A Guide to Eliminating Powdery Mildew Organically
Powdery mildew is one of the most widespread and easily recognized plant diseases. It can negatively affect most major horticultural crops. Powdery mildew is actually a family of mildews that are characterized by spots or patches of white to grayish, talcum-powder-like growth.
Oftentimes it begins on foliage beneath the top canopy or on the interior of plants, because “moderate temperatures and shade encourage the disease.”
Steps to Addressing Powdery Mildew
Understand Temperature and Relative Humidity
Improve Soil Mineral Nutrition
Monitor Nitrogen (And Other Nutrients) Through Plant Sap Analysis
Organic Spray Options
1. Temperature—the primary predictor of powdery mildew
Step 1 is easy.
Simply understanding temperature and relative humidity will make your war against powdery mildew much more effective.
Contrary to popular belief, powdery mildew spores do not require free water to proliferate. Infection can actually occur on dry leaves. Therefore, temperature is the primary factor in the growth of the disease. Universities have traditionally used temperature as the major variable predicting and treating powdery mildew on grapes.
The optimum temperature for the growth of powdery mildew is 66-77°F. Spores can germinate on the leaf between 50°F to 90°F, but growth is slowed in the upper and lower temperatures in this range.
Spores and powdery mildew colonies on grapes can be killed at extended durations above 91°F. The powdery mildew fungus is killed entirely when air temperatures rise above 95°F for more than 12 hours and if the leaves are directly exposed to light.
Track daily temperatures very closely to understand when powdery mildew growth is going to proliferate. Even better would be to pull climate data from the past five years from NOAA, and average the maximum daily temperatures in the spring/summer to give a sense of when powdery mildew will accelerate each season.
Luckily, there are numerous scientific studies of temperature as it relates to powdery mildew on grapes if you want to understand it further.
The Influence of Relative Humidity on Powdery Mildew
A secondary environmental factor that influences powdery mildew is relative humidity. It is unclear how much of a role relative humidity plays.
However tests have shown that it can be significant. On Riesling plants, the severity of powdery mildew doubled as relative humidity went from 39% to 85%.
This suggests why coastal growers often have an even more difficult time controlling the disease. With this in mind, reduce relative humidity by any means possible. This is typically done by improving airflow.
Avoid overhead watering that increases the relative humidity of the plants
Increase plant spacing
Increase pruning
Avoid overwatering
Eliminate standing water in the field
If understanding the timing of powdery mildew growth is essential for the success of your crop, check out the UC Davis Powdery Mildew Risk Index to improve management decisions.
2. Improve Soil Mineral Nutrition
It is possible to increase powdery mildew resistance dramatically with improved nutrient management.
We recommend starting by getting a Soil Audit and a Tissue Test. Collectively, these two simply tests and the Soil Dr. recommendations are the easiest first steps toward moving your plants toward greater nutrient sufficiency and balance.
After implementing recommendations from these tests, you can begin to look at more nuanced mineral interactions and how you can promote disease resistance by toggling individual nutrient levels.
Growing crops with resistance to powdery mildew requires focused attention on Calcium, Silica, Boron, and Copper.
Calcium, boron, and silica are all known as a powerful nutrients for fungal resistance due to its role in cell wall strength. Increasing cell wall strength by pushing more calcium and silica into your plant will increase resistance to fungal hyphae penetration.
Copper is well-known as a powerful disease suppressant when sprayed on leaves. However, increasing internal copper concentrations in plant leaves can act similarly as an internal disease suppressant.
Finally, nitrogen status can play a major role in the susceptibility of plants to powdery mildew. The best way to manage nitrogen is through sap testing.
3. Monitor Nitrogen (And Other Nutrients) Through Plant Sap Analysis
Once you’ve taken action from a Soil Audit and Tissue Test, the next step in precision nutrient management is to consider sap testing.
Sap testing allows you to dial in your nitrogen management to a greater degree than tissue testing. While tissue tests provide total nitrogen, sap tests provide levels of nitrate, ammonium, and molybendum in the plant sap. These three additional analyses provide much more nuance in management, specifically choosing the right form and right place to apply N.
