To the experts - I have a controlled experiment for you. The pictures show basil planted from the same packet of seeds. One shows basil plants from the original seeding, quite close together. The other shows plants that were plucked from the original pot as seedlings and planted in the much the same soil. The original plants were absolutely great: robust and dark green; I cut them back about 2 weeks ago to make pesto. The transplants look great, too. Now, as you can see, the original basil plants look yellower and as if they are being eaten. What happened? -Jim
Hi- it looks like some possible beetl feeding injury to your basil (Asiatic garden beetl?) They feed at night. You may want to go out at about 9 pm to see if you can catch them in the act. Minor pest; no need to worry about it.
Bug of the Week this week: http://bugoftheweek.com/
The yellowing could be an early sysmptom of basil downy mildew (unless you planted one of the resistant cultivars- Prospera, Devotion, Obsession. If it is down mildew you will see fuzzy sporulation on leaf undersides (more pronounced during wet, humid weather. There is no cure and plants should be removed and composted (the pathogen does not overwinter in MD gardens).
Leaves could also be yellowing due to loss of nutrients from the containers from heavy downpours and irrigation.
These night-feeding beetles are considered to be relatively minor pests, although you could see continued feeding. They will not defoliate the plants due to rapid plant growth at this time. There fine mesh insect screening materials available online that can be draped over plants to exclude pests. They don't increase air temperature or interfere with plant growth. There's always the chance of trapping some beetles under it though.