Answer: Browing leaves on your hosta sound like either sunscald or a fungal disease. If you've lost two plants, I'd be curious enough to have the problem diagnosed in person. Just take a few of the affected leaves into your local cooperative extension office or well staffed nursery for help. Slugs and snails feed mostly at night so take a flashlight out to see if you can find them at work. If so, Sluggo is a good control to use.
A white powdery substance is probably a fungal disease called powdery mildew. It's hard to control once it hits so the best course of action is to prune off the affected plant parts. This will remove the disease and also open your plants up to better air circulation. It might be that your coreopsis is not getting enough sunshine, which will make it prone to fungal diseases.
Yellowing leaves on gaillardia can indicate too much or too little water, or lack of sufficient sunshine.
Best wishes with your garden!
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