Sulfur dioxide — not sulfur itself — is the active ingredient in pesticides, and it poses health risks to farmers; potassium sorbate is a compelling alternative
Using sulfur for crop protection dates back some 4,500 years to the Sumerians. In modern times, sulfur is approved as a crop protectant for organic food production. More tons of sulfur are applied annually as pesticides to agricultural crops than any other active pesticide ingredient. In California alone, some 39 million pounds of sulfur were applied as a fungicide/insecticide to agricultural crops in 2022. In fact, agriculture has replaced the burning of fossil fuels as the largest human source of sulfur in the environment.
But hold on — sulfur is not actually a pesticide.
Forms of Sulfur
What then is sulfur? Most of us are at least somewhat familiar with the element. It’s yellow and is represented by an “S” in the periodic table, with an atomic mass of 32. Many falsely believe that sulfur has a strong odor, but elemental sulfur is odorless. Elemental sulfur is also a basic building block of life. Life would not exist without sulfur. Sulfur has low acute toxicity to animals, and there is no documented mode of action for sulfur to control insects, mites, fungi or bacteria.
Elemental sulfur is not utilized directly by plants. When elemental sulfur is incorporated into soil, certain bacteria oxidize it and convert a fraction of it into sulfate ions, which can be taken up by plant roots and incorporated into amino acids, proteins, coenzymes, and certain antioxidants that help plants deal with environmental stresses. These bacteria can further oxidize sulfur to create sulfuric acid, which has an acidifying effect on the soil. If no soil acidification is required, Epson salt and sulfate of potash are good natural sources of sulfate that are highly soluble in water and instantly available for uptake by plant roots.
Sulfur is a highly reactive element. As a component of microbial digestion, sulfur can combine with hydrogen in air or water to form hydrogen sulfide gas, which is a stinky gas we associate with the smell of rotten eggs. Hydrogen sulfide is an explosive, toxic gas that’s highly irritating to eyes, skin, and lungs. You should be aware of this hazardous gas if you do fermentations or composting in enclosed spaces. But fortunately, your nose can apprise you of such a danger, as it is capable of detecting the characteristic odor in concentrations as low as 0.5 parts per billion — well below a level that will harm you, at least if the exposure isn’t chronic.
Another sulfur compound is sulfur dioxide, a hazardous gas produced when sulfur is exposed to oxygen at temperatures above 68 degrees Fahrenheit. Like hydrogen sulfide, sulfur dioxide is a major air pollutant with significant impacts upon human health. Sulfur dioxide is the pungent odor of a just-struck match. It is also the major contributing factor to acid rain, which is associated with damaging sensitive plants, harming aquatic ecosystems, and acidifying and leaching nutrients from soils.
Sulfur Dioxide: The Pesticide
As already mentioned, sulfur is not a pesticide. But sulfur dioxide is. Sulfur dioxide damages cell membranes and interferes with essential enzyme functions, effectively causing cell death. When sulfur is applied, it reacts with oxygen in the air to become sulfur dioxide. Sulfur dioxide is the active ingredient of a sulfur application, not sulfur!
And get this: in picking up the two oxygen molecules, sulfur dioxide weighs twice the amount as sulfur. That 39 million pounds of sulfur applied for pest control in California could potentially produce 78 million pounds of sulfur dioxide! It is no wonder that researchers at Syracuse University claim that agriculture contributes more sulfur to the environment than even the burning of fossil fuels.
“The dose makes the poison” is a tenant well known to pest control professionals. Is a typical sulfur application rate of five pounds per acre enough to affect the health of the farming community? Researchers at the University of California, Berkeley followed families living in California’s Salinas Valley for some ten years and documented a statistically significant correlation between agricultural sulfur applications and reduced lung function in children living within a half a mile of application sites. No applicator produces a half mile of sulfur drift; these children could not have been exposed to elemental sulfur, and elemental sulfur is non-irritating to lungs. This exposure has to be from sulfur dioxide — a known lung irritant, and a gas that is impossible to contain on-site.
The knowledge that sulfur reacts with the atmosphere at ambient temperatures to form sulfur dioxide is of course not new. When I managed greenhouse vegetable operations for Gourmet Gardens Produce back in the 1980s, sulfur was not allowed for use in greenhouses in California, as the oxidation to sulfur dioxide in an enclosed space could create hazardous conditions. As the Superintendent of Agriculture for the Environmental Horticulture Department in the 1990s, we acidified soil for citrus using sulfur that was incorporated into the soil, as it was basic horticultural knowledge that the sulfur would gas off if sprinkled over the surface.
