple of organic options in particular.

Part 3: It’s White! It’s Growing! What Can I Do?

In the past two articles, we’ve learned that powdery mildew of roses (PM) is a fungal disease that needs high humidity to germinate, and is happiest on new, lush growth. While PM can occur at any time in the growing season, the warm days and cool nights of fall tend to produce the most infections. Good culture and plant choice helps limit mildew on our roses. But sometimes, despite our best efforts, this annoying fungus creeps in and curls leaves and whitens ripening buds.

For me, if I get . . . okay . . . once I get mildew, I have a choice: Live with some of the white stuff, or try to eliminate it. Since PM spreads so easily when conditions are right, I choose to fight back. What should we do when a rose we love starts to show symptoms like red splotches, leaf curl, and that alarming white conidal growth?

The basic truth about most fungicides is that they are best used to prevent powdery mildew. Few can offer a cure. This is a reason why so many rosarians nation-wide engage in preventative spray programs. Along the Front Range of Colorado, our normally dry climate means we don’t automatically need to become fungicidal maniacs. But most of us usually end up growing a few “mildew magnets” that pose as normal rose bushes. When the monsoon moisture arrives in late summer and fall, some of the new growth on my faves like ‘Let Freedom Ring’ and ‘Veterans Honor’ can’t stay healthy. I’m not tossing ‘em out, so I have to treat them. (Pause here while readers count off the mildew magnets in their yards. Only count the ones you intend to keep no matter what. The others may need to encounter a little “shovel pruning.” This technique is a part of good culture.)

There is no one fungicide that will wipe PM off your roses and restore order to the universe. In fact, there are far too many products out there for us to cover in this article. But they basically fall into two modes of action: 1. Contact fungicides coat the leaves and stems, and protect or cure from the outside. Contact fungicides work well, but usually need to be reapplied after rain or watering errors. 2. Systemic fungicides enter through the leaves or roots and move throughout the plant; they work from within. Also, many fungicides operate by interrupting a single growth function of the fungus. These “single-mode” fungicides are at risk of the fungus developing resistance. For this reason, best practice is to rotate most fungicides to prevent this. You can learn more at such websites as rosemania.com, or domyownpestcontrol.com

As you study your options, be aware that these products vary widely in toxicity. Read the entire label and follow all directions regarding the use of protective clothing and equipment. The content of the gear list often guides my decision whether to use a product or not.

But knowing where the hearts of most of our members are, let me suggest three organic products with lower toxicity that will not only prevent powdery mildew, but often cure an outbreak. Any curative product will always work best if you catch an infection early. These three are contact fungicide sprays, and mildew is unlikely to ever develop resistance to their broad mode of action. Expect to re-apply after rain.

GreenCure® features potassium bicarbonate, the main ingredient in baking powder. It can be mixed at different strengths to either protect the plant, or attack growing mildew. GreenCure® is OMRI certified for a broad array of plants, and can safely be used the day of harvest. This product is sold on many websites (different packages available). Also available at a handful of local nurseries, or ** BEST ** from the mighty Denver Rose Society at our meetings.

The active ingredient in Serenade® biofungicide is a patented strain of the bacteria Bacillus subtilis. Serenede® can also be used on a wide variety of plants, is OMRI certified, and can be applied the day of harvest. In my garden, Serenade® has shown the ability to knock down mild cases of powdery mildew when caught early. Available in ready-to-use or concentrate. Apply at seven-day intervals. Non-toxic to bees and other insects.

Neem oil will also control mildew. The product on store shelves is a simple oil, with the active insecticide agent (Azadiractin) removed and sold separately. Spraying neem oil protects against mildew getting started, and can also coat and dehydrate an active infection (expect to spray every seven days until controlled). There are a lot of neem oil products out there that vary in strength and purity. Pollinators can be harmed if sprayed directly. Spraying in the heat of day may cause foliage burning, and using neem oil within two weeks of applying sulfur may also cause foliage damage.

I’ve learned that any spraying is best done in the cool and calm air of early morning. Wear all protective gear listed on the label, and be aware of REI – the re-entry interval for family and pets. Remember that rugosa roses, because of the crinkled, “rugose” shape of their leaves, are intolerant of sprays other than pure water.

A few words on “home remedies” for mildew. All sorts of ideas exist on the internet, including milk, garlic and mouthwash. While a few gardeners swear by them, the science on these concoctions varies between fuzzy and none. If you try one anyway, test for foliage damage before spraying all your plants. Be prepared for disappointment.

Powdery mildew is very annoying and destructive. Grow your roses well, catch an infection early and consider trying our three options to keep your foliage clean, the flowers plentiful, and choirs singing to the glow of our Rocky Mountain sunsets!