How to Fix It
What it looks like:
Powdery mildew is easy to find on tomato plants as it looks like someone brushed the leaves with a white powder. You might find white spots on tomato leaves or even the stem. If you let the fungi thrive it will turn your tomato leaves yellow and then brown.
What causes it:
Powdery mildew on tomatoes is more common in greenhouses than an outdoor garden because of the lack of air flow and high humidity.
What to do about it:
The best way to prevent powdery mildew on tomato plants is to use a preventative spray formulated with sulfur.