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Garden Journal

Garden Journal: The battle against powdery mildew

Zweep? That’s the happy song of sweet yellow birds as they dive and swoop about the yard. Twin tall branches are set in a dense garden area, effectively hiding messy spillage. Sunshine is here for sure, and the garden area I just mentioned is brimming with early roses, yellow evening primrose, red poppies, blue catmint, white geranium and oodles of summer perennials yet to come. One of those late summer favorites is garden phlox (Phlox paniculata).

I’m sure I have too much phlox, as each year it dances and spreads throughout the garden in such a way that the yard becomes a dream-like billowing cloud of white and pink and lavender. I tell myself I will dig some up and relocate it next year, but I never do.

Phlox attracts many beneficial insects, plus butterflies and hummingbirds thrive in it. Planted in full sun it grows anywhere from 3 to 5 feet tall, and flower heads bloom profusely mid to late summer. Varieties come in every shade of magenta, lavender, purple, pink and salmon, including a multi-pink and white variety I am especially fond of that is known as “Bright-eye.”

But, wait, what is that white and gray powdery stuff on the leaves? Almost overnight, powdery mildew has crept in and covered the leaves with white spots. Uh-oh. There goes the summer show. Some of the foliage has turned yellow and the leaves are starting to curl up and turn brown. This looks a lot worse than the actual damage really is. Rarely is this fatal to the plant, so don’t panic.

If you’re like me, you want a safe and gentle way to take care of this without resorting to spreading poison and chemicals all over your pretty garden. Although it is very upsetting when you first find it on your plants powdery mildew is very common under our wet and then super-hot growing conditions.

Each plant has its own specific type of mildew. Roses get one kind, bee balm gets another, fruit trees have theirs – although to us, it all looks like powdery death. Mold spores are wind borne and affect the new leaves first. Since phlox is such a rambunctious grower, the mold is caused by overcrowded conditions. (Oops, that’s me). The good news is that you can separate them and they will recover next year.

Early detection will give the best results in eliminating or at least containing the mildew, so if you had it last year, prepare to contain it this year. Always help your plants – they need you.

Home remedies to try for powdery mildew:

∎ Potassium bicarbonate – Not for you! The Alka seltzer is for the plants! Studies have shown that, when applied properly, this stuff effectively eliminates powdery mildew. Potassium bicarbonate is a contact fungicide, which kills the powdery mildew spores quickly and is approved for use in organic growing. Use instead of baking soda in the recipe below, applying until mildew is under control.

∎ Baking soda – This is possibly the best known of the homemade, organic solutions for powdery mildew. Although it will not cure powdery mildew by itself, when combined with horticultural-grade oil or vegetable oil (such as cottonseed or neem oil) efficacy is very good when it is applied in the early stages.

Make your own solution – Add one tablespoon of baking soda mixed with a teaspoon of soybean oil or other nut oil and a teaspoon of insecticidal or liquid soap (not detergent) to a gallon of water. Adjust amounts to suit the severity of affliction. Spray this mixture on plants every one to two weeks. You will soon see results. This formula will not bother neighboring plants.

∎ Apple cider vinegar – The acetic acid of cider vinegar can control powdery mildew. Do not mix this with baking soda. A mixture of 2 to 3 tablespoons of common apple cider vinegar mixed and a few drops of non-sulphate detergent (I used Earth-friendly Dishmate) added to a gallon of water does the job. Spray plants in the early morning or evening. Direct sun will burn the foliage.

 Mouthwash – If all else fails, generic, ethanol based mouthwash can be very effective at control. Tests using one part mouthwash to three parts water worked well for the Associate Professor at the University of Minnesota, Department of Horticulture. Mouthwash kills germs and mold, but be sure not to get it on healthy foliage.

Milk – Some have had great success with milk, and a little bit goes a long way. One experiment showed good results by applying a weekly dose of one part milk to two parts water. There are naturally occurring compounds in the milk that combat the disease while also boosting the plant’s immune system.

Turn the hose on affected plants several hours after applying one of these remedies to wash off white mold, and then reapply or use a different recipe for overnight. Contact me and let me know how things turned out:

You may only get 40 percent recovery from powdery mildew, so cut the most severely affected phlox down to three or four inches and relocate them to a less crowded area.

Plan to come back next year with your new arsenal.

As with anything else, prevention goes a long way. Thin out plants and mix them in with a variety of non-susceptible perennials. A great alternate choice for the back border is a tall and elegant plant immune to mildew with gorgeous blooms of spherical flowers the color of the summer sky, not to mention it attracts certain perspicacious yellow birds.

Try planting some Globe Thistle.

Legacy Comments2

I know I'm late on this comment, but perhaps it may help someone. I know the powdery mildew is running very strong this year, and it's too late for preventive measures in most cases. Still, this won't hurt anything to try. In this story it was suggested to use potassium bicarbonate to control mildew, and alluded to AlkaSeltzer as a source of potassium bicarbonate. AlkaSeltzer contains sodium bicarbonate, as does baking soda. Baking soda is readily available at low cost, and that's not the case with potassium bicarbonate. I have no personal experience with another salt, potassium phosphate, but have read that it is quite powerful against fungi and readily available, but not as cheaply as baking soda. BTW: potassium phosphate is a component of 10-10-10 garden fertilizer represented by the last two numbers.

Correction: The correct name for the fungicide product is potassium phosphite, not potassium phosphate. Phosphate is generally used as fertilizer, phosphite is generally used as a fungicide. Potassium phosphite is the active ingredient in many commercial fungicides, including Fosphite.

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