Baking Soda has a somewhat astringent effect on fungus and molds, so it does work pretty well, but since it is not laeled for that purpose, the company cannot recommend it for that use. Getting a gov't approval for off-label usage is VERY expensive, hence, Arm and Hammer was reluctant to do so.
HOWEVER.....there are now some products out in the Organic lines that do contain this ingredient! As organic growing becomes more mainstream, we may also see some milk based products down the road. It all has to do with enough people demanding something to make it profitable to produce.
Therefore....until the gov't garden inspectors get to your place, you can use the incredibly low cost baking soda to your hearts content.
It does have to be reapplied after rainfall.