When I sowed basil seeds this year, the seedlings germinated and started to grow, but then suddenly perished.
This is most likely a disease called damping-off fungus. If starting in containers, try using a sterilized container. Try not to over-water. The soil should just remain damp. Ensure good air circulation as well.
My basil plant stopped growing and lost its color. When I dug it up the roots they appear dark and slimy.
This is root rot, caused by a variety of fungi. Try not to over-water. Water only when the soil is dry and make sure before you plant anything that you have good drainage!
My plant has developed spots on its leaves.
This is leaf spot disease. Remove the spotted leaves from your herb. To mitigate this problem, water in the morning if possible and try not to let the foliage get wet.
My basil plant is withered and distorted and the leaves have curled. Sometimes there is a black, sooty substance on the leaves.
This is the work of Aphids, also known as plant lice. See the article Getting Rid of Basil Pests for how to remove this pest from your garden.
What is the sooty black substance left behind by aphids?
This is called honeydew, a substance secreted by aphids. Black powdery mold will often grow on the honeydew. This mold can be washed off after the aphid problem has been controlled.
Holes are being chewed into the leaves of my basil plant.
This is likely the work of beetles. See the article Getting Rid of Basil Pests for how to get rid of these pests.
When I brush against the leaves of my herbs, a cloud of tiny white insects appear. What are they and are they harmful?
These are most likely an insect known as the whitefly. They will suck sap from your herbs and plants. See the article Getting Rid of Basil Pests for how to get rid of these pests.
My basil plant wilted and lost its color. When I dug it up there was swelling on the roots.
This is most likely damage caused by the nematode. Try to move your basil plants to a different location next time you plant and avoid that spot for three to four years. See the article Getting Rid of Basil Pests for how to get rid of these pests.
The foliage of my basil plant appears chewed. Sometimes there are silvery streaks present as well.
This is a problem with slugs or snails. See the article Getting Rid of Basil Pests for how to get rid of these pests.