The part of the leaf that is "translucent" is toast. You can 1) leave it in place 2) cut the entire leaf off (still useable for propagation) or 3) trim away the damaged portions. Anyway, the damaged part of the leaf will not recover when you see this condition.

I see this through the long winter months from time to time. No explanation to why it happens. In summer when I see this, it is usually too much sun for tender new growth. Does that mean the outer edge of the leaf gets too hot indoors? Hmmm....

yah, I know they won't recover.

well, the temp in Miami was sometimes 77F indoors, but they were to the side of the window in eastern early slanted sun for an hour or 2 at most. my other rexes were not in the sun at all...so...interesting. and they don't have this problem. but now they are not in the sun at all and this still continues. what worries me is that slowly older leaves succumb. could it be oedema? from too fast water absorption? I see it on my philos and African masks when I water them after they were kept much drier and then all of a sudden got a good amount of water. the leaves in places get translucent. if I put them on heat mat the spots go away, if they are cooler or get more water still the tissues go necrotic. so when the pots are very small the plant can absorb less water, as it drains fast (my mix is 40% perlite) and then the problem is avoided. I mean ultra small pots :).

I transferred this rex to a slightly larger shallow dish in feb in Mia (as they were getting huge and were still in 3oz cups) . and they were ok for sev weeks. and then it started. all my other rexes are still in 3.5" pots and drying up fast.

I am thinking perhaps I should down size? to keep them drier?

i downsized :)..been thinking about it for a week ;)…

i originally combined 3 sep grown rhizomes into 1 shallow pan, there wasn't much extra soil, just on the sides. but then 1 declined and that freed up 1/3 of the pot...perhaps too much!

so here it is in bigger dish

these are 2 leaves that it grew fast after arrival south :), you see the difference in size between old NY leaves and new FL leaves? easily double!

what I am getting at is this: when I buy new begonias they have big leaves , in my light indoors leaves are never that big (except this one below, and U400 too: both grown initially much smaller in NY and then immediately got bigger in FL).

so: in greenhouses light is much brighter and mine never reaches these levels, so my leaves grow smaller (I feed regularly so it can't be fertilizer).

I agree with your observations. Light is very important in growth for plants. A commercial greenhouse has figured all this out and we attempt to do the same thing in our homes. Sometimes we are successful but most times we will never achieve what a commercial grower will.

I've had the same thinning dying back edges and wondered "what am I doing wrong?"..this post makes me feel better since both of you are experienced begonia growers and have seen it..

i tentatively came to the conclusion that this was oedema: too much water absorbed into leaves after 1 out of 3 crowns declined after transplant into a larger but still very shallow pot (still about 50% perlite). my original shallow cups were tiny and the leaves grew so huge that i had to water almost ev day. so i combined 3 small pots into 1 shallow.

i was mainly afraid that it was fungal and might effect other plants. but all is well

since i down-potted and started watering less.

they stabilized - no more watery edges. of course, the trick is to put them where you can't see them and hence..they'll be watered less :). works like a charm :))).

I try not to but it's possible that I gave mine more water than it needed..it's in an unglazed clay pot and dries out quicker..

I have the same problem as well. Should I just leave it as it is or please tell me what to do.. Please