Research Update – Controlling Powdery Mildew with Iodine

Erisiphe (Golovinomyces), commonly known as powdery mildew (PM), is a fungal disease that can damage many different types of crops. On most plants, PM appears as white, powdery spots on leaves, shoots, flowers or fruit, which if untreated, can spread over large areas of the leaves and stems, impacting photosynthesis and damaging crops, ultimately causing reduced yields and lower quality.

One of the primary reasons for powdery mildew’s broad impact is that its fungal spores are produced rapidly and in very large numbers. One square inch of infected plant tissue contains over two million powdery mildew spores. This ubiquitous plant pathogen causes the loss of many millions of dollars per year in the agricultural market across a multitude of crops.

Powdery Mildew Treatments

There are a number of powdery mildew-targeted fungicide treatments currently available to the agricultural markets. Notably, sulfur has been used to treat powdery mildew since the 1840’s. The fungicidal action of sulfur is complex and functions primarily in the respiration process. When sulfur is applied to a plant, vapors are released. The powdery mildew fungus produces hydrogen, which reacts with the sulfur to form hydrogen sulfide, a gas toxic to the fungus. Despite its broad use in agriculture, there are two major drawbacks to sulfur. First, hydrogen sulfide gas is highly toxic to the majority of living things. Human exposure can result in damage to the eyes and respiratory tract, and higher concentrations can rapidly lead to loss of consciousness or death. Thus, use of sulfur in confined vapor spaces must be done with caution. Second, sulfur has limited usefulness on other fungal diseases. For example, attempts were made to control potato rots with sulfur, but it was not effective. Thus, sulfur must be used in concert with other fungicides to achieve broad-spectrum protection.

Myclobutanil and chlorothalonil are other common active ingredients used to treat powdery mildew. Though both chemicals have relatively low acute toxicity, they also both have significant downsides. Chlorothalonil was found to be a key factor in the decline of the honeybee population in that that it makes bees more vulnerable to parasites. Myclobutanil is listed as a developmental toxin with adverse impacts to reproduction and reproductive organs in mammals. Both chemicals are also environmentally mobile and have been found in surface water and in rain, suggesting a potential for atmospheric transport and an ability to accumulate in soil with multiple applications. Because of the drawbacks to myclobutanil and chlorothalonil, products using “organic” active ingredients such as copper, plant oils and bicarbonates are common but efficacy with these products can be an issue.

At Clarion Water, it was clear to us that a better, safer and more environmentally responsible solution to powdery mildew was needed. With this in mind, Clarion Water developed IoGreen Aqueous IodineTM, an all-natural treatment program derived from a technology originally developed by NASA for use on manned space flights. IoGreen Aqueous IodineTM offers a solution that is non-toxic, biodegradable, and highly effective across a wide variety of fungal pathogens including, but not limited to, powdery mildew.


Iodine-based iodophor products have been used extensively in medicine, water disinfection, sanitation and other biocidal applications for over 100 years. Unlike these products, which contain a multitude of other chemicals, IoGreen Aqueous IodineTM contains only free (unbound) elemental iodine saturated into water through Clarion’s proprietary process.

Free iodine (I2) accepts an electron (e-) from the molecule it is reacting with in a process called oxidation and turns the iodine molecule into the non biocidal iodide (I-) ion. When in contact with microorganisms such as bacteria, viruses, fungi (including powdery mildew) and protozoa, iodine is able to rapidly penetrate their cell walls and attacks key groups of proteins (in particular the free-sulfur amino acids cysteine and methionine nucleotides, and fatty acids) within the cells. The combined effect of these oxidative reactions is cell death.

The biocidal action of iodine is broad spectrum and rapid, even at low concentrations. When compared with another widely used halogen biocide, chlorine, iodine is 8x more effective at controlling pathogens, and 4x less corrosive. Based on this, we believed iodine showed potential for use on plants at very low dosages to combat powdery mildew without adversely impacting plant health. So, we set out to test that hypothesis.


To validate the efficacy of IoGreen queous IodineTM against powdery mildew, Clarion utilized a proprietary microarray technology from PathogenDx. Polymerase Chain Reaction (PCR) was used to amplify relevant regions of the fungal genome and obtain DNA sequences for powdery mildew from untreated samples. Then the microarray probes were used to detect the presence of those sequences on the samples.

Untreated samples were collected from powdery mildew infected cannabis plants. The plants were kept in a sterile, climate-controlled environment. Subsequently, separate infected plants were treated with 2 protocols and the results measured. No other pesticides were used at any time. The protocols are as follows:

  • Protocol AB – 50mL IoGreen Aqueous IodineTM concentrate per gallon of water, sprayed at coarse droplet size (326-400 micron), ensuring full coverage of all plant foliar surfaces, 3x per week for 1 week.
  • Protocol AB.2A – 15mL IoGreen Aqueous IodineTM concentrate per gallon of water, sprayed at fine droplet size (145-225 micron), light dusting of foliar surfaces, 3x per week for 1 week.

Figure 1, below, shows gel electrophoresis data
 for these before and after 1 week.

The samples representing the Protocol AB and Protocol AB.2A indicated a loss of fungal material looking at both gel electrophoresis of the PCR products and the loss of fluorescence signals for the probes on the microarrays. Thus, the DNA sequencing results give a very high confidence level that IoGreen Aqueous IodineTM kills PM and that Protocol AB represents an effective program for the eradication of PM. Furthermore, the data indicates that the DNA of PM organisms disappears within a couple of days after the treatment under Protocol AB. Furthermore, Protocol AB falls well within the exposure limits set forth by the US EPA for pesticides and potable water. Thus, the iodine will not persist in the environment and no reentry period is required.