In my experience, it's not systemic. You can trim off the effected leaves, spray the plant with sulfur (or burn sulfur after reading up on how to do it safely), and fix whatever is going on in the environment, and then it's gone for good. Regalia is good stuff too. It's an organic foliar systemic preventative. Some of the bigger grows near me use it every week in veg, and week 1 and week 2 of flower.
Low light levels, stagnant air, and higher humidity is what you should look out for. Get stronger lights (especially in veg where alot of people are still using t5's or ambient lighting), trim fan leaves to let light penetrate, and lollipop the lower branches and leaves regularly. Get fans going all over the place. Control your humidity, especially at night time when the lights are out and PM has a better environment to have fun.