If you experienced powdery mildew in your garden during 2024, what can you do, if anything, to ensure the fungus does not continue to haunt your garden this year?

Remember how the weather affected your garden last year? Dakota County received an extra 4” to 5” of rain and the average temperature during the 2024 gardening season compared with the average for the previous 25 years. The extra rainfall increased humidity and helped produce an abundance of powdery mildew in many Minnesota gardens. If your garden plants suffered from powdery mildew, you are not alone. Powdery mildew is one of the most prevalent crop diseases in the world,,3 affecting hundreds of types of plants including many common garden vegetables and flowers. This article explores what you can do to reduce the chances of last year’s powdery mildew returning to haunt your garden this year.

What is powdery mildew? 

Powdery mildew is a fungal disease stemming from several different pathogens that vary by host plant. Plants with early infections of powdery mildew may have yellow spots, dark blotches, discoloring and lesions on leaves, usually on the upper surface of newer leaves and lower leaves. The disease then progresses to include a ghostly-white, powder-like substance (image 1) on leaves and stems. If left unchecked, powdery mildew will quickly spread to cover the plant and wither the leaves and stems. The pathogen can spread from 10% of the leaves to 70% of the leaves in a week2. A severe infection will cause leaves to drop off and can significantly reduce fruit and flower production. 

The white spores of powdery mildew eventually mature into brown and black pinhead-sized spheres caed cleistothecia or chasmothecia (image 2). Cleistothecia will survive the winter and release new powdery mildew spores in the spring. ,,10 

Powdery mildew fungal spores infect only plants’ surface structures to take up nutrients and do not invade interior plant tissues. Therefore, it is rare that powdery mildew kills a plant but the plant may appear to be decimated. 2,10 

Minnesota weather and powdery mildew. 

Powdery mildew requires humidity (but not necessarily the presence of water on the leaves) for spore germination. Humidity refers to the amount of water vapor in the air. Humidity will increase around plants when the ground around the plants is wet and ground moisture steams into the air.

Variable relative humidity during the garden season will help promote powdery mildew. Relative humidity refers to the ratio of water vapor in the air versus the maximum water vapor possible in the air at a specific temperature. The maximum water vapor level is lower at lower temperatures. Therefore, for a fixed humidity level, relative humidity rises as the temperature falls. High relative humidity stimulates powdery mildew fungal reproduction while low relative humidity helps disperse fungal spores.  

Although powdery mildew requires humidity, the disease does not flourish in cool, rainy conditions.5 In fact, extended rainfall can help wash powdery mildew spores off leaves, inhibit germination and potentially kill powdery mildew spores. Powdery mildew spreads most rapidly in warm, cloudy12 weather (60o-80o F) and the spread decelerates in cool and hot weather.5 Note that leaf temperatures on sunny days are warmer than the surrounding air temperatures and leaf temperatures above 95o F are fatal to some (if not all) powdery mildews.2,4  

All of the weather preferences mentioned above mean that powdery mildew thrives when days are warm and relatively dry (spreads spores) while nights are warm and humid (germinates spores).5 Unfortunately, this recipe aligns with Minnesota summer conditions making powdery mildew a frequent pathogen in our gardens.

Prevent: what to do before a powdery mildew outbreak. 

While you cannot control the weather, you can minimize garden conditions that encourage powdery mildew.  Prevention of powdery mildew should be the primary focus of gardeners because the disease is very difficult to eradicate once the disease is established in a garden.5

• Frequently examine leaves and stems for a potential powdery mildew outbreak since early detection and action are critical to reducing the spread of the disease.5,12

• Choose sunnier garden spots, when possible, for plant types that are more susceptible to powdery mildew since powdery mildew will grow more in shaded areas. Highly susceptible garden plants include apples, bee balm, begonias, columbine, cucumbers, grapes, lilacs, peas, peonies, phlox, potatoes, rhododendron, roses, rudbeckia, squash and zinnias.6,,,10,12

