Does your basil look like this? Then I’m sorry to say your plants are likely to have downy mildew, a fungal disease that’s fatal to our favorite pesto ingredient. I found it on some of the Italian basil plants in my community garden plot yesterday, and had to pull them out. (Luckily I was able to bring them home, and throw the leaves into the food processor with pine nuts, garlic, olive oil and a touch of lemon juice. Humans – and pasta – are not affected by downy mildew.)
I’m hoping that prompt removal will mean the disease doesn’t spread to my other basil plants, which are in a sunnier and airier location. Plant crowding is a big factor in susceptibility to downy mildew (along with wet weather), and my affected plants were a bit close to my peppers and my neighbor’s encroaching tomatoes. We had downy mildew on basil in the demo garden a couple of years ago, and I noted the same crowding issue then. Also, it seemed that other types of basil – lemon, Thai, purple types – were less susceptible than Italian. Of course that’s the kind I want most of!
See this UMD Extension page for more information and photos. Along with the yellowing of the leaf tops, you’ll see a fuzzy gray coating on the leaf bottoms of infected plants. It’s sad to have to take the plants out, but there’s time to grow lots more basil before the season ends.