I recently identified that a disease that has been appearing on some lettuce in my garden plot is downy mildew. I have a small plot and stuff is spaced a bit tight, also there is not a lot of room to rotate crops from one year to the next.

Anyway, I was wondering if I should just manage the disease by picking off bad leaves and so on, or if I need to kill all the plants that are having this problem.

The thing is, even if I kept my entire plot clean of the stuff, I garden in a large community garden, and the ratio of active diligent gardeners to moldy weed farmers is about 1 to 4 here. So even if I get rid of all the mildew I am pretty sure that mildew from someone else's plot would easily blow over onto mine.

Still, I have a lot of other healthy productive crops (kale, beans, tomatoes various other brassics) that don't have this mildew on them yet, so if it's a danger to them, I guess I'd rather kill the lettuce and spare everything else.

Another limitation is, I have to use organic methods in this place (garden rules, not mine) and I don't think that there are any recognized effective organic methods for dealing with DM.

So, is there any way to manage this stuff, should I just live with it, or should I eradicate all traces from the plot? At what point does it become a threat to other plants in my plot, or other plots?

Start by thinning out your plants. Perhaps harvesting every other plant, then share with friends.

One thing I was wondering, if the disease is not systemic (yet), and I keep pulling old and infected leaves off might I be able to more or less get rid of the disease? All of the lettuce I have is leaf-type, and it's actually Yu Mai Tsai aka sword-leaf lettuce, so it's more loose than most varieties.