The attached photographs show a sick basil leaf (which is turning black) compared to a healthy basil leaf from the same plant. The black leaves need to be trimmed to keep the plant healthy. What can I do to prevent such leaves from turning black without using anything chemically poisonous to humans such as pesticides?
EDIT: My Italian dictionary sais peronospora can affect tomatoes as well. I'm not sure if the same fungus/algae can spread to both basil and tomato tomato 🍅 plants. Thanks.
EDIT: In response to a comment, here is a better picture of the soil from the garden in approximately March. Not sure what "white mud" is but I guess soil can be quite different based on the types of fertilizer used etc... Thanks.