It looks like sunburn to me, how much sun do you have them in? When humidity is low, you need to stick them in less sun. Here in Sacramento, where our humidity runs around %15, I have to give them mostly shade.
OR, and I am not sure about this, if quite a bit of water sits on the leaves, it could act like a lens and help to fry the leaves, like if you take a magnifying glass and start a fire with it?

My sweet basil has a few leaves that are showing the exact same signs as the one pictured. Strange though, they're exactly in the same spot they were last year and they thrived. Perhaps I need to rethink where they should be placed in the garden (container basil). I guess the same would be true of the Pesto Perpetuo, although I don't see any signs of sunburn on them as of yet.
But both Sweet and PP are in an area where they get about 9-10 hours of sunlight a day. I would say the morning sun starts to hit them around 8-9 am and they don't begin to become shaded until the sun is over to the west and the neighbors large birch tree starts casting its shadow over that part of the garden, which would be around 6pm, I'm thinking.

Will basil do well in shade, with indirect sunlight?

I agree that it looks like sunburn. Morning sun is best, then afternoon shade. Basil is a somewhat tender plant.

Thanks everyone,
They are in the sun for about 8 hours a day. I'll try to get them some shade. Should I just cut the topsvoff, and let them regrow, with more shade? Thanks.

I don't think it's sunburn, but I can't come up with an alternative suggestion.

Thing is, I have basils in full sun here in the subtropics where the sun is truly fierce (literally can fry an egg on the pathway!) and I've never had a sunburn problem with it. Not even with young seedlings. Basil is a plant that likes a lot of sun, and a lot of heat. It's also greedy for water and nutrients. I put my basils in the sunniest, hottest place in my little garden, where they never get shade except at night. They do very well there.

I think you need to take a look at your soil. Check pH, water levels, nutrient levels etc. Why take a sample of your plant to your nearest herb nursery and see what they have to say there? I wouldn't readily accept the sunburn theory.

My basils are in full sun, in very well drained soil, and are doing fine. It may very well be that when you water then from above "often" you may be allowing droplets of water to sit on the leaves and act like lenses to focus the sunlight and burn the leaves. I water mine from above, but only after the sun is very low, after 6:30 PM or so here. If you feel that yours are very thirsty during the middle of the day try watering only at the base of the plants. I'm just a newbie, but I hope this helps.

I would say probably most people understand not to water during the heat of the day when water will sit on the leaves and act like a magnifying lens on the leaf...that's just asking for it. Watering should always be at the end of the day.

I never did buy the "sunburn" theory. Sorry. Sounded stupid. Basil loves full sun, and last year's crop thrived in full, bright and baking sunlight and produced a delicious crop. Sorry to whomever gave out that bad advice. Glad a voice of reason, or someone who knew what they were talking about chimed in...that would be YOU daisy.

Perhaps trying some Safe Soap to rule out pests that may not be readily visible to the naked eye? See if that helps? Could it be as simple as overwatering?

That's fine, but in California, basil cannot take full sun. Same with other plants that can take full sun in tropical situations. In places with higher humidity, plants can take more sun. I can show you my burnt banana if you don't believe me. Also the way a lot of other plants can take full sun in northern climes but can't take it here. But I agree, it looks like water or something was on top of it. Here, when basil gets full sun, the leaves are small like that, they don't get burnt, but they don't taste as good and don't have the normal 'lush' look that sweet basil usually has.

Ugh....looking at your burnt banana makes me wretch. Please keep it to yourself, lol!

I'm sure everyone had the best intentions in giving me their opinions, and I appreciate it.
I have alot of these herbs in big Rubbermaid containers. I don't do the usual potting soil-type stuff for soil........just ground soil with aged, used chicken bedding/manure, I've grown basil for several years in this spot and it never had this problem. Perhaps I didn't harden off the plants from the nursery long enough, before subjecting them to the elements.
At any rate, I think the overhead watering might have contributed to the problem. I'll keep my eye on it and keep you all posted on what happens.
I have other plants/herbs all around it, and they don't have this problem......so who knows......

Yeah... I betcha $100 that you're right... that it's burnt because of shock. That would explain the sunburn in places that it doesn't usually get sunburn.
Steve, you don't want to see my burnt banana? :D ha ha! That does sound pretty bad! :D I should have said banana TREE!

OK....I give up. You can call me Steve as long as you don't hold anything against me that he said.....I read than entire thread. Whoa.

I guess I just had bad timing in joining the Garden Web at the time all that was happening. I wish he'd post so it could all be cleared up. "Hello, Steve...talking to you...you out there?"

Anyway, call me this gbrosteve if you must, but I prefer you call me Chris. But just call me. :-)

And even given all that, I still really don't wanna see your burnt banana. That was a good one!

Happy Gardening!

Applying insecticidal soap in the direct sunlight is asking for problems with many plants. It should always be used before OR after the sunniest, hottest part of the day. Applying it in the early evening allows the product to stay wet (thus, effective) longer.

Also, it's not advisable to use insecticidal soap unless you actually have an infestation of a pest that needs to be managed. As soon as the product dries, it effectiveness is gone...there is no residual effect. Applying it 'in case there is something there' is a waste of time and money. In other words....'bad advice'. ;-)

I doubt it's the humidity. It looks like the were subjected to some cold temperatures to me. The same thing happened to me last year after getting in a hurry and planting too early, but they did recover as the weather warmed. If it's not cold temps then I'd say it was stuck into sun too fast.
Here in Eastern WA our summers get quite hot and our humidity is little more than heathen in CA and I always have them planted in full sun in a south facing bed.

Vera

What ever happened with your Basil? Where those leaves good to use? Cause My was doing wonderful and all of a sudden it's starting to look like yours, But I was thinking that the plant was just about finished producing and is dying back

When my tomatoes looked like that they needed potassium. So I went googling for you. Found this.
I snipped this from the following website.

'Potassium deficiency is another common cause of black patches on basil plants. Basil is prone to this deficiency since it has a high requirement for potassium, particularly as the plants mature. Giving the plants some potassium-based liquid fertilizers might be worth a try. Good luck with your basil plants.'

Well, Garden web wouldn't let me post the name of the site. Got a spam alert. Anyways, check into a potassium deficiency. Hope this helps.
Sue