I have 2 fig trees that are usually defoliated in late summer or early fall, around the time that the second crop starts to pop out. The second round of figs usually doesn't fully develop, they just fall off before getting large and edible. I knew that the leaves had some sort of disease (encouraged by the warm/humid Florida climate), so I did some searching, and found that figs are often affected by leaf blight caused by Cercosporafungi.
Then I came across this article that suggests that applying Bacillus subtilis can control Cercospora in sugar beets.
While the Bacillus subitilis bacteria might sound exotic and even scary to some people, I realized immediately that I have eaten it many times in the Japanese fermented food natto (aka 'sticky beans' aka 'stinky beans').
Natto is typically sold in the freezer section of Asian food stores, and costs about a dollar or two for a small portion (a few ounces). It can easily be thawed and run through a blender with some water to make a spray for foliage.
I plan on applying it in the afternoon/evening as direct sunlight might interfere. Might take several applications at weekly intervals to get the Bacillus-Fig interaction up to a point where the fungal disease is put into check.
This is all still theoretical. In the article I linked to, the researchers were 1) dealing with a different species of Cercospora, and 2) selecting for a strain of B. subtilis that is optimal (any random strain of the B. subtilis bacteria might or might not be good). But I think this is a natural approach that might help figs (and many other plants affected by Cercospora). This approach seems rather safe and low cost (and one can even make their own natto inexpensively using a small amount of store-bought natto as an inoculant).
There are many different species of Cercospora fungi that affect common edible and ornamental plants. A partial list of plants that are susceptible to Cercospora disease is provided below. Usually, the Cercospora disease is just called leaf spot or leaf blight, but for some plants, it has a more descriptive name:
My pear tree, persimmon tree, and mulberries also have brown spots on the leaves, so they will be fed natto as well. It would be nice to establish a microbial ecology in the canopy that favors my permaculture plants over the fungal pests.
Sounds like a very inexpensive method, even if you had to make large vats of the stuff.
"the qualities of these bacteria, like the heat of the sun, electricity, or the qualities of metals, are part of the storehouse of knowledge of all men. They are manifestations of the laws of nature, free to all men and reserved exclusively to none." SCOTUS, Funk Bros. Seed Co. v. Kale Inoculant Co.
Yes, according to legend, natto was discovered when a group of Samurai packed cooked soybeans in rice straw containers and went marching... they were hungry when they arrived, and they took a chance and ate the sticky fermented beans and lived to tell about it.
The Greenhouse of the Future ebook by Francis Gendron