How to Use Hydrogen Peroxide on Your Tomato Plants to Stop Fungal Diseases: Mix Ratios, Spray Routine & Theory

Posted by Gary Pilarchik on

Hydrogen Peroxide is H2O2, two hydrogen atoms and 2 oxygen atoms bonded together. It has one more oxygen atom than water, which is H2O. Studies have shown that plants create H2O2 and similar molecules in response to fungal and bacterial attacks. You can keyword search: The Oxidative Burst in Plant Disease Resistance for detailed information. 

This an extremely simplified explanation of how Hydrogen Peroxide or H2O2 works to kill fungi and bacteria on your tomato plants. The bonds that form a molecule of hydrogen peroxide are very unstable. When we spray H2O2 onto  tomato leaves, the fungi like 'Leaf Spot' and 'Early Blight' or contacted and covered. The bond between the two hydrogen peroxide oxygen atoms change, electrons move and energy is released.This process is called oxidation. The chemical reaction or process that changes H2O2 (Hydrogen Peroxide) to H2O (water) and O (oxygen) kills the contacted fungi.  The bottom line is... that is good for vegetable gardeners.

You can't spray Hydrogen Peroxide directly onto your vegetable plant leaves unless it is extremely diluted. If H2O2 is not diluted in water, the above process will also damage the plant's leaves as well as kill the targeted bacteria or fungus.

I use 3% hydrogen peroxide for creating my garden spray. You can find that in your pharmacies and grocery stores. There is a lot of information on mix ratios and some of them are straight up wrong. I dilute 3% hydrogen peroxide down to 12 Tablespoons or 6 Ounces of H2O2 per gallon of water. I use a pump sprayer for application. Always spray the tops and bottoms of the leaves as well as the stems.

I worked this ratio up from 6 Tablespoons to 8 Tablespoons to 10 Tablespoons and arrived at 12 Tablespoons for an effective spray. I suggest starting the same way and I recommend test spraying several leaves and waiting 48 hours to look for spray damage before you try this spray or any new sprays. If there is no damage to your tomato plant leaves, spray the plants. You may find lower ratios are effective in your gardening zone as diseases vary. Here is the video that shows my hydrogen peroxide experiment.

Unlike baking soda spray, wettable sulfur spray, Serenade and Daconil which prevent diseases from from establishing and multiplying on your tomato leaves, hydrogen peroxide actually kills the fungi and bacteria. You have a new tool in your defense against fungal attacks. Hydrogen Peroxide does not stay on the leaves of the tomato plants. It is gone in about 24 hours after spraying. Sunlight, in short, also activates the oxidation process. That is why H202 is kept in brown bottles. 

This is the general spraying routine that I use for managing fungal diseases with hydrogen peroxide. Again, 12 Tablespoons per gallon of water. Spray with H2O2 for 3 days. You can skip a day in between if you want. For larger outbreaks you probably want to spray every day on days 1,2 and 3.  If the outbreak is smaller you can spray over 5 days on day 1, 3 and 5. You can experiment and see which routine works best for the different fungal and bacteria attacks. They vary in different garden zones. Here is the video that explains my general routine:

Once you spray for 3 days wait 1 day and put down the preventative spray of your choice like baking soda, wettable sulfur, Serenade or Daconil. They would be used as a prophylactic to prevent the diseases from establishing. You can reapply these sprays every 7 to 14 days based on rain and your zone.  You can spray your plants with the hydrogen peroxide the day before you reapply the preventative sprays.

The bottom line is to pick a spray routine for your garden's needs and stick to it the best you can. If you get an fungal outbreak like 'Leaf Spot' or 'Early Blight' use hydrogen peroxide to get the diseases under control. My goal with diseases is to manage them down as to still get get great production from my tomato plants. Good luck!

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  • I see a picture in your blog that shows exactly what is happening to my tomatoes as they turn red. The entire bottom half of the tomato gets eaten away, with brown/gray rings or sections left. When I picked one of these tomatoes, a moth was on it, but not sure if the moth cause the damage or came feast afterwards. Is this due to disease, or there is specific creature eating the tomatoes? Thanks! Marcia

    Marcia L Wong on
  • Is the fruit (tomatoes) effected with the use of hydrogen peroxide. Just checking that the tomatoes are good to eat after spraying or is there a time period to not pick the fruit?

    Janet Nicholls on

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