If you are noticing brown spots on your basil plants and you’re not sure why, there are four main fungal causes for browning basil. Some of these funguses are already in the seeds, but do not show up until the plant is exposed to humidity and overhead watering. Others are caused by humidity and too much exposure to water.
Four Main Funguses
Fusarium wilt is caused by a soil born fungus that makes it impossible for the plant to transport water. Plants that are infected will grow normally until they reach 6-12 inches and will begin to develop brown patches. Once these plants begin to wilt you should remove and destroy them immediately.
Bacterial Leaf Spot
Bacterial leaf spot occurs in infected seeds. The infection shows itself when water is splashed on leaves during humid weather or by using unsanitary gardening practices and tools. Avoid this by property cleaning your tools, practicing good gardening hygiene, and avoiding getting water on the leaves. If these spots do show, discard the plants.
Downy mildew is believed to be carried in the seeds and starts out as a yellowing in the center stem of the leaf and then spreading to the whole leaf. Grey fuzz shows up on the bottom of the leaf and the yellow spots on top turn brown.
Cercospora Leaf Spot
Cercospora leaf spot is the mildest of the brown spot funguses as you can simply prune the infected leaves and making sure to avoid overhead irrigation and always water from the base.
To avoid ever seeing these problem, always avoid getting water on leafs and instead water directly on the base and keeping plants away from humidity. Keep enough space between plants and applying a layer of mulch around the base of the plant will also help keep your basil plants happy and healthy.
With minor infections you can spray fungicides on a weekly basis as long as they contain potassium bicarbonate or are copper-based. Always read the labels and follow instructions to make sure you can protect your basil and any other nearby plants.