I'm growing basil in Key West, Florida because I'm a pesto lover. But I'm having a problem. I'm getting dark star-shaped spots ringed with lighter color on the leaves. See the photos.

MY BASIL PLANT FACTS
-Grown in a planter box
-I don't water them. I let the rain water them.
-They get about 30% direct sunlight throughout the day.
-High above them is a mahogany tree
-I didn't have this problem during the first three weeks after I planted them.
-I bought these from Home Depot and they were maybe 8" tall.

SUGGESTIONS PEOPLE HAVE GIVEN ME:
1. "Pull off the affected leaves."
It seems these days I'm having to pull off 8 leaves a day (total) off these several plants… but I'm not sure that the plants are capable of creating a total of 8 new leaves a day. I hope I can win the war. This has happened a few years ago when I grew basil. I didn't pull the leaves off.. and eventually this issue overtook and killed all the plants.

2. "Water sitting on the leaves is the cause of the problem and I should just water the soil, not the plant."
Truth be told: I don't water these plants. The rain is the only water source.

These spots seem to happen even throughout a period of 3 dry days. This makes me suspicious that it's not water-related.

Any help would be great. Should I pull off the leaves? Should I spray the plants with olive oil? Should I move the plants to my porch so no water can get to them? Should I go to the store, buy a coke and some chips and say it was fun while it lasted?

The Interesthelper: Welcome!! I grow a fair amount of different basils, and all of them sooner or later, get leaves like yours, which Is from a bacterium or fungi. I would rather just pick off affected leaves than spray anything on my basil. A soap spray would have no effect! Cheers!

Here is a timelapse photo showing the speed of the bacterium/fungi growth over 3 days. It's faster than the leaves can grow.

As an experiment, I just sprayed one variable plant with olive oil to see if it can prevent it. I don't know the consequences other than people get sunburned faster with oil on their skin. My hope is that the oil suffocates the bacterium/fungi or somehow creates an environment that is not condusive for it's growth.

Other note: The issue seems to be isolated to the top part of the leaf, not the underside.

The attached photo isn't too large. If you'd like to see it in detail, let me know and I'll send the higher-resolution image. I tried including a link to the image on my server, but this forum doesn't allow links.

theinteresthunter: I looked at your pictures (4) and I only see the same two leaves with the black spot advancing on each, do I need new glasses?? LOL. Probably! There is a rationale for spraying some plants with oil, "Neem oil" in particular. But I would still just pick the two affected leaves off and put them in the garbage. The plants certainly look healthy. I have Basil growing in the basement right now under lights, regular and Thai and I can assure you that there are some "blight" issues, but I think you may be losing some sleep over a non-issue, except that as an "Interest Hunter", the quest goes on!!! Cheers!

Thanks, fwmosher, for the recommendation of neem oil! I'm going to look for it.

In the time lapse images, you'll also find some small spots on some bottom leaves that also advance over time.

The problem is I'll find new spots literally every day. The fungi growth is faster than the plant growth. I'm trying to cure the issue so I can win the race (and go sailing for a few days without worrying about my basil).

I'd agree these plants look healthy. They'd look less healthy had I left all the leaves I've plucked off them due to the spots. For example, if I didn't pluck any leaves off the plant shown in the photo, probably in about 5 days I'd have at least twice the amount of leaves affected.

And yes, I am indeed "hunting" down this "interest"!

Thanks for your help everyone!

Hi Interesthunter,

I think that you have an insect attack of Blotch Leafminers rather than fungus. To verify, try carefully peeling up the top layer of the leaf in affected spots and you may find the very tiny larvae that are causing the damage.

http://blogs.ifas.ufl.edu/monr...

Danita,

I checked out the blotch leafminers. I think you are right, thank you!

I'm going to dig in deeper and see what kind of defense I can do.

Thanks!

Wow! Good call @Danita. I've never seen this before in all of the years I've grown basil. Learned something new today

This is attack of fungus. You can Pull off the affected leaves.

This is probably very late, but I thought it'd be worth a try. I had a similar problem with my basil plant, so I made a baking soda spray, sprayed the plant, and then took off the affected leaves. Haven't had any problems since.

I had the same problem with my Walmart bought basil. I spotted it early and removed the affected leaf. Didn't have a problem with blotches since.

Hey guys ! Sorry I am new and don't know if this forum is still active but giving it a try.

I have the exact same issue. Same spots on basil leaves. Very frustrating.
Plant is not dying but black spots are growing with the plant . Can't barely have any leaf without it.
Just bought a new plant from Home Depot and 3 days later, barely any sun, new pot with fertilizer, same thing.... took it inside now and will see. I cannot spot any larvae in the leaf. And it's dry not moldy. Plant not dying, just grows with it .

Does anyone has found the source of the issue?

I have to say that I've never seen this before, and I grow a ton of herbs.

Did it possibly get any fertilizer or herbicide damage from wind drift nearby?

The pattern of damage really looks like Blotch Leafminers.
http://blogs.ifas.ufl.edu/monr...

Wow! It looks exactly like it. I had never seen that before.

Florida has the best reference site. Thanks for sharing that link to this problem.

Taking affected leaves out does not work. It's coming back and every day I have 3 or 4 more black leaves.
Trying the baking soda spray!
Thank you

I'm having this same issue from a plant a coworker gave me. It is currently inside and there are no leaf miners present. We have already pulled off the diseased leaves. The only thing I can think of is mold or fungus.

It looks exactly like Blotch Leafminer damage. Leafminer larvae will often exit the leaf and drop to the soil to pupate. Some types pupate in their tunnel but will fly away once mature.

Fall cool weather, humidity, looks like the powdery mildew I get, I have to harvest good leaves one at a time to have any of that variety of basil at all. Instead, here near Houston, I try to grow the African varieties of basil and expect to lose them at the first frost. Those I lose as soon as temps hit 70's, like Fl I can have no rain and 60% plus humidity every single day. There are a few varieties of that sweet basil that have some resistance to the mildews.