I have a thai basil plant and a mint plant in pots on my windowsill. They thrived for about a month, but in the past 2-3 weeks their leaves have gotten covered in tiny black specks and dead white patches. The basil is in much worse condition. I have several other plants in the same location which seem ok. What could be causing this? Anything I can do?
That looks like insect damage to me. The two biggest culprits with indoor plants are mites and thrips. My advice is to go get some insecticidal soap or a product called “Nuke ‘Em” and spray the plant completely, especially stems and undersides of leaves.
If you want to confirm what it is for sure, go online and get a 40X jewelers lens and/or a cheap handheld pocket microscope, and look closely at the underside of the leaves. You can also clip one of the newer, but still damaged leaves and put it on a white paper plate.
If you see anything that looks like teeny tiny cobwebs or fuzz, or grains of sand, you probably have mites. If you see clusters of black dots, and have a hard time finding any bugs at all, check crevices and along leaf veins looking for fast, tiny, rather long bodied bugs. They are far larger than mites, which you cannot see with the naked eye. Thrips larvae are small and kind of translucent/green, so I have trouble seeing them without magnification either. Larger adult thrips are easy to see with the naked eye, maybe half the size of a thistle seed, and usually darker. The problem is, they move fast, are great at hiding, and some varieties can jump. I find them more easily when I don’t disturb the plant, and I come in close with my jewelers goggles and lift a leaf gently and inspect along the veins.
