Marshall. Yes, it certainly seems like it’s Fusarium Wilt. Sweet Basil is susceptible to Fusarium Wilt and the symptoms are a match for the disease, yellowing leaves at the bottom the plant and progressing to more yellowing and browning as the infection spreads in the plant. Once a basil plant has the disease, there is no treatment for it.

Fusarium Wilt is a fungal disease and is spread via contaminated soil, infected seeds, or by water splashing contaminated soil up onto a susceptible plant.

Not all leaf yellowing is caused by Fusarium Wilt. Yellowing can also be a result of heat stress, under watering or nutrient deficiencies. But, you mention you’ve been watering and use fertilizer, so the symptoms are likely caused by Fusarium Wilt.

To control the disease you need to remove the plant and dispose of it, don’t compost it. Any leaves or other debris from the Basil may harbor the infection and need to be disposed of also. The spores of Fusarium Wilt can survive in the soil for years, so I recommend of getting rid of the potting soil and thoroughly cleaning the container.

Prevention is the best control for this disease. Since you purchased the plant consider buying future plants from sources who understand how to monitor their plant stock for disease and who purchase their stock from reputable sources. Also there are now Sweet Basil cultivars that have resistance to Fusarium Wilt which would reduce the chances of the disease reoccurring, see the link for more information. If growing Basil in a container again, use fresh potting mix and make sure the container has a hole for good drainage.

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Maryland