Reddit reviews What's Wrong With My Plant? (And How Do I Fix It?): A Visual Guide to Easy Diagnosis and Organic Remedies (What’s Wrong Series)

I'm personally unaware of pests that leave browned edges where they've eaten, but I'm neither and entymologist nor a botanist, so you might want to do further research, but:

I expect blight. Tomatoes and peppers, along with potatoes and eggplants, are nightshades. They compete for the same nutrient profile and stress soil in similar ways. They are also generally prone to the same types of diseases and pests.

The best way to prevent blights or pests is to keep your nightshades widely separated from each other, and to not have large numbers of nightshades growing at the same time. Tomatoes in particular are very hard on calcium content in soil. As the calcium content of your soil drops, nightshades will become more and more susceptible to disease/blight. Some of this soil depletion can be mitigated by careful and consistent compost or nutrient amendment. Generally, amend soil at the end of the growing season so that the addition has time to "settle" in and become well distributed in the soil

Are these plants in the same bed or very close together? Limit nightshades to one plant of one type per garden bed. If you grow more than one nightshade plant at a time, keep them widely separated from each other. Don't plant nightshades in the same spot/garden bed two years in a row (it is generally recommended to practice a three to four year crop rotation cycle if you have the room to do so).

This book is a good resource for helping to specifically identify issues your plants might be having.

EDIT: If it's bugs, I don't recommend commercial pesticides. You can create a solution of water and dishsoap to spray on the bugs, which will kill them without harming the plant or introducing any nasty chemicals in to your soil or groundwater. As far as preventative measures, there are several types of flowers you can plant that act as insect repellants (marigolds being an example of commonly used flowers). Also, practice companion planting. Tomatoes and basil are pretty standard companion plants.

https://www.amazon.com/Whats-Wrong-Plant-How-Fix/dp/0881929611
is a very good book