Basil is an herb in the mint family that can be used in many different cooking styles. Hailing from India originally, basil is a key ingredient in many ethnic cuisines, including Northeast Asian, Southeast Asian and, of course, Italian. Like all plants, basil can be susceptible to certain pests, diseases and other disorders. If your basil plant’s leaves are turning brown and curling, take action quickly to save the plant. These leaves are unappealing in appearance and flavor, so remove them from the plant and discard them as you attempt to determine the problem's cause.
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Brown, curling leaves can be a symptom of a soilborne disease known as cucumber mosaic virus. Your best course of action is to treat the plant with a commercial fungicide. If that fails to remedy the situation, remove the infected plant and don’t replant in the same area. Brown foliage may also be the result of wilting diseases, such as fusarium or verticillium. Check the plant for other symptoms of these pathogens, such as wilting and leaf drop. Remove the infected plant and treat the remaining ones with an antifungal spray.
If the leaves have a scorched appearance, in addition to being brown and curly, a pest infestation may be the cause. Aphids, thrips, beetles and mites are the most common pests attracted to basil plants. Insecticidal soap sprays, either commercial or homemade, should rid the plants of these pests.
Excess fertilizer can burn the basil plant’s leaves and ruin their flavor, especially since excess nitrogen can be harmful to the plant. Compost is the ideal fertilizer as it provides a slow-release form of nitrogen. If you prefer to use a commercial formula, fertilize with a slow-release liquid product every three to four weeks, at the rate suggested on the fertilizer label.
Water and Sun
Dehydration can be another cause of curled, brown basil leaves. Keep the plant well hydrated by ensuring that the soil remains consistently moist. A good rule of thumb to determine if it's time to water is to stick your finger two inches into the soil. Water if the soil is dry to that depth. If you're growing the basil indoors, provide it with at least five hours of sun each day. Insufficient sunlight may also brown foliage and stubby plants.