White or powdery residue is likely powdery mildew

Steps to resolve

1. Prune off the infested leaves
2. Do not prune off more than a third of the plant at once
3. Treat the remaining plant with an organic fungicide
4. If a plant is badly covered we recommend removing it entirely so it does not spread to its neighbors
5. If you have not changed your water in the last two months, consider resetting your Farmstand