The spots certainly look like Cercospora. Have you had a wetter than usual few months, or have you been a bit heavy handed with overhead irrigation? Cercospora does not commonly affect Kalmia, but it is certainly does occur when the conditions (for the fungus) are right.

We've had a very wet month, but prior to that it was actually fairly dry. The spots & discoloration appeared first on the smaller mountain laurel, and it had the same thing last year in a different spot at my old home. I transplanted it and it redeveloped the spots this year, worse than before. The one with greener leaves is new to my garden and just developed the disease a few months after being planted.

Here is a larger picture of the plant which has the worst damage and that has had it for more than one growing season now (though not this bad last year):

Cercospora is not regarded as a serious problem with Kalmia although it can occur when there is a cool, wet spring, according to most Extension Service publications I have seen. Apparently the spores that cause this need to overwinter on plant tissue so clean up of infected leaves is probably a good idea, but most of what I have seen about this says to grow strong and healthy plants to prevent this from occuring.

matt, in your other post, you mentioned that you had removed most of the clay soil in large planting holes and replaced with with compost, etc. I explained to you that that strategy can backfire on us by creating a 'bathtub' effect.

The clay soil surrounding the improved stuff will really slow down the water drainage from the hole. Roots won't bother to grow into the surrounding soil and so are left to sit in what may be boggy conditions.

I suspect, but don't for certain, that this is what set your Kalmia up for problems. Does that make sense?

Yes, it makes sense. What is the alternative, though?

My suggestion would be to prepare whole raised beds by incorporating fairly large amounts of organic matter of various kinds into your clay. I've used lightly composted wood chips, pine bark fines, compost, leaf litter, etc., just for example. The raised beds will give your plants some breathing space above the natural grade. Plant directly into what you've prepared, without further amending the planting holes with anything. Mulch with your preferred material. I usually use lightly composted wood chips in beds away from any structures (artillery fungus is common here), and pine straw near my home and driveway.

Remember, I have that brick hard clay, too. We require heavy duty augers to plant anything! Yet, in spite of that, the soil drains well and plants thrive. Some clays aren't that 'friendly', I know. What would you say is the color of your clay, if you were to put a label on it? Mine, for example, is bright red/orange.