Powdery mildew and black spot are common garden diseases and one of the most common home remedies is a mixture known as the ‘Cornell Formula’, a mixture whose main ingredient is baking soda.
This is an example of how and why myths get started.
In this post I will discuss the Cornell Formula, explain how it got started, and have a look at the efficacy of using baking soda as a DIY fungicide. Does it work? Will it harm your plants? Are there better options?
Cornell Formula for powdery mildew – does it really exist?
What is the Cornell Formula?
If you do a search for this formula you quickly realize that there is no ‘one’ formula. In fact, there are many versions of the formula but they are mostly similar and consist of the following key ingredients.
- 1 gallon water
- 1 tablespoon backing soda
- 1 tablespoon vegetable oil
- 2 drops dish washing liquid
These ingredients are mixed together and then sprayed on the plant that is under fungal attack. It is popular because it’s simple and most people have easy access to the ingredients.
Other ingredients can also be added and various types of oils and soaps are recommended. Many recipes do not include the oil.
Baking soda is sodium bicarbonate, a simple salt that can be found in the grocery store. Some people substitute potassium bicarbonate (available on Amazon).
But why are there so many versions of the formula? If the Cornell Formula exists and it is based on scientific studies, does it not make sense that there should be one preferred formula? The lack of one formula is a good hint that the Cornell Formula is a myth.
This stuff must be great – one site claims, “It’s as good as any chemical fungicide you can buy – the only thing stronger is fermented compost tea.” Compost tea is not fermented, and it certainly is not very good for fighting fungus infections. What nonsense!
Find our more about Powdery Mildew Treatments
The Invention of the Cornell Formula
As with most myths there is some level of truth to the story. In this case, there is real research involved and some data was reported. Then someone added in some bat wings, and eye of newt and came out with the Cornell Formula.
We know the basis of this myth because the original scientist, Dr. Ken Horst, has commented on the myth in a press release dated May 18, 2006, to try and set the record straight. Or at least we are led to believe this press release happened.
In 1985, Dr. Horst, working at Cornell University, started testing the effectiveness of sodium bicarbonate on garden fungi, like powdery mildew and black spot on roses. “During scientific trials, incomplete information was reported in a gardening publication that ultimately found its way into the public domain”, (ref 1). Other writers picked up on the information, filled in the details of the formula and gave it a name – the Cornell Formula.
Dr. Horst says, “There is no such formula.” “With the advent of the Internet,” says Dr. Horst, “The myth of the so-called ‘Cornell Formula’ continues to spread and I feel that the record needs to be set straight. Many of the formulas that are promoted in articles and forums are simply inferior and may have adverse impacts.”
That seems clear – the Cornell Formula is an invention by writers and general public, and spread by social media.
But like any good mystery – there is more to this story.
The Ingredients in the Cornell Formula
To better understand if and how this mixture works it is valuable to have a look at the ingredients.
Sodium Bicarbonate
Sodium bicarbonate when mixed in water is fairly alkaline. Fungal spores germinate best in acidic conditions. So sodium bicarbonate prevents or slows down the growth of fungi. This sounds simple, but all of this is very dependent on the dose. To be effective the right amount of material needs to be on the leaf. Too much can be phytotoxic (harms the leaf), and too little is ineffective.
Effectiveness also depends on the disease. We think of powdery mildew as one disease, but it is actually hundreds of different diseases. Black spot is a completely different disease. Each fungal disease is controlled to a different degree.
How much sodium bicarbonate should you use? J. Howard Garrett—a well-known horticultural columnist and radio personality in Dallas, Texas, recommends baking soda sprays at a concentration of 4 teaspoons (1.3 tablespoons) per gallon of water for control of powdery mildew, blackspot, brown patch, and other fungal diseases (ref 2).
Vegetable Oil
Many formulas suggest using vegetable oil, probably because it is readily available to home owners. It is much more likely that horticultural oil was used in the research and it is also a common recommendation.
The oil is important since it makes the sodium bicarbonate more effective. It also helps the bicarbonate stick to the plant.
Dish Soap
Lets face it, every home remedy includes some dish soap – so why not add it? In this case it helps the water stick to the leaf and may also help the bicarbonate stick better. It also helps the oil and water mix together.
Does Sodium Bicarbonate Work?
Sort of.
Sodium bicarbonate on its own is not very effective, but does work to some extent. When mixed with oil it becomes more potent, but it is not a cure. All it does is control the spread of the disease.
Here is the problem. The internet has many formulations. Which one works best? Since these formulas are not based on scientific testing nobody really knows. Which oil is best? How do different types of soap affect things? How much bicarbonate is the ‘right’ amount?
