Jim Murphy & Sons

You can call it ‘powdery mildew,’ but it’s really a fungus among us

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Powdery mildew is the nemesis of any gardener worth his Epsom salt (magnesium sulphate) and one of the most widespread plant nudniks. Thus, there is no shame in finding powdery mildew, only a strong feeling of shared frustration when the spores win.

Mildew spores look like a white fuzzy powder that accumulates on leaves and stems, predominantly in spring, and again to a lesser degree in fall. It is actually a fungus that is spread by millions of microscopic spores, imbedding in tender new growth to feed on sap. By the time the naked eye can see the white ‘powder,’ it has already invaded the plant tissue and is feeding and reproducing at a rapid pace. As it spreads itself on the surface, it eventually kills the cells of the plant leaf, leaving the leaf rippled and curled.

Some plants are more resistant than others, and plant researchers go to great lengths trying to develop varieties that are resistant to mildew. It is worth looking for cultivars that have been shown to be “mildew resistant.” There’s nothing new about mildew spores in nature, only new gardeners learning the hard way that there is no easy way.

Grapes can be affected by powdery mildew.

Grapes can be affected by powdery mildew, downy mildew and black rot, the latter being the most important factor of grape production in Missouri vineyards. In the Ozarks, you can turn to the Missouri Commercial Grape Pest Control Guide (MS-19, revised annually) for current pesticide recommendations.

Powdery mildew tends to affect flowering plants and fruit trees or bushes (roses, apple trees, grape vines, etc.). This is why many gardeners advise watering the ground under a growing plant rather than showering the leaves. Best of all, use drip irrigation if it’s in the budget.

Even so, with spores always in the air to contaminate plants in the garden, mildew spores and can even be drawn inside to impact certain house plants.

The cannibus industry in Colorado and Washington, where growing it is legal (despite federal law), has discovered that powdery mildew can take over a crop under cultivation in tight places.

In online forums, you will find treatments that suggest a complete recovery at all stages of mildew infestation. But most writers (including this one) have rarely tested these products themselves, and the compounds are seldom differentiated between those applicable only for growth, and those safe to use during flowering, another reason to read carefully and ask plenty of questions.

So if you take away nothing else, remember there is no ultimate “cure” for powdery mildew, and affected growth is not going to recover as such. While not usually fatal, there is only prevention, and unaffected new growth – and wise selection of plants resistant to powdery mildew.

Actually, powdery mildew it is a fungal disease, the result of many species of fungi that affect a wide range of plant life. The same forces of nature that cause a proliferation of mushrooms in ideal conditions (mostly, non edible in the Ozarks),  powdery mildew can often spread like talcum powder after a shower.

Powdery mildew is one of the easier plant diseases to identify, usually appearing on lower leaves first, but the fungi can appear on any above-ground part of the plant. As the disease spreads, spots get larger and more dense as large numbers of asexual “spores” are formed, and the mildew may spread the length of the affected plant.

Powdery mildew grows well in environments with high humidity and moderate temperatures, but grows on dry surfaces. In an agricultural setting, the pathogen can be controlled using chemical methods, genetic resistance, and careful farming methods. Greenhouses provide an ideal moist, temperate environment for the spread of the disease, so “when in doubt, throw it out” and “whack it back” can be useful bromides.

Here’s some helpful points:

Description
A gray, talcum powder-like coating that covers the leaves, flowers, and even fruit of some of your vegetables, perennials, and shrubs, powdery mildew can affect plants in all regions of North America and the Ozarks.

Lifecycle
Fungal spores are spread by wind and overwinter on plants and in plant debris. Unlike mildews that appear in bathrooms or basements, powdery mildew does not need direct contact with water in order to grow. The warm days and cool nights of late summer create an ideal climate for spore growth and dispersal.

Plants affected
Powdery mildew is the blanket name for a few different species of fungi that infect many ornamentals, such as beebalm (Monarda), lilacs (Syringa), zinnias, roses, and garden phlox (P. paniculata). It also affects vegetables, including beans, cucumbers, grapes, melons, and squash.

A magnified look at powdery mildew (uncinula tulasnei) spores.

A magnified look at powdery mildew (uncinula tulasnei) spores.

Damage
Powdery mildew is unattractive and it can affect the flavor and reduce yields of some fruits and vegetables. Although plants are unsightly and can be weakened by an infection, they do not usually die. Powdery mildew on ornamentals is usually an aesthetic issue, and not usually worth treating. Prevention and control is more important for vegetables.

Organic control and prevention
Powdery mildew can be prevented, and it can be controlled once it appears, but it can’t be cured. The key to preventing it is planting mildew-resistant or mildew- tolerant varieties. Resistant varieties get less mildew than susceptible varieties; tolerant varieties may get some mildew, but it shouldn’t affect the performance of the plant. Prevention always includes locating plants where they will have good air circulation, and exposing as much leaf surface as possible to direct sunlight, which inhibits spore germination.

To control minor infestations, simply pick off leaves from affected plant parts and either compost them in a hot compost pile or bag them tightly and put them in the trash.

When found on produce

While soaking leaves in hydrogen peroxide will kill powdery mildew eating leaves with powdery mildew on them is not recommended. Although not toxic to humans, fungi cause allergic reactions in some people.

Homemade sprays
Spraying leaves with baking soda (one teaspoon in a quart water) will raise the pH, creating an inhospitable environment for powdery mildew. Research on infected zucchini showed that spraying cow’s milk slowed the spread of the disease. To try this, mix one part milk with nine parts water and spray the stems and tops of leaves with the solution. Reapply after rain, a requirement that can seem never-ending during a wet year like most of us have had in the Ozarks.

George Freeman is a veteran journalist and photographer. An award-winning writer, editor and columnist in Springfield, Mo., with more than 50 years experience. His preference is for positive and uplifting stories about people, places, traditions and trends that make the Ozarks one of the most livable regions anywhere. A member of the Garden Writers Association of America, he is a past-president of the Society of Professional Journalists of Southwest Missouri, the Kansas and Ohio AP societies; a board member of Friends of the Garden and a member of the Rotary Club of Springfield. In 1976, he traveled to India as a member of a Rotary Foundation Group Study Exchange Team.

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