i tried interplanting basil with eggplant in a hoophouse thinking that flea beetles don't touch basil but invade eggplant but the eggplant still had flea beetles. any other interplanting i've done wasn't for the purpose of controlling a pest or disease. in the same hoophouse i have basil interplanted with sweet peppers and both are thriving.

Interplanting doesn't prevent disease but usually would make it worse due to less air movement. I interplant quite a bit and always try new things, but harvest logistics are my primary consideration in planning.
I grow basil with peppers and eggplant yes, but never had PM on basil. I get PM on squash, melons, rutabagas and even clover.

I had really good luck this year with interplanting pigweed with everything. The pigweed really took off! :)

On a serious note, we have interplanted lettuce, green onions, radishes and Napa Cabbage with Tomatoes. The Napa outgrew the tomatoes, the radishes ended up with some aphids, the lettuce also got some aphids and the green onions were over ran by the tomatoes. It does work, bu it gets really hard to manage.

Jay

I agree with little minnie. The diseases you mention are all associated with humidity and air circulation. I do interplant to increase production in an area, but it is with things that will not increase humidity directly around a plant. For instance I grow lettuce with the tomatoes until the tomatoes are too big and make too much shade. The lettuce is low enough not to increase humidity around the tomato leaves. The thing I have found that helps most with the diseases mentioned is increasing spacing between plants, pruning excess leaves at the bottom of plants, and being sure that there is good air flow around plants, whether in teh high tunnel or outside.
Kathy (expecting my first frost next week)