There are many species of fungus that cause powdery mildew on plants. Most only infect the leaf surface or stems and do not attack the leaf tissue of the host plant. Powdery mildew is not usually a serious problem, but to avoid severe damage to plants, quick control methods need to be taken.
Symptoms of powdery mildew:
Powdery mildews are observed in late spring and early summer as a white or gray powdery growth on leaves, stems, flowers, and fruit. As the fungus developments, buds fail to open, leaves can become distorted, turn yellow, brown or become chlorotic, or may drop prematurely. Fruits may develop blemishes or abort early.
Powdery mildew grows predominantly on leaf surfaces and does not require water to infect the plant. Powdery mildew fungi overwinter in tiny black bodies called fungal threads, which can be found in leaf litter, twigs, and dormant buds. In Spring, the fungal threads produce spores that start the cycle, especially during periods of high humidity when days are warm and nights are cool, ideal temperatures range between 60F to 80F. Vulnerable plants are most susceptible while new shoots and leaves are expanding. Fungus is host specific, meaning the powdery mildew on phlox does not infect crabapples.
How to manage the mildew!
Many powdery mildews, especially those that attack woody plants, are more unsightly than destructive. Good sanitation is highly important to reduce infections the next season. Powdery mildews can hibernate through the winter on dead and living plant tissue.
- Be proactive and purchase disease resistant plants.
- Space the plants properly, in-well drained soils where plants receive good air circulation.
- Dispose of diseased leaves as soon as they drop.
- Do not compost or use as mulch.
- Always avoid working among plants with wet foliage. Stay inside on rainy days!
Since most powdery mildew symptoms occur late in the growing season, it is usually not considered serious enough to justify chemical control. However, some plants may warrant protection and successful chemical control requires applying a fungicide properly and at the right time. Fungicides are a prophylactic, meaning it has to be sprayed on the plant before the infection occurs. Depending on what species or part of the plant (leaves, flower or fruit) you are trying to protect, spray times may be different.
One of my favorite, efficient fungicides to use is the Bordeaux mixture. In the early 19th century, many of the European grape vines were infected with blight caused by the aphid Phylloxera vastatrix (argued to have come from American grapes), but also mildew and other diseases caused by fungi.
In 1885, botany professor Pierre-Marie-Alexis Millardet of the University of Bordeaux studied powdery mildew in the vineyards of the Bordeaux region. He noticed the vines sprayed with copper and lime to keep nibblers away along the roads showed no signs of the disease. To this day, his solution is widely used in vineyards.
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