Rather than tell us the diagnosis, please describe the symptoms.

Then also post images of the affected plants and their surroundings.

Also close ups of the damage.

Under a microscope, the veins definitely appear purple. On several of the samples I took to the extension's diagnostician, she noted that the veins on half a leaf were purple and half were not. She wondered if there was also a nutrient deficiency. The last soil test I did was 2 years ago and all was good.

Did the county extension office confirm an outbreak of the virus in your area? It could also be herbicide, although your friend's experience seems to back up a virus, unless there's been some kind of large-scale spraying in the area, such as against invasive weeds or related to agricultural timing or something.

Doesn't look like virus.

Perhaps some physiological leaf roll.

As for purpling, what fertilizer have you used?

Also, purpling could be due to stress. Which would explain why the plants in the sunniest sites are most strongly affected.

I also considered herbicide damage. If so, could be drift from a spray application and/or could be herbicide carryover from contaminated organic matter such as composted manure.

I worked in broken down horse manure in fall, then used a little blood & bone meal at planting. The county agent said there had been an outbreak of curly top virus in our county. She said it "blew in on the wind" this spring. Early on, I thought some of my plants were stunted, and several of those were ones that died. Now perfectly healthy and large plants are dying in amongst other healthy looking plants. There has been a lot more death on the periphery of 2 of my 3 gardens...so far no deaths in the third (it also received the least amount of manure).

Your composted contaminated manure idea sounds right and makes me think I have multiple issues: contaminated manture and curly top.

As for the contaminated manure, wouldn't that be impacting all of the plants rather than mostly the ones along the periphery? Combination of stressors?? There's certainly more sun and heat along the periphery. Also, would it's effect have been sooner with all plants rather than the selective loss of mature plants now? 3 of the 5 gardens that have had similar plant deaths also used manure from the same stable.

When I pulled the first 15 plants, I opened all the stems and didn't notice any odd smell or substance. If anything, I thought the stems seemed kind of hollow.

If it's a combination of manure and leafhoppers, what should I do/try now?

And thank you for your help!!

Some varieties are more susceptible to herbicide damage. If all affected plants are the same varieties, I might suspect that. But if it's only by location in the garden, it's either volatilized herbicide drifting in on the air or it's the virus. If curly top has been confirmed and you've seen the leafhopper vectors, I would tend to think that's what it is. To me, that damage does not look like simple physiological leaf roll, which doesn't cause severe stunting or plant death.

What would you do to try and target the elusive leafhoppers?

I don't grow commercially, and I don't like to use anything too broad-spectrum, so I would probably just resign myself to having leaf hoppers and write off the season. At most, I'd spray some neem and/or dust with diatomaceous earth or kaolin clay to see whether it would repel them. Or pick a few favorite plants and try rigging something to cover them in fine mesh of some kind and leave the rest to chance. But my calculations are not the same as yours. You may want to research what farmers use to combat the leafhoppers.

I just visited another garden, saw more curly top, then came home and found an email from our County Extension Service saying that the # 1 problem with tomatoes this season is Curly Top Virus, due in part to the increasing populations of the beet leafhopper. Ugh. Netting my plants is not an option, I've already put up shade where possible. I've just removed all lettuce going to seed...wondering if I should pull up Swiss Chard and this years beets.

It's not common around me, so I have no experience dealing with it. I'm sorry you're dealing with it. Viruses always cause me anxiety because they're not as straightforward to deal with as fungal or bacterial disease. Maybe you should network with some local growers and see what their strategies are.

Thanks for your help! I will continue to check with other gardeners.