It is also fairly well known among Pest Control Advisors that sulfur does not work as a fungicide below 68 degrees Fahrenheit, the temperature at which sulfur starts to oxidize to a gas. Additionally, the oxidation rate of sulfur to sulfur dioxide increases with an increase in temperature and will burn foliage at temperatures above 90 degrees Fahrenheit. This is why labels for sulfur pesticides instruct applicators not to apply their product at temperatures above 90 degrees Fahrenheit. Still, the pesticide labels and associated Safety Data Sheets don’t acknowledge the hazards of sulfur dioxide and instead provide the hazards of sulfur, which are minimal.
I knew of the hazards of sulfur for years, but I still used it for pest control. I didn’t see good alternatives. So many of the synthetic fungicides are long lived and make their way into streams and aquafers and are later implicated as potential carcinogens. Paraffinic and mineral oils sound natural enough, but they are highly refined crude oil and are inherently phytotoxic and potential surface water contaminants. Neem oil is more phytotoxic than the petroleum oils and more damaging to aquatic life. Essential oils work well, but they are phytotoxic and expensive. To date, mycofungicides have shown low or inconsistent efficacy in field trials.
An Alternative: Potassium Sorbate
Without an economically competitive alternative, it is difficult to see a path for reducing farmers’ reliance upon sulfur. One possible candidate to replace sulfur, though, is potassium sorbate.
Potassium sorbate was isolated from mountain ash trees in the 1850s and is now one of the world’s leading food preservatives. It is a component of ecosystems of pristine forests. It doesn’t degrade into a toxic gas. Its modes of action in controlling bacteria and fungi are complex and well documented: degrading cell walls, working at the cellular level in shutting down respiration, and compromising the integrity of spores. At the cellular level, potassium sorbate inhibits dehydrogenases, sulfhydryl-containing enzymes, catalase enzymes, acetyl CoA formation, carbohydrate uptake, and amino acid uptake, along with eliminating the proton gradient that energizes membrane transport and uncoupling active transport from cellular energy production.
Research has also documented absorption of potassium sorbate into foliage and systemic movement within plants. In one study, foliar treatments of potassium sorbate were made to tomato seedlings, which were subsequently inoculated with a variety of root pathogens. After sixty days the plants were evaluated, and the single foliar application provided a measure of protection against all pathogens. It was particularly effective against fusarium, in which case inoculated plants had statistically the same dry weight as plants free of any disease inoculum. This makes for an interesting strategy for controlling root pathogens, whereby a disease-suppressive agent, potassium sorbate, is introduced via foliar application without disturbing soil biology.
In 2018 I began testing formulations of potassium sorbate for controlling plant diseases. I found a synergy in combining potassium sorbate with the food additive carrageenan. Carrageenan is extracted from certain species of seaweed using lye and then precipitating the carrageenan from solution with alcohol. Carrageenan provides multiple functions in these formulations. It has a basic pH. This matters because the effectiveness of potassium sorbate is strongly influenced by pH — it is least effective at a pH of 7.0. Most scientists have used potassium sorbate at a low pH, but studies show it is equally effective at a high pH. I focused on formulations at a higher pH, as this is more common in well water that farmers might be using to fill their spray rigs.
Aside from pushing the spray solution to a higher pH, carrageenan is a natural polymer that can become infused with potassium sorbate and then envelope hyphae, fungal spores and foliage, drying to an anti-fungal/anti-bacterial film surrounding them. The polymer also reduces leaf drip so that more of what is sprayed stays on the foliage. Furthermore, carrageenan has a thickening effect on the spray solution, increasing the drying time on foliage and giving potassium sorbate more time to compromise cell walls and membranes and to enter cells to disrupt metabolic processes.
Additionally, carrageenan is a plant biostimulant. In multiple crops, foliar-applied carrageenan has been shown to induce a defense response and to increase biomass production over non-treated crops.
The most promising of these potassium sorbate formulations has been field tested in multiple trials conducted by UC Davis, the Cal Poly Strawberry Center, and Collins Ag Research. It has performed better than sulfur in every field trial that has included sulfur, and in controlling powdery mildew has done so at a rate that would cost growers only around $10 per acre.
In addition to controlling powdery mildews in a variety of crops, this potassium sorbate formulation was tested to control peach leaf curl significantly better than copper with dormant oil, and it was a top performer in controlling downy mildew and brown rot. It is a cost-competitive alternative to sulfur and is compatible for use in rotation with sulfur. Such a rotation could result in cutting sulfur use in half.
Ronald Lane is the owner/operator of Circadian Crop Sciences, LLC. He is an inductee into the Gamma Sigma Delta Honor Society of Agriculture, a past Superintendent of Agriculture for the Environmental Horticulture and Plant Sciences departments at UC Davis, a former Vice President of the American Greenhouse Vegetable Growers Association, a former commercial greenhouse vegetable grower, and a California Licensed Pest Control Advisor.