• Increase airflow around plants to help reduce humidity and reduce moisture on plant leaves. Air flow will improve when you thin out plants, pull weeds, prune perennials, stake or trellis plants, and keep a space between plants and solid structures (e.g. keep plants 8”-12” from buildings).3,5,10,11

• Avoid over-watering of plants, especially in areas with slow draining soil. Water at the base of the plants, not on the leaves, and water early in the morning to give the plants time to dry out before night fall.3

• Plant garden varietals resistant to powdery mildew.5,12 Garden catalogs, seed packets and garden stores should provide information on the disease-resistant characteristics of plants.10,11 In addition, Cornell University provides a database of disease-resistant vegetable varietals. For example, Cornell reports that Avalanche snow peas and Cascadia snap peas are resistant to powdery mildew. Cornell’s 2025 information is based on 2018-22 data and should be confirmed, if possible, with plant-specific information for the current growing season.

Control: what to do during a powdery mildew outbreak.  

If you identify powdery mildew in your garden, you should follow all the powdery mildew prevention recommendations and also take the following actions to help control the outbreak.  It is important to act quickly when you diagnose powdery mildew in the garden since the pathogen spreads so rapidly.

• Trim off infected plant leaves and stems and remove the debris from the garden. Consider removing the entire plant if the infection has progressed significantly or if nearby plants are at risk of becoming infected.5,11

• Trim off leaves near the ground and newer leaves and stems of the infected plants. Newer leaves are more susceptible to powdery mildew. 

• Avoid fertilizing late in the season which encourages new plant growth.3,10

• Sterilize tools that were used to trim plants infected with powdery mildew before using the tools on healthy plants.

• Tolerate small outbreaks of powdery mildew.  The best management course may be tolerance of powdery mildew when the plant is beyond peak production and surrounding plants are not at risk. You may want to harvest full-grown pumpkins or squash in this situation to avoid powdery mildew attacking the fruit. Pumpkins and squash should last a couple of months if stored in a cool, dark spot after harvesting.11

Fungicides should only be used as a last resort since fungicides can harm both plants and insects.5 There are many fungicides that people have applied in attempts to control powdery mildew including milk, garlic, baking soda, neem oil, horticultural oils, potassium bicarbonate, sulfur and copper. Before choosing a fungicide, be sure to understand if the fungicide is a protectant (e.g. sulfur) or eradicant (e.g. oils) fungicide.  Protectant fungicides only work if applied prior to a powdery mildew outbreak and usually require repeat applications.  Eradicant fungicides work best if applied after a powdery mildew outbreak but before the outbreak becomes severe. Fungicides will not eliminate powdery mildew after the majority of leaves are infected.10 Be sure to follow any instructions provided on the fungicide packaging.3,6

There are a few specific situations where fungicides should be avoided even though powdery mildew is present in the garden. Fungicides should not be applied to late-season powdery mildew outbreaks when the autumnal leaves are close to dropping since the infection is not as harmful to the plant at that point.8 Fungicides should also not be applied to infected annual flowers or low-value perennial flowers since the fungicides can kill pollinators.10

Note that supporting research is very limited with regards to the efficacy of home remedies (e.g. milk, garlic, baking soda) for powdery mildew prevention or eradication. In addition, preventive and control activities should always be completed regardless of whether or not fungicides are applied.  Fungicides are rarely necessary.,

Minimize: what to do after a powdery mildew outbreak. 

Powdery mildew can survive winter on both live plants and plant debris. Therefore, powdery mildew is likely to recur once it is in your garden unless you take precautions to control the disease.

