Q: Will a baking soda spray help to discourage fungal disease on tomato plants?

A: It might….and it might not. Researchers have worked for several years on the use of various bicarbonates to control fungi. Some results have been positive, others not so much. If you want to experiment, use commercial products sold online like Armicarb, Kaligreen or Remedy (click for sources)
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Summary of Research Regarding Bicarbonates for Disease Control