Avocado plant with a problem
I have an avocado plant that is about 12 inches tall, 2 years old, and living in an 8 inch pot in a filtered sunny southern position. Over the last 4 months, the lower leaves have developed brown spots and are curling in at the sides. The spots generally appear first at the tips of the leaves and more spots gradually appear along the sides and in the lower center. So far, none of the leaves has dropped off. There are no signs of fungus, mold, or insect infestation. The plant is watered with a dilute houseplant fertilizer whenever the soil feels dry...usually once a week. Since the spots started to develop, assuming dry air was the cause of the problem, I've been spraying the leaves daily. Is there anything else I should be doing?
It's difficult to diagnose your plant's problem without seeing the plant in person but there are a few things you can do to make sure your plant is healthy. These may or may not be related to your browning leaves.
First, is there a possibility that at any time the leaves were exposed to either direct sunlight or very cold air? Is the plant near a forced air heat vent?
Since your plant is two years old, have you tried turning it upside down and gently removing the rootball? If it has been in the same pot for this time, you may see roots circling around the outside of the rootball and it could be root bound. In this case, repotting in a slightly larger pot with some fresh soil may help your plant to be healthier.
You also may have a buildup of soluble salts from your fertilizer. A somewhat effective way to remove some of the salts from the soil is to put the plant in your sink and run a small stream of tepid water into the plant, allowing it to drain out the bottom. You can allow the water to run through and "wash" out the soil for about 10 minutes. There is also a product available at garden centers that removes salt and mineral deposits from your plant's soil. Most garden centers probably carry this product. I think it's called Sledge Hammer. This will also contribute to a healthier plant.
When you check for moisture, be sure to push your finger into the soil at least 1" deep to make sure you are monitoring the moisture level where the roots are below the surface. Your plant is growing and it's demand for water may be increasing with its size. Be sure to keep your plant damp but not wet.
If you are able to take several pictures of your plant and attach them to a reply email, it may be helpful also.