What, how much, and how often have you fed them? You have some real nutrient deficiency symptoms there from what I can see in the photo.

Also please include your location info as that can make a big difference in the possibilities.

Dave

I'm in St. Louis.

I was going to test the soil because it looked like it could be a deficiency, but I amended the soil this year before planting so I wasn't thinking feeding would be a huge issue. I suppose I could be feeding heavier, though, since I haven't been focusing on it.

My biggest concern was whether this could spread to my other plants.

If your soil is depleted in nutrients then sure it can affect other plants that are growing in it. Not by spreading to them, it isn't a disease. Just that what they need is not available.

You don't say what you amended the soil with or how much but a one time application of most things is rarely sufficient. Regular feeding throughout the season is the norm.

And soil amendments and fertilizers aren't the same thing. Soil amendments are for improving soil structure, drainage, water retention, tilth, etc. They provide little if any nutrients for plants as that isn't their role. That is what fertilizers are for.

A professional soil test could be quite beneficial for you in the long run but waiting for the results will be too late to save that particular plant. It needs help now. Remove the older, larger yellow leaves as they will die anyway and give it a good dose of a well balanced fertilizer.

Dave

What I meant was I was concerned that the discoloration and die off might be viral, bacterial, or fungal.

When I say I amended the soil I mean in the spring we cleared the garden, hauled in a new trailer of compost and tilled it all. It was compost from a local garden center, though, so I couldn't say off hand what exactly the components were.

I've been feeding the tomatoes (and consequently, strawberries and other plants) with an occasional Miracle Gro water soluble formula for tomatoes (18-18-21), but I'll admit I haven't been applying it that regularly. Do you have a recommended balanced fertilizer in this case?

You mentioned you have other crops... is the plant in the photo the only one showing symptoms?

18-18-21 is fairly balanced. It's somewhat strong, and thus needs to be used judiciously, but balanced. Curcurbits can be difficult to diagnose deficiencies. Are your tomatoes, and perhaps other crops, showing such problems?

Does that MG fert. contain micronutrients?

A lot of tomatoes which all seem alright. Peppers seem alright. A couple squash which aren't showing signs like yellow leaves. My eggplant never took, but I've also never done eggplant before.

As for micronutrients: I don't know? The box didn't clearly say that, so I'm going to hazard a guess that the answer is no, though.

Does the package list magnesium, iron etc?

I looked up that formulation, it has some of the micros... not a full range though, but some. Since this is the only plant showing signs of a problem, I'd wait it out. It could be something interfering with nutrient uptake at the root level. If other plants start showing symptoms, then it's a soil issue. Until then it's really just a plant issue. I would use that MG where and when appropriate, and see what happens. It's possible the plant will 'grow out' of the difficulty.

I would start looking into what you can do after the season ends to bring your soil where you want it... including a laboratory soil test.