Powdery mildew was a big problem in the greenhouse last growing season. I’ve never had a greenhouse before and, amazingly enough, have never had a problem with powdery mildew in my outdoor food or ornamental gardens. So I was unprepared to recognize and remediate it. I was always behind and despite trying a few remedies, I never caught up before removing the last tomato plants in November. This is what I’ve learned and am adding to my greenhouse ‘hygiene’ going forward.
What is Powdery Mildew
Powdery mildew is a common fungal disease that poses challenges for greenhouse growing and living. It manifests as a whitish, powdery substance on leaves, stems, and petals.
Environmental Conditions
It grows best in areas with high humidity and moderate temperature.
How is it Different From Toxic Mold
Although both powdery mildew and toxic mold are both fungi, they have distinct characteristics and potential impacts.
| Powdery Mildew | Black (aka Toxic) Mold |
| Many different species of ascomycete fungi in the order Erysiphales. | Stachybotrys chartarum |
| Generally appears as whitish powdery substance on plants | Often appears as a fuzzy growth on walls |
| Spreads outward on surfaces | Eats into its base making it harder to remove |
| Although it may trigger allergic responses, it is generally not harmful to humans | Can cause serious health issues for humans due to the mycotoxins it produces |
What Plant Families are Most Susceptible
It’s good to know what plant families are most susceptible to powdery mildew so I can stay vigilant everywhere these are planted. These are only the fruits and vegetables I intend to grow during the 2024 growing season:
- Nightshades: tomatoes, lettuces, potatoes
- Cucurbits: cucumbers, melons
- Legumes: beans, peas
How Does it Spread
Spores can spread to nearby plants via air currents. Spores in the soil can splash onto stems and lower leaves from over-head watering.
Greenhouse Hygiene
I’ll be using a multi-pronged approach to powdery mildew control, including:
Bioenvironmental Control: humidity level and air movement
Even though powdery mildew spores can germinate at low humidity levels, I will strive to keep relative humidity much lower than it has been. The recommended relative humidity level to prevent powdery mildew growth is 50%. However, the recommended optimal humidity level varies by both temperature and plant growth stage.
| Seed germination | 90%-100% | To encourage seeds to sprout and establish roots |
| Vegetative stage | 60%-70% | To encourage stem and leaf growth |
| Fruiting stage | 40%-50% | To encourage flower and fruit growth |
I’m also add a large dehumidifier to the greenhouse. A Moiswell 250 pint commercial dehumidifier is on its way and should arrive the end of February 2024.
Last year I left the ridge vents open 7×24 from spring until fall but kept them (mostly) closed through the winter to try to keep heat from solar gain inside the greenhouse. However, I’m now opening the ridge vents (almost) daily throughout the winter. Without opening the ridge vents, the relative humidity ranges between 85%-95% throughout the winter. Just by opening the ridge vents, the relative humidity has dropped into the 70s.
I’m also working on finishing the irrigation installation of soaker hoses and getting the irrigation set onto timers to limit irrigation to mornings only.
Plant and Soil Treatment: spray and remove
The internet provides recipes for many plant-safe and human-safe home-made sprays. Last summer I tried mixtures with hydrogen peroxide, milk, and NEEM with varying success. This year I’m trying potassium bicarbonate mixture to compare the results.
As each plant ended for the season, instead of composting, I removed that material (including the bamboo stakes) from the greenhouse and didn’t add to the compost bin.
Overall cleanliness: wash greenhouse and tools
Last year was the first greenhouse clean since the end of construction. I washed all the greenhouse walls from top to bottom by spraying with the hose, then scrubbing, rinsing, and a final squeegee. In hindsight this may have been a mistake that washed ‘live’ powdery mildew spores from the walls to the soil and added a lot of moisture.
This year I’m using a different approach. To kill the powdery mildew and limit washing it into the soil, I’m soaking a microfiber mop in a mix of water and potassium bicarbonate, wringing well, wiping the walls, and letting air dry.
Also keeping bottles of hydrogen peroxide and rubbing alcohol at the potting bench to clean tools after use on infected plants.
Still a lot to do to be ready for starting seeds in a couple weeks!
Hi Shawn,
Thanks for sharing information about the joys and challenges of living in a greenhouse. I’m working my way through your blog and the various videos, but I thought I’d share some possible solution that have worked for me with regards to the problem of powdery mildew.
Like you, I had powdery mildew problems in my free standing glasshouse in the first couple of years after I bought my home in the northwest Oregon Coast Range mountains. The mildew was affecting the quality of my salad greens, tomatoes, and chile peppers.
After discussions with a commercial greenhouse operator in the Portland area about how to minimize mildew issues, I implemented the following remedies that have eliminated these issues:
– Installed a pair of horizontal air flow fans, which improved ventilation and helped reduce excessive humidity
– Used vented humidity domes over my 1020 trays during the seed germination phase in my propagation area.
– Reduced the amount of water being delivered to plants in the greenhouse by reducing the output of drip irrigation emitters and dividing drip irrigation into multiple zones controlled by an irrigation timer/controller and associated electric valves. Instead of all plants being irrigated simultaneously and for the same duration, I now have 4 separate irrigation zones, each set for specific irrigation duration, as required by the type of plants – tomatoes, peppers, salad greens, and strawberries (I grow salad greens and strawberries year round in the greenhouse).
I also have a number of cold frames over raised beds outside of my greenhouse. The cold frames are built using the same 8mm twin walled polycarbonate used to sheath your greenhouse. In spring, when the cold frame covers stay closed until the the temperature inside the cold frame reaches 70° F., the lack of air movement inside the cold frames was causing some occurrences of powdery mildew on my chard, kale, and salad greens.
Once I replaced the “soaker hose” style drip irrigation in those raised beds with 15 mil drip tape with emitters spaced 8” apart, and reduced the duration of the daily irrigation cycle, the powdery mildew problem was eliminated. While using the “soaker hose” irrigation in multiple rows in each raised bed, I was planting many of my greens with a 6” spacing. Changing to drip tape with 8” emitter spacing reduced the number of plants I could place in each bed, but it improved the overall quality of the plants due to increased airflow around each plant, and it reduced the amount of water being pumped from my well.
I hope that your greenhouse produces a bountiful harvest for you next year!
Best regards,
Tony Mrozinski