when I see it starting. because I'm not sure how effective it is as a prophylactic... also, I usually forget. :) cukes got it bad one year & spread to my comfrey. mixed up a half gallon milk with half gallon of water... sprayed the leaves liberally, soaked the soil around the plants. did it again a few days later. didn't have any sign of it this year, but I can't say for sure it lasts.