How to Get Rid of Black Spots on Herbs

Observe your herbs for signs of disease. Different bacterial, viral and fungal diseases can affect herb plants. Fungal diseases, such as mildew, and bacterial diseases, including leaf spot, may be the culprit behind black spots on herbs. Take a sample of your herb plant to the local agricultural extension office and have an expert help you determine the type of disease that has infected your plant. Follow the diagnosis with the recommended treatment for the specific disease.

Check your herbs for pests. Mites, worms and gnats can leave black spots on leaves or leave plants open to contracting diseases that result in spots. Look for signs of pests, including droppings, holes torn into leaves and scorched-looking leaves, along with the black spots. Pests themselves may also be visible on the plant. Use an insecticide to control and eliminate the pests. Choose a broad-spectrum pesticide that is safe for herb plants and follow the label instructions carefully.

Water your herbs properly. Both over-watering and under-watering cause black leaf spots. Herbs, such as basil, dill, parsley and mint, require lots of moisture and need daily thorough watering. Keep the soil for these herbs moist but never soggy or muddy. Check herbs that require less water by inserting your finger 1 inch into the soil. If the soil feels dry, water the plant immediately. Herbs in this category include rosemary, thyme, chives and marjoram.

Fertilize your herbs to prevent black spots. Herbs generally require little to no fertilizer; however, if the soil is deficient in nutrients, black spotting may occur. Work an organic fertilizer, such as peat moss or compost, into herb-growing mix before you plant or replant the herbs. Work compost into the soil each year to provide a slow-release fertilizer to keep the herbs healthy.

Remove black spotted leaves from the plants. If the cause of the spots is infectious, such as disease or pests, affected leaves should be removed to prevent spreading the problem. Furthermore, removing spotted leaves means removing unhealthy leaves and allowing your plant to spend its time and nutrients producing new, healthy leaves.