Improved nitrogen management—and more specifically, improving nitrate reductase—is an important step in helping plants reduce their susceptibility to all fungal pathogens.
Before starting sap testing, be sure to read The Grower’s Guide to Plant Sap Analysis to avoid several common mistakes.
4. Organic Spray Options
Different powdery mildew spray products have two primary modes of action:
Suppression Through Biological Competition and SAR (systemic acquired resistance)
Suppression Through Reducing Powdery Mildew Populations
Organic Products to Increase Biological Competition or SAR
The most common targeted biological sprays include biological species such as Bacillus pumilis, Bacilus subtilis, and Streptomyces lydicus. These species are intended to populate the leaf surface and provide strong competition against many fungal pathogens. Common brands used as preventative sprays include: Sonata, Serenade, Activate, DoubleNickel 55, Cease, and Regalia (through SAR).
While branded products can work very effectively, a less expensive option would be to foliar apply compost extracts or teas high in a broad spectrum of beneficial microorganisms. This is a less-targeted, but more holistic and inexpensive approach when disease pressure is relatively low.
Organic Products to Reduce Powdery Mildew Populations
There are various compounds that reduce powdery mildew populations before they get out of control. These products have various modes of action.
Sulfur: Products with sulfur have long been the go-to material for two main reasons: it’s inexpensive and effective. Additionally, little resistance has been developed over 150 years of use. The major sulfur products to consider are wettable sulfur, lime sulfur, or sulfur dust. Apply sulfur products with caution, especially things like lime sulfur that are caustic.
Sterilizing Products: Products such as Zerotol (a combination of Hydrogen Peroxide and Peroxyacetic Acid), straight Hydrogen Peroxide (about 3% is best), Potassium bicarbonate, and Potassium silicate will kill powdery mildew quickly and aggressively. Their modes of action are different, but all of them are intended to kill powderly mildew instantly. Zerotol would be considered the “big gun” approach when things get out of control. Check with your certifier, follow the label, and re-inoculate plants with beneficial biological foliar after using.
Vegetable Oils: Across the board, vegetable oils are effective in reducing powdery mildew, likely through reduction of spore germination. One study found: “When tomato leaves were sprayed with 0.1% emulsified canola oil, corn oil, grape seed oil, peanut oil, safflower oil, soya bean oil or sunflower oil, the severity of powdery mildew caused by Oidium neolycopersici was greatly reduced. Among these edible oils tested, sunflower oil was the most effective in the control of powdery mildew. When sprayed with 0.5% sunflower oil, powdery mildew on tomato leaves was reduced to a negligible level.” Emulsifying vegetable oils with egg yolk has also been found to be highly effective.
Other Plant Oils: Various oils can work in the suppression of powdery mildew. Hyssop oil was found to be effective in one study. Products such as Thyme Guard offer broad spectrum suppression utilizing thyme oil. Products such as Cinnerate offer suppression utilizing cinnamon oil and potassium oleate. Various products utilize neem oil to reduce powdery mildew colonization (ex: Trilogy is a brand name product utilizing neem). However, there are two winners with consistent results across multiple plant species.
Jojoba Oil (shown in this study, this one, and explained here)
Japanese Knotweed. This one is much more expensive, but has been shown to be very effective (ex: this study and another).
Daniel Fernandez suggests the following recipe: “To prepare the organic fungicide for the prevention and treatment of PM, use 1g/gal of yucca extract, 45mL/gal of Jojoba oil, and 25mL/gal of sunflower oil. Add the yucca extract first and mix till it’s all dissolved, then add the oils and mix well before application. Ensure the oils are completely emulsified before performing an application. You can apply this as a foliar spray once per week.”
Mineral Oils: Products such as SuffOil-X have been used successful to reduce PM since it “interferes with the attachment of the pathogen to the host and acts as a suffocant.”
Comfrey Extracts: Some research suggests that comfrey extract can “markedly diminished infection in plants with powdery mildew”. This is from both protective action as well as photosynthetic activation within the plant.
Quillaja Extracts: “The Q” is an expensive but natural and effective product. A grower I trust says it works very well. Application rate is 2 tsp/gal as a foliar.