By this time of year, most of us have smelled something strange in our gardens or woods. What is that stinky, mushroomy scent in the air? Never smelt it? Count yourself lucky indeed! It’s another one of those garden nemeses, ranked right up there with Moose for difficulty to control. Powdery Mildew, the ‘Witches Wilt’ of the Middle Ages, is one of the oldest recorded plant diseases. And with a name like Witches Wilt, need I say more?
Like moose, powdery mildew is always ready to attack, is rarely seen approaching and is always harmful if allowed to wallow in your garden. And they are both equally impossible to chase out once they’ve entered the gate!
Powdery Mildew is a common name for a small group of closely related mildews that manifest themselves in a similar manner. The primary sign that these microscopic beasts have invaded your territory are grayish white, powdery blotches on leaves, stems and buds. If left unchecked, the growth will soon devour its victim. Once a plant is infected, leaves can turn yellow and drop prematurely. Young growth is especially susceptible with twisting and deforming, sometimes before the mildew appears. Flower buds may also develop abnormally or not open. Even before the plant is swallowed up in white, stinky yuck, it can look like it was abducted by aliens and used for some horrific experiment. Worse yet, it can leave you feeling desperate to know what in the world (or other-world) has hit your beloveds.
Powdery Mildew on Rose Leaves
Without introducing too many dull and sleepy details, it is important to know this tiny enemy. First, realize that the spores are carried by air currants so they cannot be kept out of your garden. Once settled onto your plants they lie in wait for the perfect day to pull out their fangs and spring to life! The optimum germination temperatures for these mildews are between 68 to 77 degrees F with optimum relative humidity above 40%. It is a popular belief that water on the leaves encourages germination, however dry leaves under ideal conditions are just as susceptible. Low light also seems to favor powdery mildew development. From the time of infection to the sign of symptoms takes about a week or a little longer, and by this time the secondary spore production is well underway, reproducing every 48 to 72 hours since the initial infection! What all this means is that once you’ve sniffed out the problem or noticed that your once-favorite rose is in deep trouble, the stuff could be anywhere!
While I will be the first to tell you that it can sometimes be a pain to keep up with this miniscule life sucking force, it is not impossible.
Sulfur is a traditional combatant with two major positives: it’s cheap and effective. Furthermore, it’s been used effectively to control Powdery Mildew throughout the world for nearly 150 years, with no development of resistance, however, fungus are nasty and sometimes so is the cure. There are many sulfur containing fungicides on the market but the solution has to be at lease 33% sulfur to be effective. Because sulfur acts largely through vapor, its activity is temperature-sensitive and does not work too well if the temperature is below 65 degrees F. That limits some of our application times right there! It is also somewhat phytotoxic (in other words, it is toxic to foliage and will defoliate your plants) at temperatures above 85F. In spite of these limitations, I have been nuking these mildews with sulfur for years. If you’ve ever had a disgusting infestation reducing your favorite perennial to a lump of dried up trash, losing a few (or a lot) of leaves along the way is an acceptable liability.
Potassium bicarbonate works in a similar fashion, with similar limitations, but must be present in the fungicide at least 85%. There’s a bit more information on both of these weapons online at dirtdivasgardening.blogspot.com.
A more drastic (or is it?) approach would be to yank the plant out by the root, put it in a plastic bag and throw it out, but perhaps a gentler, though less thorough approach would make you happy. If so, try mixing up the following and attack with a spray bottle.
2 ½ T. light olive oil
1 gallon water
A few drops of liquid soap
6 t. baking soda
Most importantly – think wicked whiteness and beware! It’s out there and waiting to pay you a personal visit!