Oomycetes and Powdery Mildew Fungus

powdery mildew on roses


 No,‭ ‬it’s not a typo.‭  ‬Oomycete really does start with two o’s.
It’s pronounced oh-oh-MY-seat,‭ emphasis on “my” ‬although the presence of oomycetes in the garden is more a matter of uh-oh than oh-oh, and I would rather that they not associate with any plant of mine, nor with any of yours, for that matter.
The most devastating plant diseases are caused by oomycetes.
Oomycetes were once thought to be fungi.  In Latin, oo = egg and mycete = fungus.  Thus, oomycetes were named for their egg-shaped female reproductive structures that are found in many kinds of fungi, too.  The difference is that oomycetes, unlike fungi, have sturdy cell walls made of cellulose, just like plants, and their cell nuclei contain two sets of genetic information, while fungi nuclei have only one set.  What this means is that oomycetes are both tougher and reproduce more readily than fungi, which makes oomycetes such formidable adversaries for farmers and gardeners everywhere.
The most common garden oomycete is Phytophthora, the plant destroyer.  (In Greek, phyto = plant and phthora = destruction.)  All plants are susceptible to Phytophthora.  While there is a very short list of Phytophthera tolerant or supposedly resistant plants, the list comes with a warning that in poorly drained soil, these plants, too, may become infected with the ubiquitous oomycete.
The most infamous Phytophthera tale concerns potato blight, a potato disease that ravaged Ireland in the 1840’s.  The Irish had become so dependent on potatoes for their sustenance that when the potato crop failed on account of Phytophthera infestation, nearly one million people died and another million and a half left Ireland, mostly for the United States.
Phytophthera typically enters plant roots, clogging them and reducing water uptake until wilting of leaves and death of the entire plant occurs.  I have seen plants that were perfectly healthy for years — such as rosemary — wilt and die practically overnight as a result of heavy late winter rain followed by warm temperatures.  Azaleas, too, are known for quickly dying where soil drainage is poor or where soil pH is insufficiently acidic, perfect conditions for Phytophthora growth.  Phytophthora is nearly always the explanation for the sudden death of any plant.   If your California native plant suddenly dies, which happens when soil drainage is inadequate, the culprit is probably Phytophthora.  Once a plant’s water supply is cut off on account of dead feeder roots — the ones near the soil surface that are primarily responsible for water uptake — that plant simply cannot live another day.
Many plants and trees, however, including avocado and citrus, will show a gradual, rather than instant, decline as a result of Phytophtora infection.  Incidentally, it has been shown that allowing avocado leaves to decompose on the soil surface may mitigate against Phytophthera infection since the rotting leaves release humic acid and anything acidic, in lowering soil pH, is good where Phytophthera deterrence is concerned.
Downy mildew, a notoriously damaging disease of grapevines, is also caused by an oomycete.  Downy mildew, observed as angular grayish white patches on leaves, nearly destroyed the wine industry in France.  Downy mildew is not native to France but arrived on the foliage of grapevines imported from the United States in the 1850’s, an era when farmers throughout the world engaged in a frenzied inter-continental exchange of crop varieties.  The imported American vines had varying degrees of susceptibility to downy mildew.  Meanwhile, however, the downy mildew was passed along to French vines, which were more prone to becoming infected with it.  French vineyards were in danger of being wiped out by this disease until, in Bordeaux, in the southwest corner of France, a remedy was found totally by chance.
Grape growers in Bordeaux whose vines were planted along roadsides had a major problem with pilferage.  In order to deter passersby from picking their grapes, the growers concocted a mixture made of copper sulfate and lime.  Not only were the grapes speckled white and unappetizing to look at following application of this formula, but their taste was bitter to anyone bold enough to try and eat them.  Yet when downy mildew began infesting the vines near roadside vineyards, vines that had been sprayed with the copper sulfate and lime did not develop the disease.  Thus was chemical control of plant disease successfully implemented for the first time, even if it was entirely by accident.  The grapevine saving potion was called Bordeaux mixture, a name it bears until today.  Bordeaux mixture, which is widely used in organic agriculture, has also proven useful in preventing the potato blight mentioned above as well as in combating fungus diseases such as apple scab, peach leaf curl, and powdery mildew.
Powdery mildew, a fungus, is the most prevalent plant disease, although more unsightly than life threatening, and it seems that most leafy plants are susceptible to it.  Unlike the spottier downy mildew, powdery mildew may appear as a soft layer of white on leaf surfaces.   On sun loving plants such as roses, powdery mildew is often an indicator of not enough sun exposure although, as I learned from Loren Zeldin, Reseda’s master gardener, morning dew alone can lead to powdery mildew on roses.  Zeldin found that by watering with overhead sprinklers early in the morning, which knocks fungal spores off foliage before they can germinate, he could prevent powdery mildew from taking hold.  Fine horticultural oil and neem oil are both effective in control of powdery mildew, too, as is baking soda (1 tablespoon) combined with liquid soap (1/2 teaspoon) in a gallon of water.  Be aware that the baking soda formula may cause certain foliage to burn so try it on a few leaves before dousing a whole plant with it.  The formula also loses strength over time so discard any that is left over following application.
Getting back to Phytophthora, I thought of writing about it after receiving two emails, one  regarding a wilting grapefruit tree with cracked and peeling bark in Valley Village and another concerning a cedar tree losing its needles in Silverlake.  The most likely explanation for these symptoms is root invasion by Phytophthora brought on by wet soil followed by warm weather.  Following last winter’s heavy rains, the root zone of these trees became saturated.  The warm weather of this past spring, while soil was still wet, encouraged release of Phytophthora zoospores.
Zoospores, microscopic in size, have tails called flagella that allow them to swim through water-filled soil pores until they make contact with, grow into, and kill roots.  Zoospores can remain dormant in fruiting bodies called sporangia for many years until saturated soil and warming soil trigger their release.  Poorly drained soil favors these conditions since its continued saturation with the onset of warm weather presents the perfect environment for Phytophthera proliferation.
Tip of the Week:  As part of your fall tablescape, you might want to consider ‘Orange Spice’ poinsettia, with its bracts that match the color of autumn foliage on liquidambar and Chinese tallow trees.  When picking out your plant, select one whose flower buds are still closed.  The brightly colored leaves you see on poinsettias are bracts.  While signaling that the  plant is ready to flower, bracts are not themselves floral appendages.  Focus on the center of your poinsettia plant where the bracts meet and you will see a cluster of golden yellow flower buds.  You want to find a plant with tightly closed buds since the opening of those buds coincides with the decline of the poinsettia’s colorful bract display.

Leave a Reply

Your email address will not be published. Required fields are marked *

This site uses Akismet to reduce spam. Learn how your comment data is processed.