Autumn and spring clean-ups are critical to breaking the cycle of last year’s powdery mildew outbreak. All annuals that were infected should be pulled and removed from the garden, preferably in autumn but spring cleaning is acceptable. Perennials should be pruned to remove infected plant material and to increase air flow. In addition, any plant debris in and around infected plants should be collected and removed from the garden. Do not till infected plant material into the ground as powdery mildew will continue to live in the plant material. Infected refuse should be burned or composted by a commercial composter. Home composting does not get hot enough for long enough to kill powdery mildew.3,5

Powdery mildew requires plant material to thrive. Therefore, you should not need to treat or remove wood chips or soil from powdery mildew areas. However, if the wood chips appear moldy after a powdery mildew outbreak, you may want to remove the wood chips as a precautionary measure.

Because powdery mildew pathogens vary by host plant type, rotating plants (i.e. do not put the same plant type in the same space as last year) is key to minimizing the recurrence of powdery mildew.  The following table provides groupings of common garden plants that are affected by the same specific powdery mildew pathogen2. If you had one of the plants shown in a group in your garden last year and those plants were infected by powdery mildew, you should avoid planting any of the plants in the same group in the same spot in your garden this year. Note that some plants are included in multiple groups indicating that those plants may be impacted by more than one powdery mildew pathogen.

Photo Credits: Cherise Skeba (1), Michigan State University, http://www.canr.msu.edu/news/late- (2), University of Minnesota (3), University of Minnesota, M. Grabowski (4)

1 National Centers for Environmental Information, May-September 2024 compared with May-September 1999-2023, http://www.ncei.noaa.gov/access/monitoring/climate-at-a-glance/county/time-series/MN-037/tavg/5/9/1999-2024?base_prd=true&begbaseyear=1999&endbaseyear=2023

2  EOS (Earth Observing System) Data Analytics, “Powdery Mildew: Effective Prevention And Treatment”, http://eos.com/blog/powdery-mildew/#:~:text=Powdery%20mildew%20is%20among%20the,would%20kill%20most%20other%20fungi.

3  Colorado State University, “Powdery Mildews”, http://extension.colostate.edu/topic-areas/yard-garden/powdery-mildews-2-902/

4  Michigan State University, “Late-season control of powdery mildew in grapes”, Image 2 and content from http://www.canr.msu.edu/news/late-season_control_of_powdery_mildew_in_grapes_focus_on_inoculum_managemen

5  Penn State, “Addressing Downy Mildew and Powdery Mildew in the Home Garden”, http://extension.psu.edu/addressing-downy-mildew-and-powdery-mildew-in-the-home-garden#:~:text=In%20some%20plants%2C%20such%20as,oily%20spot%20on%20the%20leaves.&text=Powdery%20mildew%20looks%20like%20flour,across%20the%20top%20of%20leaves.

6 University of California, Davis, “Powdery Mildew on Ornamentals”, http://ipm.ucanr.edu/home-and-landscape/powdery-mildew-on-ornamentals/pest-notes/#gsc.tab=0

7 Western Massachusetts Master Gardener Assocication, “Powdery Mildew”, http://www.wmmga.org/content.aspx?page_id=22&club_id=101643&module_id=2289318 Western Massachusetts Master Gardener Assocication, “Powdery Mildew”, http://www.wmmga.org/content.aspx?page_id=22&club_id=101643&module_id=228931

8 Iowa State University, “Powdery Mildew- Ornamental plants”, http://yardandgarden.extension.iastate.edu/encyclopedia/powdery-mildew-ornamental-plants

9  Cornell University, “Disease Resistant Vegetable Varieties”, http://www.vegetables.cornell.edu/pest-management/disease-factsheets/disease-resistant-vegetable-varieties

10 University of Minnesota, “Powdery mildew in the flower garden”, http://extension.umn.edu/plant-diseases/powdery-mildew-flower-garden#:~:text=How%20to%20identify%20powdery%20mildew,to%20red%20around%20the%20infection.

11 University of Minnesota, “Powdery mildew in flowers and vining vegetables”, http://extension.umn.edu/yard-and-garden-news/powdery-mildew-flowers-and-vining-vegetables

12 Oklahoma State University, “Pumpkin and Squash Diseases”, http://extension.okstate.edu/fact-sheets/pumpkin-and-squash-diseases.html