An Unlikely Pair: “Heavy Metal” and “Organic Produce”

  • Published on September 27th, 2010

I don’t know, but I’m guessing that there is not a big overlap between the people who are fans of “heavy metal” music and the people who buy a lot of Organic food.  But that is not the “unlikely pair” I’m talking about. I’m guessing that most people who buy Organic produce wouldn’t like the idea that they are getting quite a bit of copper (technically a “heavy metal“) as part of the bargain.  Why copper?  Because fungicides based on copper (copper hydroxide, copper sulfate pentahydrate…) some of the limited options that an Organic farmer has to control plant diseases caused by fungi and bacteria (there are also biocontrols that can help but which are often used along with coppers).

Why Even Bring This Up?

This certainly isn’t “news” since copper fungicides have been used in agriculture since the late 1800s.  I bring it up as a way to make three points:

  1. We need to be careful to avoid being mislead by emotive words like “toxic” or “heavy metals”
  2. Yes, pesticides are used on Organic crops
  3. The pesticides that are used on Organic are not necessarily safer than those used by “conventional” growers

1. Beware of Emotive Words

When I describe these fungicides as “toxic heavy metals” I am being technically accurate, but also being misleading because I’m not providing other important information.  Yes, copper is a “heavy” metal and it is toxic at high enough doses, but this particular use isn’t nearly as scary as that sounds.  Leaving it at “toxic heavy metal” would be an example of employing misleading language  – the sort of thing that is standard practice for anti-pesticide groups like the Environmental Working Group.  It is the sort of disinformation that reporters/bloggers pass along when they uncritically print something like the “Dirty Dozen” list.  To be informative, rather than manipulative, I need to tell you that copper is also an “essential mineral” in our diet and we would be sick without it.  I also need to tell you that even though these fungicides are toxic to mammals at certain doses,The EPA’s risk analysis finds that there is an acceptable safety margin between how much we can get from eating the produce and how much it would take to actually hurt us. I also need to tell you that copper is far less hazardous than the really dangerous heavy metals like lead or cadmium.  So when you read about topics like pesticides, consider whether there is more information needed before you let someone alarm you.

2.  Yes, Pesticides Are Used On Organic

Many consumers believe that by buying Organic they are eliminating exposure to pesticide residues.  This erroneous concept is often encouraged by some of those who market Organic products or those who advocate for Organic. There is a very long list (1700 products) of the materials allowed on Organic published by OMRI.  The pesticides on that list (including the copper fungicides) are definitely real pesticides (they kill pests) and so they have to be registered for use by the EPA like any other pesticide.

3. What About the Relative Safety of Organic and Conventional Options?

First of all, there is quite an overlap between the pesticides used on Organic and Conventional crops, it is just that the “Conventional” grower has more options.  Safety has nothing to do with which products are allowed for use on Organic – only the fact that they are deemed to be “natural” as opposed to synthetic.  This is a “philosophical” limitation, not a scientific one related to safety since some of the most toxic chemicals known are “natural” (aflatoxin, cyanide…).  50 or 60 years ago the Organic list may have been “safer,” but there has been a huge advance in the safety of synthetic pesticides since that time.

The various copper compounds that are approved for use in Organic (and widely used) are actually fairly toxic to mammals and to aquatic invertebrates by modern pesticide standards.  The measure of how poisonous a chemical is to mammals is known as the “acute oral toxicity.”  It is measured by how much chemical it takes to kill rats by feeding – so, the bigger the number, the less hazardous the material.  On this scale the Organically approved copper fungicides have toxicity measures in the range of 300-800 while many “synthetic” fungicides have values over 5,000 (this is also true for dozens of insecticides and herbicides).  That means that the Organic options are 5 to 17 times more hazardous on a pound-for-pound basis.  Not only that, the copper fungicides are applied to crops frequently and at several pounds per acre each time.  Many synthetic alternatives are applied at a few ounces per acre and less frequently.  The best synthetic products not only generate far less hazard, their exposure is also far smaller, and they actually work better for the control of most diseases. Acute toxicity like this is only one dimension of safety, but coppers also have issues with aquatic invertebrate toxicity and environmental persistence.

So, should you avoid Organic because of this toxicity issue?  No, but you also shouldn’t buy Organic based on false assumptions related to pesticides.  Whatever you choose, you can feel comfortable that the EPA does a good job of risk assessment.  Even with their “issues”, these copper fungicides meet the strict risk thresholds that the agency requires.  They are  “safe used according to label restrictions.” Many conventional alternatives are objectively safer, but the EPA does not allow products to be marketed on the basis of relative safety.  Copper fungicides are “safe enough,” and they have their place (particularly for some bacterial diseases where there are not better alternatives). Beyond those cases, it would be good for farmers to be using the best synthetic alternatives.

Heavy metal fan image from antjeverena


The discovery of copper as a fungicide is an interesting story.  A disease called Downey Mildew was spread from the New World to Europe in the mid 1800s.  In the New World it caused a mild disease on the wild grape species (Vitis labrusca …, think Concord grapes).  When it got to the European grapes of the species Vitis vinifera, it became a very severe disease that was threatening the viability of the wine industry. Alexis Millardet, One of the very first “plant pathologists,” (people who study diseases of plants – what I did for graduate school) was walking along a road in the French countryside.  A grape grower who was tired of people helping themselves to his grapes that grew by the road had somehow decided to spray them with a mixture of copper and lime to make the grapes look less appealing.  Millardet noticed that those grapes had less of the Downy Mildew disease.  He stopped to ask what the farmer had done. That was the beginning of the use of copper as a fungicides and fungicides in general. (though the Egyptians may have used sulfur as a fungicide 5000 years ago).

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About the Author

Born in Denver, now living near San Diego. Agricultural scientist for 30+ years with a Ph.D. in Plant Pathology. Have worked for Colorado State University, DuPont and Mycogen and for the last 13 years consulting for all sorts or companies, universities and grower groups. Experience in biological control, natural products, synthetic chemicals, genetics, GMOs and agronomic practices. Have given multiple invited talks on the interaction between agriculture and climate change (both ways)