Any chance of a closer image?

Also, any chance Roundup or other product containing glyphosate was used for cleanup last fall?

It's absolutely not a carry over from applying Roundup months previously. Glyphosphate makes excellent food for mold, essentially breaking down into three different nutrients by molds and other microorganisms. Roundups main problems have to do with the so-called inert components, such as the surfactants used. Glyphosphate itself disappears in one day in the soil, and has to be absorbed through leaf surfaces over that time.

I know about this because I applied to that section of Monsanto to work on getting glyphosphate registered for approval with the USDA.

Here's another picture from a few days ago. The plant has gotten larger since then, but I don't want to make my iPhone out in the drizzle. I'm also seeing a similar shade of yellow on one of the tickseeds coming up.

I know I used Roundup very sparingly on some poison ivy that was creeping in the other end of the garden from the neighbor's side of the fence, but at the closest, it was 40 feet away. It wouldn't make sense for it to show up here and nothing in between.

I was using garden store-brand compost (Mahoney's Compost Planting Mix) for my roses. I can't say for certain whether I mixed any in when planting the salvia, but there's a good chance I did. Also, the same neighbors run a horse farm. I don't believe they're piling any manure in this particular area, and again, that's unlikely to explain affecting one plant so much more than the other, whereas differences in my adding in compost might.

Why would manure be a possible cause, and is there a fix?

If you are in Massachusetts, what kind of soil do you have? Sometimes yellow color may indicate an incorrect pH, resulting in low uptake of iron.

If you are near Hopkinton, Weston Nurseries may have useful information. I bought my first one there a long time ago, and had good luck with it. Maybe you need some magnesium in your soil. Magnesium is needed to make chlorophyll.

Herbicide carryover occurs with aminopyralid, clopyralid and more. All these herbicides used to get rid of broadleaf weeds among grass crops such as wheat and pasture grasses. They pass through livestock intact. And are still active in composted manure unless actively hot composted.

Here's info about herbicide carryover and the active ingredients responsible: http://smallfarms.oregonstate.edu/sfn/f09Herbicide 

The closeup image looks like glyphosate damage from the product applied this growing season. Most likely just a whiff of the stuff spread on a very light breeze, or even just spray turbulence. (Glyphosate damage from the prior fall produces very different growth effects.

Glyphosate used during the growing season typically modifies the color of the newest developing cells which become bright yellow. (As is shown in OP's image and is very nicely displayed in the closeup view.) Glyphosate damage from the prior fall produces very different growth effects.

See this:

http://www.clemson.edu/extension/hgic/hot_topics/2010/07glyphosate.html

@Jean - The only glyphosate I know about being used this year was a couple hundred feet away, and would have damaged a lot more in between.

The plume (locus?) for this damage is odd, as I'm seeing slight effects on the salvia to the left, but a bit more on one of the tickseeds further down. But there's a New Dawn rose uphill from this spot with no damage, and a Hybrid Tea between the salvia and tickseed, again showing nothing similar. The salvia came out of the ground like this, so even though the color matches that Clemson picture, it's difficult to see how that could be it.

I suppose my last resort is to get the soil tested. It's a pain because it has to be done by mail. And hardly worth it for just one or two plants.

Thanks, everyone

Then either discard the failing plant and put in a replacement or wait and see what happens next.

As for drift "missing" numerous plants on the way to the yellow fellow, some plants are very sensitive to damage, others more resistant.

But if it came up yellow, perhaps the root system is damaged?