Lilac blooms are one of the best parts of the growing season, but these bushes can also bring dramatic heartbreak when they get sick. Powdery mildew on lilac bushes is one of the most common problems of these beloved plants; learn how to eliminate it from your garden inside.
There’s nothing like the scent of lilac in the spring, but the amazing smell that reminds so many of us of our grandmother’s and their gardens often comes with tricky diseases like powdery mildew. If you see white powder on lilac leaves, it doesn’t mean your bush is at the end, but unless you correct the underlying cause, it can become a chronic condition that weakens your bush over time.
Powdery mildew on lilac bushes isn’t caused by a single fungus, but is, instead, caused by several different species. As the name implies, the primary symptom in lilacs is a powder-like coating on the leaves of the bush, but this coating can spread to leaves, stems, and flowers if conditions are right. The fungus can also cause cupping, crinkling or other distortion of leaves in younger growth on your lilac bush. It often grows aggressively during warm, damp summers, resulting in entire sections of plants covered in the dust-like fruiting bodies.
For established bushes, a mild infection of powdery mildew won’t cause permanent damage or death, so often the best treatment is to simply ignore the problem. Sometimes it’s just a matter of bad luck that your summer is unusually moist or long, giving the pathogen a better chance to really set in. However, if you have powdery mildew fungus of lilac year after year, you may want to consider making some changes to how you garden.
Treating powdery mildew on lilacs is rarely necessary, but there are several things you can do to eliminate or reduce the effects of the environment on those fungal spores. Instead of heading straight for fungicide that is often a costly and endless battle, try these tricks to reduce powdery mildew long term:
1. Trim your bush. Thinning your lilac and trimming the bottom branches can greatly increase interior air circulation, which in turn makes it hard for powdery mildew to survive. It can also help to thin other plants that may be close by to encourage aggressive air flow so your plant can dry completely, robbing powdery mildew fungus of much needed humidity.
2. Remove plant debris. Powdery mildew spores tend to overwinter in the dead leaves of seasons past, so instead of letting that debris simply pile up, remove it and compost or bag it. This takes more powdery mildew out of play and helps to prevent reinfection.
3. Reduce fertilizer. Although it might seem counter-intuitive to withhold fertilizer from a sick lilac bush, it’s actually a great way to fight powdery mildew. Since this fungus loves soft, tender new growth and has a harder time infecting older, tougher growth, reducing or withholding fertilizer is another way to reduce the opportunities for a lilac to go wild.
How can I control powdery mildew on my garden phlox?
Powdery mildew is a common disease of garden phlox (Phlox paniculata). The fungal disease produces a grayish white coating on the leaves. Infected leaves eventually turn yellow and then brown. Initial symptoms appear on the lower leaves with the disease progressing upward.
Powdery mildew is most commonly found on plants growing in shady areas and in crowded plantings with poor air circulation. Optimal conditions for powdery mildew are cool nights followed by warm days.
Cultural practices can reduce the severity of powdery mildew on garden phlox. The amount of disease inoculum can be reduced by cutting off and removing diseased plant debris in fall. Plants growing in shady locations should be moved to a sunny site. In overcrowded plantings, improve air circulation by digging and dividing perennials.
While cultural practices are helpful, fungicides may be necessary to control powdery mildew on garden phlox. To be effective, fungicides should be applied at the first sign of the disease and repeated on a regular basis.
Selection of powdery mildew resistant varieties is another option. ‘Shortwood’ (rosy pink flowers), ‘David’ (white flowers), ‘Katherine’ (lavender blossoms), and ‘Robert Poore’ (reddish purple flowers) possess good resistance to powdery mildew. (The resistance or susceptibility of garden phlox varieties to powdery mildew varies within the United States. A variety that possesses good resistance to powdery mildew in the Midwest may be susceptible to powdery mildew in other regions of the country.)
The white or gray powder-like substance that afflicts plant foliage, stems and fruits is powdery mildew. Powdery mildew is a fungus that is an issue with a wide range of plants, including desert annuals, fruit trees and vegetables. The fungus spreads by spores and thrives in humid and low-light environments. Your first defense against powdery mildew is to select plant varieties that state they are resistant to this fungus. However, if you decide to grow plants that are not resistant, several cultural habits can help you prevent powdery mildew from becoming a serious problem in your landscape.
Select the sunniest spot that susceptible plants can tolerate. For example, if a plant can be grown in full or partial sun, choose an area that is exposed to at least six hours of direct sunlight a day. The sunlight helps lower humidity, which in turn reduces the likelihood of powdery mildew.
Space plants far enough apart so air can easily flow between plants, even at full maturity. Look at the spacing recommendations for the plants you are planting and use the widest distance listed, at a minimum. Good air flow helps lower the humidity in the middle and base of the plants.
Water plants susceptible to powdery mildew with a garden hose, spraying the plants from overhead. Keeping the foliage wet helps decrease the chances of powdery mildew spores from germinating. It also washes spores away. Because pests can become an issue with overhead watering, the University of California does not recommend this type of irrigation when growing vegetables where pest damage can ruin your crop. Also, if powdery mildew actually becomes a problem, stop overhead watering because it increases the spread.
Clean up plant debris around susceptible plants on a regular basis. This task helps remove spores that might be on the debris. Debris also stays wet longer than the soil, thereby raising humidity around the plants if left in place.
Adopt a regular pruning regimen that includes thinning. This cultural habit keeps the air flowing and it helps sunlight reach the center and bottom of susceptible plants to reduce powdery mildew. Also, prune and discard plant parts that are infested with powdery mildew to prevent spread.
Apply a ready-to-use fungicide labeled to prevent powdery mildew and listed as safe for your plants. Wait until winds are calm and rain is not forecasted for 24 hours. Spray the fungicide until all plant parts are thoroughly covered. Reapply as instructed on the label, often seven to 14 days during the growing season. If the product contains sulfur, do not use when temperatures exceed 90 degrees Fahrenheit.
Melissa Lewis is a former elementary classroom teacher and media specialist. She has also written for various online publications. Lewis holds a Bachelor of Arts in psychology from the University of Maryland Baltimore County.