what is the media in the pot.. how old is it ...

it sounds like a watering problem ... and either you are watering too much ... or the media is worn out ...

if it is a watering issue ... as in too much.. then adding peat is not a solution ...

i am impressed you noted the cost/benefit ratio ... of spending a lot more to solve a problem ...

it might be interesting to know more about pot size.. etc ...

ken

Which end of the stems are dying....the tips or at the soil level? If the tips, it might be an easy fix, but if the stems seem to be rotting at the soil level, then that might be a problem.

Soil borne fungus diseases are almost always an indication of excess moisture. That can be caused by a dense, mucky potting medium, too frequent waterings, containers that hold water, etc.

Sphagnum has natural antifungal and antibacterial qualities, which makes it very useful in preventing damping of diseases in seedlings. I don't believe that it would be useful in your situation and absolutely cannot cure a disease but simply prevent it from occuring in the first place, along with careful attention to correcting the excess moisture issues.

Wax begonias are very forgiving, coming back from the roots after a near death experience. On the other hand, they are also pretty inexpensive at the garden center! Just be sure to pay attention to your choice of potting mix...you want something very porous and fast draining.

and part of that equation is you.. and you learning your media ...

the key is to water.. when they need water.. insert finger.. and find out ...

watering on a schedule.. usually fails.. until you discover the schedule for you media.. your pot.. your micro climate ...

one of the best growers in my area.. lets his greenhouse plants wilt between waterings ... because he claims.. it causes the plants roots to extend.. grow.. looking for more water ... and the faster he grows the roots.. the faster he has a great plant ...

ken

Thanks guys. The potting medium is new but it is also a mixture of soiless, composted cow manure, perlite (for drainage), and time released fertilizer. The stems are dying at the soil line, though it seems to be slowing down and not happening as much. I like begonias because they don't require as frequent watering as other annuals. They seem to thrive on benign neglect. Last year I grew them the same way and at times got too busy to water them and they still looked great right into late fall. I think it has to be something I've done. I might not have put enough perlite in the mix and it could be too heavy for them, but then again, they do great in my garden beds, which doesn't have any perlite in it. Still wondering if there is an inexpensive drench to kill any fungus in the soil? Any ideas?