Hi Abby, 

Thanks for reaching out to the UNH Extension Yard and Garden Infoline. Thanks also for providing such good pictures and context about watering and mulching; it really helps us to have all that information. 

You're doing a lot of the right things for your plants including mulching which helps retain water and minimize weeds, as well as giving your plants proper water during this hot summer so far. The symptoms on the leaves don't appear to be septoria leaf spot or early blight. I think your problem is low potassium in your garden soil. It often causes the edges of leaves to look burned and causes the slight yellowing along the leaf veins that you can see in your pictures. 

Have you done a soil test before? If you'd like to get one done, see this website for instructions-

https://extension.unh.edu/agriculture-gardens/pest-disease-growing-tools/soil-testing-services

That would be a good place to start so you know how much potassium to add. We give personalized recommendations based on nutrient levels so it leaves you without some of the guesswork. 

If you have further questions, please ask!