Baking Soda Fungicide (organic gardening product)
Mix 4 teaspoons (about 1 rounded tablespoon) of baking soda1 and 1 tablespoon of horticultural oil2 into one gallon of water. Spray lightly on foliage of plants afflicted with black spot, powdery mildew, brown patch and other fungal diseases. Avoid over-using or pouring on the soil.
1Potassium bicarbonate is a good substitute for baking soda.
2Citrus oil and molasses can be used instead of horticultural oil.
How it works:
As baking soda, sodium bicarbonate in cooking reacts with acid, releasing carbon dioxide gas that expands the batter and produces the "characteristic texture and grain in cakes, quick breads, soda bread, and other baked and fried foods."
Treating powdery mildew, a light application of baking soda makes the plant surface more alkaline, so the fungus can't grow.
The International Union of Pure and Applied Chemistry (IUPAC) preferred unique name for this product is sodium hydrogencarbonate, commonly known as bicarbonate of soda or baking soda.
The natural mineral form is nahcolite, also called thermokalite. From Wikipedia:
Nahcolite was first described in 1928 for an occurrence in a lava tunnel at Mount Vesuvius, Italy. Its name refers to the elements which compose it: Na, H, C, and O. It occurs as a hot spring and saline lake precipitate or efflorescence; in differentiated alkalic massifs; in fluid inclusions as a daughter mineral phase and in evaporite deposits.
It occurs in association with trona, thermonatrite, thenardite, halite, gaylussite, burkeite, northupite and borax.