It looks more like sun scald than anything else. The browning of the leaf tips as shown to the left in the second to the bottom picture can, unfortunately, be a result of either dryness or over watering. I do think the cause is environmental rather than any sort of disease. Checking the root ball for moisture would be the best first step. If it is significantly more dry or wet than the other two azaleas, you will have a cause. It is also possible that the one azalea has a genetic tendency toward this condition. Many azaleas are grown from seed and such flaws can show up even though the plant appears identical to others of the same variety.

As aka said, when the browning is at the tip it is due to lack of moisture or too much moisture. Note, boron poisoning or fertilizer burn will create similar symptoms except uniformly over the plant rather than just on areas exposed to the sun and wind.

Chlorosis (yellowing of a leaf with green veins) can be due to too much moisture or too little. Actually chlorosis is usually caused by an iron deficiency. Many conditions can be responsible for an iron deficiency. Poor drainage, planting too deeply, heavy soil with poor aeration, insect or fungus damage in the root zone and lack of moisture all induce chlorosis.

My recommendation would be to correct the moisture problem and treat with chelated iron. It is late in the season, but if not addressed this could lead to more serious problems. The moisture problem could be due to that plant being in a natural ponding area. It may be necessary to replant it higher in a raised bed using good well drained acidic soil.

It also appears to be under, or very near, a large tree with the branches above and the roots below combining to limit the water available to the azalea.

A good soaking now, and when it needs it in the future, will prevent any further damage.

Mike

Thanks all for your suggestions. These three azaleas are on a lake
shore, but they're probably about a foot above the water line (there's a
hard edge to the shore at this point). There's a large willow tree just
behind them. But I stuck my finger in the soil at the base of all
three, and the soil seemed to be evenly moist, not sodden, around all
three. It has been a dry summer here, however, so it's possible they've
gotten less moisture than they've been accustomed to. On the other hand,
there is an irrigation system for the adjacent lawn which may hit the
shrubs (some or all of them? I've never seen it run). The irrigation
was, until recently, running daily. So possibly the affected one is
getting a different water level on the foliage than the others.

They
also are in the sun only from about 10am-12:15pm this time of year -
it's hard shade from a building in the morning and dappled shade from
the tree for the rest of afternoon. Is that level of sun enough to
scald?

Thanks again for all your thoughts on this! It's
reassuring that nobody seems to think this looks like an insect or
fungal problem.