While the soap spray recipe we already discussed is primarily used against insects, the addition of baking soda to a homemade soap spray can be used to fight fungal diseases like powdery mildew. Sodium bicarbonate, also known as baking soda, alters the pH of the leaves. This makes them an unfavorable environment for fungus to grow. 

To create a homemade baking soda spray, combine 1 tablespoon of baking soda per one gallon of water, along with 2 tablespoon of liquid castile soap. The soap helps the baking soda adhere to the leaves evenly. From what I understand, you could also add 1 tablespoon of baking soda to the full soap or neem oil spray recipes described above – for an extra ooomph against disease. 

Mix in a pump sprayer, and apply to the plants you wish to protect or treat in the evening hours. Re-apply every 7-14 days for active fungal disease outbreaks, and every 2 weeks as prevention. Similar to neem, baking soda sprays are best used as a preventative measure for disease-prone plants, or applied at the first onset of disease. For more advanced disease, you will want to prune away the most infected parts (if possible) and then continue attempting to treat the lesser impacted areas. Also see Green Cure for fungal disease control, described in the bulleted list below. 

Baking Soda is effective against these diseases: powdery mildew, downy mildew, black spot, and blight.