Powdery Mildew

Powdery mildews live on the surface of leaves and prefer dry, drought-like conditions (Rateaver, 1993). Often I have read that simply spraying the plant down with water can help. However, I tend to work towards prevention and finding the root of the problem.

I must say first that choosing varieties of plants that thrive in our climate is the smartest decision I have ever made! Attempting to bend nature to my will has proven time consuming and frustrating in most cases. While our climate tends to be quite unpredictable, I believe we experience a wide variety of pests and disease based on plant health, selection, and soil quality and not so much on climate. What impacts the health of a plant is much to be debated but in general, I find success in looking to the soil.

The elements that impact a plant and cause stress are relatively simple and any time we see pests and disease, that plant is under stress for sure. Plants need water, sunlight, air, and nutrients. I believe most of the time the easiest and most common issue tends to be water; whether it’s too much, too little, or poor quality water (chemicals, alkalinity, etc.), water problems can quickly create stress on a plant. Next in line is soil structure; the ideal structure for a healthy soil according to the Florida Nursery, Growers & Landscape Association is 45% minerals, 5% organic matter, 25% air, and 25% water. I’ve read several opinions but I think those percentages are pretty close to what it considered a good standard. In grade school science class, I remember our teacher showing us how to look at soil structure using a quart jar filled with soil and water; we shook up the jar and waited 24 hours for it to settle. The layers were clear and we could see each level of sand (minerals), organic matter, and water (the air being replaced by water). While I’m sure that isn’t a perfect method, and some may find it a bit inadequate, I belive it’s simple enough to give us a good idea of the soil profile. I tend to see a specific weeds growing in areas where aeration is a problem in soil; nutsedge, foxtail, thistle, dogfennel, bindweed, nightshade, and peppergrass are common weeds I see that are primary indicators of soil lacking air (McCaman, 2013); our sandy soil tends to compact easily and adding organic matter and improving the soil biology helps reduce soil compaction. I can recall several examples where plant problems were resolved by choosing the right amount of sunlight for each plant; a relatively easy resolution if sunlight is either too much or too little. There are plenty of references to consult about hours of sunlight needed for each variety. I find the more challenging part of diagnosing plant problems is nutrients.

Powdery Mildew is caused by an imbalance of calcium, phosphorus, and vitamin C available to the plant according to Dr. Arden Andersen in his book, The Anatomy of Life & Energy (2014). The calcium and phosphorus are commonly understood, not so much with Vitamin C. The presence of the mildew indicates there is a problem with the plant’s ability to produce Vitamin C, a substance that reduces plant stress. While I’ve not yet found anything concrete, it is my thought at this time that improving vitamin C involves a balanced soil microbiology. I come to this conclusion through several sources I have studied including Dr. Ingham’s Soil Food Web information (2006), Lowenfel’s Teaming with Microbes book (2010), Dr. Andersen’s work, the teachings of Dr. Howard Garrett (known as The Dirt Doctor), and numerous scientific articles such as the Horticultural and Food Research Institute Laing and Bulley (2007) article about what controls vitamin C levels in plants. I would simply add elements to the soil that improve the soil biology; a simple first step would be a good compost tea application containing plenty of molasses.

Most people don’t spend the time and money for a soil test and prefer guessing; my routine method until I realized how much time and money I was spending. Now I’m not saying we can’t look at the plant conditions and make decent choices because I believe that is helpful in many ways but to accurately diagnose an issue, testing is a critical component in my experience. Additionally, the soil tests where the results indicated the nutrients in the soil without their availability to plants were leading me down the wrong path and certainly not producing the results I expected. Last year, I discovered information about the Reams soil testing that uses a combination of methods taking into consideration nutrient availability, soil structure, and soil microbiology (Andersen, 2007). So far, I’m very pleased with the results and customizing amendments based on facts rather than my former and expensive guessing methods.

In the case where guessing is the preferred method, and since it’s typical that we have enough calcium in the soil here but it’s just not available, I wanted to share some ideas:

–  Try adding some epsom salts (magnesium sulfate at 1 cup per 100 square feet) to the soil and watering that in. We commonly see a lack of magnesium availability in our soil; magnesium helps the calcium become available in plants just like it does in our bodies.

–  Improve the soil! A healthy soil microbiology will encourage nutrient availability so adding quality compost (finished compost) and carbohydrates like dry and liquid molasses would improve the soil biology and thus the plants ability to absorb nutrients. I think many organic gardeners tend towards the idea that there can never be too much compost but I find it important to remember that balance is more important and keeping to around 5% organic matter in the soil profile is essential. In my opinion and experience, unfinished composts and cool manures with shavings may be beneficial over long periods of time but in the short term, simply causes more problems because while that carbon material is breaking down, nutrients like nitrogen are used and needed by the biology but taken away from the plant which can lead to deficiency. If unfinished compost, mulch, or manures are applied, I think adding a good nitrogen source, like cottonseed meal, along with other soil building materials can help mitigate the nitrogen problem.

