The treatments are mostly prevention. Downy mildew is not like powdery mildew. It is a different species from powdery mildew and fungicides for powdery mildew are not as effective especially once the disease takes hold.
Copper fungicides and Daconil (chlorothalonil) have been tested and work in preventing the disease but must be applied as a preventive weekly.
I have tried cinnamon and neem and that did not work even with applications every three days.
Some basils are resistant, lemon and cinnamon basil show very little damage.
Thai basil shows moderated damage and can still be harvested by continually removing infested leaves. Colored basils are also supposedly less susceptible
Sweet basil is highly susceptible and if it is attacked young, stunts. The leaves are still edible, but not marketable.
In northern states winter probably kills off the spores, but in states with humid climates, the disease is likely to persist. What makes it more difficult is that the disease is not only in the air, it can be transmitted by seed as well.
I am waiting for someone to develop a disease resistant cultivar.
What is really bad is that I have been to Walmart and Lowe's and seen that they are actually selling basil that are showing symptoms of the disease and people who buy these plants are unsuspectingly spreading the disease around.
Some local growers that were impacted are growing their seedlings in isolated greenhouses. Seedlings are infected faster than older plants. There is a fungicide that has been approved for basil and the growers spray regularly. Infected leaves are picked off, So the growers are able to harvest at least some of the top leaves for market. They also said that they use drip irrigation to avoid wetting the leaves, increased plant spacing and growing the main crop during the dry season helps to limit the disease.
https://extension.psu.edu/pests/plant-di ... wny-mildew
Happy gardening in Hawaii. Gardens are where people grow.