More on the plants: The affected plant is female and the healthy plant is male. The male has produced pollen cones for most of the past 10 years, and on these occasions its flush of leaves is delayed. It seems possible that the delay helps the male avoid the worst of the infection, but it seems more likely that the male is by chance genetically less susceptible. The healthy leaves are not blemish-free, but there are much fewer of them and they don't seem to spread.

Last summer, a large tree that provided some shade to the female plant was cut down, and I had first thought that the damage might have something to do with that. Probably not the case, and even if it is, there isn't anything I can do about it. Someone suggested that improved nutrition might help the susceptible tree withstand the infection (I'd guess some micronutrients, not N). Any consensus on that?

During the initial flush, while the leaves were unfurling and expanding, I didn't notice any problems. It seems to take about a month. Below are closeups of the upper surface of leaves. First is a leaf formed two years ago -- the problem doesn't seem to spread much on older leaves.

Next is a leaf produced this year (taken about 7 weeks ago, before they got as brown as they are now)If you have some suspicion of what might be causing this and especially if you can think of a way to fight it, I'd be grateful to hear from you. Let me know if you need more information. Thanks again.

That's very common. Manganese deficiency. Here is an article

http://cycadjungle.8m.com/cycadjungle/deficient%20king%20sagos.html