All good questions – with no answers.
Another important question is, “which formulations are phytotoxic”? No point in trying a cure if it harms the plant.
We know that home soaps are harmful to plants, even the darling of home remedies, Joy dish soap. If you make this formula, at least use insecticidal soap which is less harmful.
Horticultural oil would be better than cooking oil, but even it can be phytotoxic if used too frequently or in high doses.
The sodium in baking soda is also toxic to plants. Too much added to the garden will kill them. Lessor amounts might desiccate them. I would not use it in the garden. Potassium bicarbonate is 25% to 35% more effective than sodium bicarbonate, and much less toxic to plants. It is harder to find – self brew stores might carry it (available on Amazon).
An alternative is to use a commercial product.
GreenCure®
I am not recommending this product. I have never used it, nor do I know if it works. I am including this only because it allows me to continue the story of Dr. Horst’s work.
Dr. Horst continued his work for several years and published several papers on the subject. He also obtained several patents for commercial products and concluded the following (ref 1).
- Sodium bicarbonate is not very effective. Potassium bicarbonate works much better.
- Horticultural oil does not work very well and too much of it is phytotoxic.
He studied 350 different ‘spreader-sticker’ systems (oil + soap equivalent), on many different plants and diseases. This resulted in him developing a special formula which is now sold to home owners under the brand name of GreenCure®.
A Myth Inside a Myth – Maybe?
My investigation started with a report by the NC State Extension office claiming that the Cornell Formula did not exist and that “this was confirmed by a press release by Dr. Horst” (ref 3) . This report is incorrect. The press release is from GreenCure – not Dr. Horst. As far as I can tell, Dr. Horst never made a press release claiming that the Cornell Formula is a myth. I did contact GreenCure and asked them to supply his ‘so-called press release’, but got no answer.
Dr. Horst is associated with GreenCure.
The GreenCure press release, (ref 1), quotes things that Dr. Horst apparently said. So we have a company, which is heavily marketing a new product, making statements in a press release that imply home remedies that compete with its own product, don’t work. They say, “Many of these so-called formulas have very limited benefit and some of these recipes can even result in phytotoxicity”(ref 1). They then go on to say that Dr. Horst’s previous working formulas are no longer effective. If they worked in 1990, why do they not work today?
The press release claims that “Inconclusive information that originated out of isolated results from scientific trials has found its way into the public domain where it contributes to the myth of the Cornell Formula.”
This is certainly possible but results of his three year research work using sodium bicarbonate and insecticidal soap (no oil was added) were published in 1990 in Greenhouse manager. This article reported suppression of both powdery mildew and blackspot on roses. Is Dr. Horst responsible for publishing the so-called ‘inconclusive information’ as fact? If so, he played a role in starting the myth.
Clearly some of the things Dr. Horst says about his research would lead us to believe that home remedies are not going to work as well as his commercial product. But how biased are these statements?
For example, GreenCure reports Dr. Horst as saying, “horticultural oil was rejected as a spreader-sticker, in order to find a safer, more efficient additive”, (ref 1). However, several studies by others using bicarbonates and SunSpray horticultural oil report good results (ref 2).
If you read the press release carefully, what GreenCure actually says is, “baking soda and water by itself has limited benefit because it does not spread evenly across the surface of the leaf and easily washes away. The use of oils or soaps to spread and stick the bicarbonate can lead to an unwanted build-up of chemicals, alter soil pH levels and increase the potential of phytotoxicity. ” They do not say home remedies do not work – just that their product is better.
I have no reason for not believing Dr. Horst, nor GreenCure, except that they have financial interests in promoting their product and in trying to convince others that home remedies don’t work. Their spreader-sticker may be better than SunSpray, but how much better is it? GreenCure does not provide specific data on it’s web site to support their claims. If the difference is small, a home brew using a horticultural oil and potassium bicarbonate may work just as well.
Is this a case of someone claiming a myth exists in order to sell more product? Maybe the real myth is the claim that the Cornell Formula is a myth?
References:
1) Dispelling the Myth of the “Cornell Formula” Garden Fungicide; http://www.greencure.net/press_releases.asp
2) Use of Baking Soda as a Fungicide; https://attra.ncat.org/attra-pub/viewhtml.php?id=126
3) The Cornell Formula Fungicide; https://newcropsorganics.ces.ncsu.edu/2010/07/the-cornell-formula-fungicide-an-example-of-why-you-need-to-check-out-your-internet-information-sources/
4) Photo source, Cornell logo added: Jeff Kubina
