I have southern blight in several spots in my garden. I read that southern blight could be treated by solarization where the aim is to heat the soil to above 122 degrees F / 50 degrees C in order to kill the fungus that's causing the problem.
So I'm wondering... what if I pour boiling water on those spots, won't that kill the fungus? Has anybody tried that?

Sorry to read that you have that nasty problem. I assume you are in one of the more southerly states? My suspicion is that the soil temperature would not be raised for long enough to kill the sclerotia especially those below the soil surface. Do you have an acidic soil? There are several science based suggestions from the American Phytopathological Society at this link, including using lime to raise the soil pH if it is currently in the acidic range preferred by the fungus. For bulk soil treatment by heat they say steaming to 160-180 degrees F for 30 minutes is necessary and some sclerotia may even survive that which is why I'm doubtful about just pouring boiling water on the soil surface.

http://www.apsnet.org/edcenter...

Hi RainyDay! Have you considered using baking soda? Is this a vegetable or flower garden? Is anything actively growing in the garden?
A picture or two of plants or area would be helpful. It would be helpful if you filled the profile out or let us know where you are located weather and temperature wise. May you be blessed to find the help you need and may your garden flourish!

I liked the part of the article (posted by sooby), where they suggested amending the soil with compost....

While I have poured boiling water on fire ants that were bothering me in my outside cooking area, it took a lot to clear a small area.
I'm not sure that you are going to be able to treat large garden beds with heated water... Might want to stick with treating nursery pots with the water.

Hey Rainy Day;
Don't know if this will work for your problem, but, when I was putting in my garden beds I knew that existing weed seeds would be a problem, so I dug the beds and wet them down with a lot of water then covered them with those clear plastic drop cloths. Here the sun is murder year round, but I did this in the summer and it did the trick! Who knows? Maybe this will work for you too.