Baking soda with oil beats vinegar every time for this fungus. The choice of vinegar or baking soda for powdery mildew is easy once you know the facts. Vinegar can burn your leaves at the strength needed to kill fungus. Baking soda won't cause any harm and still slows the infection when you mix it right with a bit of oil and soap.
I tested both on my zucchini plants last summer to settle this debate once and for all. I sprayed baking soda mix on half the row and vinegar solution on the other half. After three weeks the baking soda side had healthier leaves and less white fuzz. The vinegar side showed brown burn marks on the leaf edges where the spray sat too long in the sun. That test made my choice clear.
The baking soda powdery mildew treatment works by raising the pH on your leaf surface. Powdery mildew spores need a certain pH range to sprout and grow. When you coat your leaves with a baking soda solution, you push that pH too high for the spores to get started. Adding a few drops of oil helps the mix stick to your leaves so it lasts longer between rains. Without the oil, your spray washes off with the first morning dew and leaves your plants exposed again.
Cornell University came up with a simple formula that gardeners still trust. Mix 1 tablespoon of baking soda with 1 teaspoon of liquid soap and 1 gallon of water. Add a tablespoon of oil to help it cling to your leaves. Spray your plants every 7-10 days during humid weather for the best results.
Vinegar spray for plant fungus sounds like a good idea until you see what it does to tender leaves. You need at least a 5% concentration to have any effect on the mildew. But at that strength, the acetic acid burns soft leaf tissue on hot days. Your lettuce, squash, and cucumber leaves are all too tender for it. I've seen gardeners strip leaves bare trying to fix a mild infection with straight kitchen vinegar.
If you still want to try vinegar, dilute it to 2-3 tablespoons per gallon of water. Spray in the early morning before the sun gets strong. Test on a few leaves first and wait 48 hours to check for burn damage. At this low strength, you're getting very little fungus-killing power. You're better off sticking with the baking soda recipe for real results.
I tried full-strength apple cider vinegar on a patch of infected phlox as a test. The mildew died but so did the top layer of leaf tissue on every plant I sprayed. Your plants can't make food from burned leaves. That one mistake taught me that vinegar is too harsh for most garden use.
Here's what I tell every gardener who asks me about home remedies. Start with the baking soda formula because it's cheap and safe. If you want something stronger, grab potassium bicarbonate from your garden center. It outperforms both baking soda and vinegar in every test I've run. You'll spend a few extra dollars but your plants will thank you with a much stronger harvest.
I made the switch to potassium bicarbonate two years ago. I tested it next to my usual baking soda mix on the same row of squash. The difference was clear within the first week. My treated leaves stayed clean and green while the baking soda row still had spots popping up. If you're serious about keeping your garden healthy, this upgrade is worth every penny you spend on it.
Read the full article: Powdery Mildew Treatment and Prevention
