Blame It on the Rain
Identification and Control of Powdery Mildew
September 2, 2017
What is this white coating on the leaves of my chokecherry tree and what you would recommend to get rid of it?
-via Colfax County Ag Agent, Boe Lopez
The likely culprit for those symptoms at this time of year in New Mexico is powdery mildew.
Powdery mildew is a commonly occurring fungal disease across the country. Warm temperatures combined with high humidity in the plant canopy create the perfect conditions for the fungal spores to germinate and infection to spread. Therefore, the monsoon season provides ideal conditions for a powdery mildew outbreak in New Mexico. I once noticed the symptoms on rose leaves and buds earlier in the summer, but it turned out a nearby sprinkler head was spraying too high, unnecessarily causing higher humidity in the bush.
Different species of fungi cause the powdery mildew disease on different species of plants. However, the symptoms are very similar: white, powdery-looking film either in spots or completely coating leaf tissues. Distorted leaf features like curling and browning may also occur. If you think your plants may be infected with powdery mildew, but are unsure, you can always contact your local County Extension Agent for help. Your Agent may decide to send infected leaf samples to the NMSU Diagnostic Plant Clinic in Las Cruces for a diagnosis. These fungi do not infect people and they do not kill their host plant. But the disease can cause leaf damage, early leaf drop, and it is unsightly. Photos of the chokecherry tree featured in this week's column can be found at my blog: Desert Blooms Blog.
Some species and cultivars of plants are more susceptible or resistant to powdery mildew infection than others. Rose, zinnia, euonymous, lilac, locust, and crape myrtle are a few of the more susceptible species. But there are cultivars within these species that tend to have more resistance, like 'Hopi' crape myrtle or 'State Fair' zinnia.
We cannot stop the monsoons, not that we would want to, but we can slightly modify the plants' environment making them less susceptible to the fungi. One way to lower humidity in a shrub or tree canopy is to keep the plant pruned so that air can flow freely past the leaves. If a small part of the plant canopy appears to be infected, it may be best to prune that single branch out now. Pruning is a subject for another column all on its own, but for now just remember that the best pruning times and techniques are dependent on plant species. Chokecherry trees and shrubs should be pruned in late winter or early spring, so pruning too much now may cause more stress than the problem we are trying to control. Think ahead for how to best lower canopy humidity of your trees and shrubs next season.
Rake up leaf litter around the plants that showed symptoms after the leaves fall. This is another great way to minimize future infection of powdery mildew. These leaves should be discarded rather than composted or mulched. Get rid of them because they may harbor fungal inoculum that could re-infect the plants next season!
Healthy plants have a better ability to resist powdery mildew. This is true with other potential pathogens as well. When you see start to see disease symptoms on your plants, think about how you can best boost the plant immune system so that it might tackle all of the potential infectors on its own. From under watering to over fertilizing or forgetting to prune, these actions (or inactions) take their toll one by one.
While fungicides are available for control of powdery mildew, the proper timing of application is crucial for successful disease control. Since the likelihood that powdery mildew will kill the infected plant is very low, and our monsoon season is relatively close to the end of the growing season when leaves of deciduous trees will begin falling anyway, my recommendation is to sit back and enjoy the monsoon season while you make plans for cultural control of the fungus with proper pruning and leaf litter cleanup in a few months.
Got powdery mildew? #blameitontherain Post pics of your infected plant tissues on my Facebook page (DesertBloomsNM) or Twitter feed (@NMDesertBlooms)!
Keep the questions coming!
Marisa Y. Thompson, PhD, is the Extension Horticulture Specialist, in the Department of Extension Plant Sciences at the New Mexico State University Los Lunas Agricultural Science Center, email: firstname.lastname@example.org, office: 505-865-7340, ext. 113.
For more gardening information, visit the NMSU Extension Horticulture page at Desert Blooms.
Please copy your County Extension Agent and indicate your county of residence when you submit your question!