Fig trees are quite susceptible to nematodes. These microscopic worms feed on the smallest roots of the tree and inhibit its ability to absorb water. As nematodes are tiny, they can feed on a fig tree's roots for years undetected, until their population becomes large enough to do enough damage to the tree to become noticeable. Fig trees infested with nematodes gradually decline and may even drop their fruit prematurely.
Make sure that nematodes are the culprit--fig trees may decline for many reasons. Nematodes are too small to see with the naked eye. But if you uproot a small section of your fig tree's roots, wash them off and observe them they should be covered in nematode galls or swollen areas that can reach 1 inch in diameter.
Send a soil sample to your local county extension office. While root galls are a sign that your culprit may be nematodes, there are several root fungi that cause similar types of damage. Your local county extension office will be able to correctly diagnose your fig tree's problem.
Apply a post-plant nematicide, like Nemacur 10G, to the soil according to the manufacturer's instructions.
Spread a 5-inch layer of organic mulch over the soil, out to the drip line of your fig tree. As the mulch decomposes, it will fertilize the soil. Nematodes thrive in poor soil and these nutrients will discourage nematodes from resettling near your fig tree's roots.