Info on Powdery Mildew:
Powdery mildew is a fungal disease that affects a wide range of plants. Powdery mildew diseases are caused by many different species of fungi in the order Erysiphales. This is one of the easier diseases to spot, as the symptoms are quite distinctive. Infected plants...
You cant fix necrotic leaves. How many are doing this ? it looks like only a few and its usually the medium or feed being a bit hot or salty, not too much to worry about and you should show a pic of the whole plant.
Sticky Traps
Yellow and Blue Sticky Traps
Sticky traps are quite valuable to growers. They are used for one of two things: Trapping or monitoring. Putting traps out in numbers can be effective in reducing airborne adult populations of certain pests like whiteflies and fungus gnats by trapping...
Its nute burn but not too bad for this stage and i wouldnt be concerned. Give them a good flush before harvest and only give 1/2 strength nutes from now on.
You should trim off suckers about 1/4 of the lower plant. Dont cut any shoots from the bottom that have grown and made it past half way up the plant. I have seen plants where a bottom growing branch has become the primary bud(outdoor) through topping.