– I find compost routinely high in phosphorus and potassium and since there has not been a soil test but we know an imbalance exists from the presence of powdery mildew, I would add zeolite in an effort to bring the balance back. You can read more about zeolite here in Dr. Garret’s articles in the reference section of this paper. I buy zeolite at Sutherlands General Store in the form of horse stall freshener called Sweet PDZ in which the single ingredient is Premium Grade clinoptilolite or another name, zeolite.

– I would also add dry or liquid molasses to this area to feed the soil biology and likely improve the plant’s ability to produce Vitamin C as a protectant against the mildew and other stressors. On a side note, since I started adding molasses to my routines around the garden, I see fewer fire ants, almost none as a matter of fact; you can read more about this topic on Howard Garrett’s web site (www.dirtdoctor.com). I didn’t believe it when I first heard it either but so far it’s been working for me!

– Spray the whole plant and soil with liquid seaweed at a rate of 1 ounce per gallon of water. Seaweed contains bio-stimulants that help reduce plant stress (Arioli, Mattner, and Winberg, 2015).

– Add calcium. While calcium presence may be shown in our soil, it’s availability to plants in an absorbable form is often low. Thus it would make sense to apply calcium in the form of gypsum because carbonized calcium and limestone tended to increase alkalinity where gypsum did not in my experience. I use to apply bone meal but after I started soil testing, I found that bone meal is not available to the plant quickly like the gypsum. The areas where I have added the gypsum are showing great improvement. It is recommended for citrus soil to have about 3000 parts per million (ppm) calcium and mine is up to 2940 ppm.

Sprays of baking soda and sulfur, biologicals (like bacteria), 3% hydrogen peroxide, and corn meal tea are simply temporary and I think beneficial to help the plant while working on nutrients; in the long run, choosing the right plants and addressing the soil biology and nutrients will reduce cost and labor and have potential to eliminate powdery mildew.

I always try to remember that spraying something to remedy a problem may keep sick plants alive but it can also kill beneficial microbes and lead to a variety of unintended consequences. I personally don’t want to eat from sick plants and would rather learn to hear nature’s message and provide what is needed.

I hope you have found these ideas interesting and helpful; your constructive thoughts and comments are welcome!



Arioli, T., Mattner, S. W., Winberg, P. C. (2015). Applications of Seaweed Extracts in Austrailian Agriculture: Past, Present, and Future. 5th Congress of the International Society for Applied Phycology. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4584108/pdf/10811_2015_Article_574.pdf

Andersen, A. B. (2014). The Anatomy of Life & Energy in Agriculture 3rd edition. Acres USA, Austin, TX.

Andersen, A (2007). Carey Reams’ Testing & Evaluation Methods. Acres USA Febrary 2007 Vol 37 No 2. https://aglabs.com/pdfs/Feb07_ReamsTesting_andersen.pdf

Bear, F.E., & Toth, S.J. (1948). Influence of calcium on availability of other soil cations. Soil Sci. 65:67-74. http://journals.lww.com/soilsci/citation/1948/01000/influence_of_calcium_on_availability_of_other_soil.7.aspx

Eckardt, N.A. (2008). The Plant Cell, Chitin Signaling in Plants: Insights into the Perception of Fungal Pathogens and Rhizobacterial Symbionts, American Society of Plant Biologists, Vol. 20: 241–243. http://www.soilfoodweb.com/Plant_Infection_Sites.html

Garrett, H. (2017). Zeolite. https://www.dirtdoctor.com/garden/Zeolite_vq848.htm

Garrett, H. (2016). The Organic Manual: Natural Organic Gardening and Living for Your Family, Plants, and Pets. Ogden Publications, Topeka, KS.

Ingham, E. R. & Rollins, C. A. (2006). Adding Biology for Conventional, Sustainable, and Organic Growing Systems. Sustainable Studies Institute, Corvallis, OR.

Lowenfels, J. & Lewis, W. (2010). Teaming with Microbes: The Organic Gardener’s Guide to the Soil Food Web. Timber Press, Portland, OR.

Laing, W. & Bulley S. (2007). What Controls Vitamin C Levels in Plants? Horticultural and Food Research Institute of New Zealand Ltd.

McCaman, J. L. (2014). When Weeds Talk. Sand Lake, MI.

Rateaver, B. & G. (1993). The Organic Method Primer Update Special Edition. Bargyla and Gylver Rateaver, San Diego, CA.