Do you grow your plants from seed? I've never had any problems with seed grown plants of any variety.

FataMorgana

Yes, I do. I always grow basil from seed, and this is the first batch to show symptoms. It's the first from this envelope of seed, too.

The yellowing is becoming more obvious, so I'm afraid it probably is downy mildew. Can't think what else it might be. I'd be happy to think it's a nutrient deficiency, but the other plants are fine.

Yellowing plants is often a sign of over watering. There are many people that literally love plants to death and water too much. We see it as a common problem people post about here. Could that be your problem?

FataMorgana

I'd like to think so, FataMorgana, but these are on the same regimen as the older plants that have never shown any yellowing, so I'm not sure how that could be.

I'll letting them dry out more and see if that makes any difference, though. Thanks for the suggestion.