Like many other plants and herbs, rosemary is susceptible to fungus. Rosemary is particularly sensitive to humidity and water. More often than not, when fungus begins to grow on a rosemary plant, the two main culprits are over-watering and high humidity. You have to remove fungus from rosemary as soon as possible so it does not spread to the tender new growth and kill the entire plant.
Cut off dead or infected leaves. Dead and infected leaves can spread the fungus to healthy parts of the plant. Cut them off with a clean pair of garden shears and dispose of them immediately.
Pull up the rosemary. Soggy roots from overwatering often lead to fungus. If you have potted rosemary, pull it out of the pot, allow the roots to dry completely and re-pot.
Make a baking soda spray. Baking soda is effective for killing fungus on the leaves of the rosemary plant. Mix ½ teaspoon of baking soda with one quart of distilled water in a spray bottle. Spray the plant once a week until the fungus is gone.
Spray on chamomile tea. Chamomile is naturally antimicrobial and antibacterial. Make a cup of chamomile, let it cool and pour it in a spray bottle. Spray the rosemary once a week until the fungus is gone.