I agree with John. For powdery mildew prevention and control I use a brew of 3tablespoons of baking (bicarb) soda, 1/2 teaspoon non detergent dish soap to 6litre of water. The soda changes the ph and destroys the spores, soap acts as sticking agent. MUST test spay, leave 24-48hrs observe for chemical damage. May need less soda. Do not add anything to brew like fertiliser or neem oil, as this will change the ph and the brew will either become ineffective or cause damage. When ok with mix, spray everything (plant, container, soil, stakes etc) all over. Give sprayer a good shake prior and during application. Don't pour leftover brew into soil - ph change can cause growth problems of young plants. I do this weekly from planting out regardless of need; this minimises powdery mildew starting up. My cucs, courgettes and toms get this treatment but the amount of soda required may vary. I keep a note of the brew(s) and results for future reference. Best done in the evening so it has time to do its thing overnight without the risk of sunburn. Make a fresh batch of brew for each spray session. Remove some additional leaves to enable good ventilation throughout plant. What variety of triphid are you growing?

- Karen