Sorry about the rot Keith.
I haven't found anything that totally prevents it other than spraying with BT to kill the caterpillars that can be one cause. I will say that I hardly see it anymore since I started using citric acid (.25% in distilled water as Mel mentioned) but I won't say it's definitely a preventative. Works on PM for sure though.
Don't use copper fungicides on your flowers as Captain mentioned since they're not safe for smoking. ↓↓
I use a tiny amount of yucca powder and find it works really well, but if you use dish soap stay clear of Dawn since it contains petroleum.
No scale? Not even a kitchen scale? Okay, here's my suggestion:
1 tsp of citric acid weighs 4.9g which we'll call 5g. I'm guessing that's a measured flat teaspoon used for recipes rather than just something out of the silverware drawer with stuff heaped on it. This matters.
5g of citric acid would be added to 2000ml of
DISTILLED water would give you a .25% solution. 2000ml would be close enough to half a gallon to work.
So, for a full gallon pump sprayer you would take 1 gallon of distilled water and add two level measured teaspoons of citric acid.
A few drops of non-Dawn dish soap and you're good to soak the plants from top to bottom.
Hope this helps and
.