My Tulasi plant (basil) leaves are turning black, collapsing into themselves (like folding into a small bulb shape very close to steam) and then dying. In the row, there are 7 plants with gaps, in random positions, 5 of them died/dyeing like this and 2 are only fine and green.
Ground water is used from many years only, no change in water source.
Please advice, below are the pics.
I suspect bacterial infection. The very first thing you must do is move the diseased plants away from all your plants, and at a minimum the two Tulsi plants that are still healthy. If they came from the same lot, you will be lucky if they survive.
If you used the same soil, or if they all germinated from the same set of seeds, you should also separate the two healthy Tulsi plants from the rest of your other plants.
Discard the 5 and do not reuse the soil or pots without sterilization. Do not throw in a compost pile either.
If you tell us more about how or where they were purchased/germinated, we may be able to help better. Can you try to get us clearer close-ups of the leaves, if possible?
I can see a lot of evidence of spider mites Whilst probably not the main reason it certainly has an affect on the plants health and how it can respond to infections.
It looks like it had/has a very bad case of powdery mildew. Powdery mildew attacks a lot of plants. It takes advantage of moisture on the plants leaves to live. This moisture often comes from humidity in the air. Some herbaceous perennial plants get powdery mildew every year with very little damage. But, a very bad case can kill the plants, especially evergreen plants. They have not way to shed the disease and grow again the next year. Plants that often get powdery mildew are roses, phlox, some maples, and Rudbeckia. These plants are able to tolerate it due to their seasonal changes.
There are some fungicidal sprays that will slow down the disease, but most you do not want to spray on leaves that you plan to eat. There are some bacterial sprays that actually kill the disease. Those might leave behind a strong taste, so again no a spray you want to uses.
The best you can do is prevention in the future. Make sure you plant has plenty of air flow. Keeping it out in the open vs against a wall will help prevent the disease. Keeping your plant in good health is the best thing you can do. Don't under or over water. It takes advantage of weak plants.
