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I noticed that the Lettuce leaves, Basil and the Honey Dew Melon leaves have this black spot that appears on the leaves of the plant. The back of the leaf looks the same as the front. I have fans blowing to keep the air circulating in the greenhouse where they are growing. They are growing in the same grow bed. I had to pull out the Basil because they were affected the most. I dont want this to get out of hand. Do you think Potasium Bicarbonate spray would work on this problem? 


Rich K

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Your lettuce looks like you have a thrip problem on your hands. Take a really close look at some of your leaves (preferably with a magnifying glass or microscope) and keep and eye out for tiny, elongated green bugs.

As far as sprays go, I've had immense luck with using spinosad products to control thrips. It's an organic, selective insecticide and it's quite safe for fish.

Thanks Alex,

Upon initial inspection, I did not see any bugs. But I will give it a more thorough inspection after work today.

Rich K

They are TINY. I'm a young guy with 20/20 vision and I have a hard time making them out without a magnifying glass. Check the less damaged growth on your plants.

It's also possible that it could be spider mites, but I'm leaning towards thrips, especially if the leaf damage has a silvery glint.

If you live in the Illinois area, you're welcome to come over with some leaf samples. I can put 'em under my fancy, schmancy microscope and we can figure out what exactly is going on :)


That is very kind of you. Unfortunately I live on Long Island, NY. It would be a bit of a trek.   I have been looking at all of the plants (Pak Choy, Peppers,Lettuce and Honey dew) growing in that particular growbed and I did find some Earwigs. But thats about it. The Pak Choy and Pepper Plants are not being effected. I have been searching the web and my gardening books but nothing definitive so far.

Perhaps a microscope purchase is in order.

Thank you  

It didn't look exactly like that but humidity has caused black spots on my basil with leaves eventually dropping off.


I had this problem last year with black spots and I thought it had to do with humidity because the green house had poor ventilation. This year, I screened in the green house to open it up a bit and bought some green house fans to see if it would fix the problem.

There is lots of air movement and less humidity but the black spot problem came back. I have been spraying the effected plants with a Potasium Bi- Carbonate and Molasses mix for the past few days and I have not seen any new black spots. So thats a good sign.

George said:

It didn't look exactly like that but humidity has caused black spots on my basil with leaves eventually dropping off.

Richard, I believe you are on the right track.  I've also seen worm casting spray recommended.

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