What Is Wrong With My Basil Plant?

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basil plant

Sweet basil infected with basil downy mildew.

If you’re planning to make pesto from your summer basil crop, do it sooner rather than later. This is because basil downy mildew, a fungal disease that attacks basil leaves, is now active in North Carolina.

The leaves of basil plants infected with downy mildew initially turn yellow and then brown. Symptoms show up on older leaves first. Upon close inspection, you may notice dark fuzzy spores growing on back of infected leaves. Spore production is most prolific during warm, humid and wet weather.

All types of basil can get this disease, though sweet basil is the most susceptible. The good news about basil downy mildew is that it only infects basil plants and will not spread to other types of plants in your yard and garden.

Once basil plants are infected, the best course of action is to remove them from the garden. Basil downy mildew requires a living host to survive and will not persist on dead plants or in plant debris, which means infected plants can be tilled into the soil or composted.

Basil leaf

Fuzzy gray spores may be seen on the back of infected leaves.

Do not overwinter infected plants indoors as this will provide a living host for the disease to persist in our area. Basil downy mildew can be passed on through infected seeds so don’t save seed from plants showing symptoms. Remove any volunteer seedlings that come up where infected plants were growing as these will likely also have the disease.

Applications of fungicides have little impact on preventing this disease. If your basil plants are still healthy, you can reduce the chance of them getting downy mildew by keeping the foliage dry. While you can’t prevent rain from wetting your plants, you can help them dry off quicker by thinning and pruning plants to increase air flow around them. Also, when watering plants, avoid wetting the leaves by applying water at ground level or use a soaker hose.

Basil plant

Symptoms develop on older leaves first.

There is little point in making new plantings of basil this year because downy mildew will continue to be active until frost. There are several options for garden areas left empty by the removal of basil plants. In the piedmont and coastal plain there is still time to sow a second crop of squash, zucchini, cucumbers, or green beans for fall harvest. You can also sow a warm season cover crop such as buckwheat to be tilled into the soil in 30 to 45 days. Wait until late August to plant cool season vegetable plants such as broccoli, kale, collards, cabbage, lettuce, and spinach and to seed carrots, beets, turnips, mustard, and parsley.

Learn more:

Visit your local Cooperative Extension center to learn more about gardening and landscape care. Find your county Extension center or post your questions to be answered online via Extension’s ‘Ask an Expert’ widget.