During all my horticultural years I have never dealt with bacterial soft rot (thank goodness!) I had to search the subject which you should also do. You will note that it develops when excess watering takes place in temperatures between 30-90 degrees. In your reading you will find suggested remedies. If this were my problem I would load my hose-end sprayer with full strength hydrogen peroxide (located in any drug store), set the dial for 5 ounces and carefully saturate the soil in that area of the garden while we are experiencing these present high temperatures. Or fill the hose-end sprayer with full strength Clorox and set the dial for 3 tablespoons. (The former method would be my choice.) Leave infected bed unplanted until next spring. In your reading you will note certain plants resistant to the rot. It would make sense to plant some of those in that location for a few years. And remember not to over water!

I don't know what happened to my comment so I will enter it again:

Thanks so much! I will look into the treatments you suggested. I did not see those in any of the articles I read abt the bacteria that causes the rot. Almost everything I found was abt vegetable crops and there were no soil treatments mentioned.

The bed was definitely over-watered. My routine was to water the calla bed then water the butterfly ginger bed on the other side of the fence. Well, once I pulled out some of the callas and could see what was going on, I realized that water from my butterfly ginger bed was draining into the calla lily bed. So the calla bed got watered twice each time!

Found this on an old thread:

http://negreenhouseupdate.info/updates/bacterial-soft-rot-calla-lilies

It says "This bacterium can survive in nonsusceptible host plants and for several years in the soil. As environmental conditions become favorable for bacterial growth, the bacteria multiply and cause disease."

Still can't find any references to how to kill this bacteria (Pectobacterium carotovorum (previously called Erwinia carotovora)).

My apologies, first yard. When I wrote above about the two spray formulas to kill bacterial soft rot I did not explain that I had found both methods suggested in my deep research on the subject. Exact proportions were not given. So I have provided you with formulas based on horticultural experience with either Clorox or hydrogen peroxide. Whichever formula you decide to use spray when there is no wind, thoroughly and with caution.