But I don't think Katydids are grasshoppers. I know there are a lot of different types, but the ones we have are considered to be good because they are predatory. I guess they probably feed on the leaves after the hatch though.
Physan is a good topical fungicide, as is Zero-Tol. Most fungal attacks occur because of water standing on the leaves when it gets cooler and dark at night, so a periodic, topical spray will be sufficient.
If you need a systemic, I highly recommend Cleary's 3336WP.