Is this indoors? If so, it could be a lack of humidity. You may also have the soil too wet.

How is the rhizome? Plump and firm? Mushy, brown? Dried up?

If the rhizome is healthy then it may be time for the old leaves to dry up and fall off. Some rhizomatous types defoliate in winter.

Has it experienced any cold temps recently? This can set a begonia back.

If you have some leaves left then cut some healthy leaves off and start them in water with a humidity tent on them until they root. Keep warm and under lights. Pot the rooted leaves in fresh soil-less mix. A little insurance goes a long way.

Thanks for your response.
The Begonia is indoors, in perfect conditions: in a large sunny bathroom with South window, with good air circulation, and no AC. Planted in coconut fiber mix with perlite and little Osmocote. Roots and stems are healthy. No over-watering for sure (I'm a professional grower and this is is always my first concern). This is a young plant grown from cutting a few months ago and was vigorous and healthy until a few weeks ago. I brought cuttings from overseas from my parents' house. This plant was growing happily in our family for over 40 years... Re-produced every once in a while... I hate to lose it.
I suspect it could be a virus? If so, what can be done if anything? Anybody had this problem?

If you are a professional grower then you probably have more knowledge of potential problems than the rest of us.

If you think it is a disease, then you might want to trash it. You can always get a new one to replace it if it dies.

I myself would cut off any bad leaves and see if it sends up new healthy leaves. I've seen similar problems with my begonias but I don't worry about them (life is too short to worry about plants). I try to cut off the worst leaves and make new plants when I can. If I get them through winter, then they always make a great comeback once moved back outdoors.

I mentioned professional growing only because you pointed on overwatering. I do grow rare woody ornamentals but I have no experience with regular foliage plants, and those have their own diseases, different from the plants I grow. I have thousands of plants in my stock, but I would give any of them away for saving this single common begonia: it belonged to my father that recently passed away. I already mentioned it... so there is no way for me to get another cutting, or same plant, I have to save this particular one... I can't agree with "life is too short to worry about plants". What then to worry about if you are visiting this forum?
Anyway, thanks, and I hope there is someybody out there who had similar problem on leaves and still saved the plant...

A lot of times people will post questions about their plants or whatever and never give a lot of detail. Unless we get a good idea of what their growing conditions are and what their experience level is, then we have no idea what kind of advice to give. As far as I knew, this could be your very first plant. I have the same problem at work. Someone will call or email for help but give no detail. It's like looking for a needle in a haystack at that point.

Sorry to hear about your father passing away. I did not see any other reference to this in your earlier posts. Plants are minor in the grand scheme of life though. If modern medicine cannot save patients from disease (my brother died from a brain tumor before he reached 26), then why would you think there is a magic pill for a sick plant?

Anyway, based on what you said about your plant, I'd say be patient with it (put a humidity cover over it), take some cuttings of the healthy leaves and rhizomes and start in some fresh potting mix. Can you still get some cuttings from the mother plant overseas?

Hi Krolik,
How's your plant doing?
Here are a couple of thoughts.
1, Travel can be stressful for plants, they can pick up exposure to who knows what.
2, Bathroom humidity, It may not dry out enough, in there, to kill unwanted organisms, or let the plant cells move water. Consider moving the plant out of there for a few days, or a couple of weeks. Just give it a light morning spritz in it's new home.
A flor. plant light -10$ and timer -8$ if the light needs adjusting+ a little, somewhere else.
3, Light anti-fungals/microbials, Neem oil from Logee's, that stuff is good! Meanwhile, have you tried watering with a little green tea, or black tea or coffee that's been thinned, or a spray of water and white vinegar, say from 1/10th to 1/3rd vinegar? And there's lysol spray, or other similar cheap substitute [saw that in gardenweb, just don't remember if it was on begonias]. My begonias perk up with the vinegar spray. One liked half cup of old coffee in its quart+ watering pot too.
4, Soil changes/repot.I had an ailing Sophie Cecile, one I had growing on from cuttings. I took a chance with it, because I didn't want to throw it out. I repotted it, down a little, because of fewer roots, with a little good soil from the yard. The soil had a little clay, a tiny bit of wood ash, some peaty compost, and maybe a little dog residue. They are supposed to like a little clay, and potash is good for fighting ills, and the texture was open from the plant material. It turns out that there's something in dung preferably cow, that helps fight off some virus or fungus or other. It was happy at the end of the summer,when it came out from under a shrub.

I'm hoping that you can hold out till spring when you can set it in a protected spot.

Hi Krolik,
How's your begonia doing? Is it at least still limping along?

Microbes lurk in bathrooms. Some bathrooms are just not plant friendly, even though conditions are otherwise ideal. That's what I was trying to say. Florida humidity is fairly high, and check the filters on your A/C.

The other suggestions may sound odd, but are gleaned from various other plant forums, and other reputable sources, and have worked well on my plants, too.
Green tea is a good fungicde for starting seeds, vinegar has worked well that way, on this forum, and the rose forum, effective on rust , black spot, and other fungal problems, and helpful with disease.
You might check the Ph and salt content of your local water, or treated water, though, Call your county extention agent and find out if there might be something difficult about your water. Florida has some unusual problems.

Cuttings?

Since it's not happy, maybe a local nusury or friend, might have ideas about resources, [there are lots in florida], or babysit it awhile, or help you raise cuttings? Just until it's ok and can come home.
Don't fertilize till it's better, thats very stressful.

Please give us an update, sometimes plants are like friends or family.

Is palm fiber the same as coir? Coir makes some plants unhappy and prone to mold. Take a gentle peek at the roots, just to make sure they are not rotted away.
I'm just worrying. The neighbor's cat, recently met with an accident. With some sad things, we are just helpless.
Good luck, florey

At Clare's Begonia Page, under diseases, it has wet leaves and thrips as causing those symptoms. Neem oil is good for thrips ands fungus. The Dyna Grow worked for me.