We have enjoyed operating a hydroponics store for several years now. During this time, we have helped many customers with a wide range of problems relating to their gardens. Many times in the past you have provided us with solid advice, and so we thought this time would be no exception. Our question is this: What would you recommend to be the safest and most effective way to get rid of powdery mildew in the late stages of bloom without any risk of damaging the plants. P.S. This is probably one of the most commonly asked questions in this industry.
Brian Hasebe and Beau Sherman, California Hydroponics.
Thanks for your question. As you well know, growers have tried just about anything to cure or remedy powdery mildew in the late flowering stage. The solutions range from some pretty heavy duty agricultural chemicals to home-made “potions”. It is worth noting that typically, the problem could have been avoided by keeping relative humidity below 60% (especially during dark cycles) and having good air circulation between plants. This means no plants leaning on each other or on surrounding walls. Watch for moisture pooling on large fan leaves. As far as safe remedies go, the best one I have heard of (but have not tried) is from Ted of Omega Garden. Apparently, spraying a solution of one part un-pasteurized whole milk to 10 parts water on the plants will stop the powdery mildew with just one application. Note that the milk is UN-pasteurized. The milk at most grocery stores is pasteurized, so you may need to look for it in a health or specialty food store. There are likely natural compounds present in the milk to stop the powdery mildew. Alternatively as a control and preventative measure, evaporative sulfur burners work very well. They produce sulfur gas from elemental sulfur (a natural product) which circulates throughout the crop and re-condenses leaving a very fine film of sulfur on the plant material to combat and prevent powdery mildew. The quality and effectiveness of the process relies largely on the sulfur being evaporated at the right temperature. Some models do this better than others. Thanks for taking the time to write in on behalf of your customers. Sincerely,