In regards to seaweed treatment for powdery mildew, some trials show that it could be helpful in the treatment for powdery mildew on different plants and trees. However, from what I have observed and read, it’s not the silver bullet. In combination with other things it could be helpful. I believe that both Accelerate and micro 5000 have seaweed in them. I read recently that someone used seaweed to prevent powdery mildew of the mango. However, they were only able to achieve success by spraying every other day. That might be something to try, but it would certainly use a lot of time and labor.