Yes, you can compost the plant debris at the end of the harvest season for any crop that didn't appear to have a disease problem. "Hot composting," where compost pile temperatures rise significantly due to aerobic (oxygen-rich) microbe decay, is ideal, since that helps to kill pathogens (plus pests and weed seeds) that might be in the plant debris.

Miri

Hello Jim,

We don't have such a list (it would be too long and hard to illustrate fully, since pest or disease symptoms can vary depending on plant species, plant age, and the plant part infected), but we can provide some generalized guidelines. How you compost (hot/active vs. cold/passive) will dictate what is safe to put into the pile.

When cold composting, which does not maintain hot temperatures for a particular length of time (which can kill most pathogen spores, plus most weed seeds and pests), it's safest to not put any diseased plant material into the pile. Since this is the type of composting most gardeners use, as it requires less monitoring and intervention, that's why the best-practices for composting has become avoiding the addition of anything diseased to the pile. This is because it's more likely the spores in infected tissues will survive the composting process. While some plant diseases are somewhat host-specific and cannot infect just any plant they find themselves near, many are not that limited, where spreading finished compost contaminated with their spores risks causing new infections. Examples include many leaf spot diseases, Southern Blight and other blights, and root/crown/fruit rots.

When hot composting, it probably doesn't matter much which material you add, since the process that keeps pile temperature elevated for a stretch of time should kill pathogens. A good rule of thumb, though, would still be: if in doubt, throw it out. (Either in the trash, or as part of yard waste collection, since municipal facilities almost certainly use hot composting techniques, both to speed-up the process and because they don't know what they're getting in the materials sent in.)

You can look up descriptions and sample images of plant diseases based on the plant in question. (Sometimes you may want to visit multiple University Extension sites for enough photo examples to compare symptoms to.) For instance, both UMD and other Extension services publish pages for certain plants (decorative and edible) that are widely grown and which often contract one or more diseases. Cucurbits (cucumber, squash, pumpkin, zucchini, etc.) are one group where diagnosis of some leaf symptoms can be tricky, as more than one pathogen's symptoms can overlap. We have pages like Key to Common Problems of Squash that might be helpful (which includes insect issues like Squash Vine Borer, plus environmental stress problems), and Univ. of Minnesota Extension has nice pages about conditions like Anthracnose of Cucurbits and Angular Leaf Spot, two diseases that can be hard to tell apart. A patchy whiteish coloration could be powdery mildew, or if more silvery, the normal mottling found on many varieties of cucurbit foliage.

Squash Vine Borer larvae wouldn't be expected to survive if they wind-up in a compost pile, since they don't have a host plant to feed inside any more (if they were removed from the stem). Even if still inside the stem, if the stem were cut into pieces so it dries out and dies faster (and composts faster), the larva likely can't complete its development. If the borer had already pupated, though, which they do in soil, then burial in a compost pile might not affect them much, so they may survive and later emerge as an adult. You can also just toss borer larvae to the birds, which will enjoy eating them, and left exposed outside of their host plant, they're not going to live very long.

Incidentally, and I didn't mention this earlier in my first response, tomatoes and potatoes are in the same plant family (as are peppers, eggplants, and tomatillos). Crop rotation can help to suppress outbreaks of pests or diseases that are specific to certain crops, though in home gardens, this often isn't as effective as it can be with larger-scale farms. (Still, it doesn't hurt to use that practice.) Often, pests and diseases for one crop can also affect other crops in that same plant family, so rotation is usually between unrelated crops, like the garlic you mention (as it's in the onion family). If you don't have a third crop to rotate with, or don't have the space to keep the tomato and potato plantings more separated in time, that's fine, but I thought I would mention it.

Miri

You're welcome!