Is There an Insect or Disease That Affects Basil Plants?
Several insect pests and plant diseases can affect basil (Ocimum basilicum). Basil is a flavorful herb and an essential ingredient in many Italian and Thai dishes. All varieties of basil, grown for culinary purposes or ornamental value, thrive as annuals in U.S. Department of Agriculture plant hardiness zones 2 through 11. Basil is very frost sensitive but may grow as a perennial in USDA zones 9 through 11. Growing basil requires fertile soil, warm temperatures, and full sun or partial shade conditions.
Japanese beetles (Popillia japonica) are 1/2-inch-long, metallic green beetles that chew holes in basil and many other plants. A large number of Japanese beetles can skeletonize leaves by eating every part of a leaf except for the larger veins. Japanese beetles emerge in summer and are derived from C-shaped white grubs that live in the soil of garden beds or underneath turf grass. Strategies to avoid Japanese beetle damage include hand-picking of beetles and ridding the lawn of grubs.
Aphids (Aphididae) and whiteflies (Aleyrodidae) are pests of basil. Aphids are tiny insects that suck sap from new tender growth, and infestations can be identified by a shiny secretion, called honeydew. Whiteflies are especially a problem in greenhouses. A strong stream of water can dislodge aphids and whiteflies. Insecticidal soap sprays also are effective against these soft-bodied insects.
Nematodes (Meloidogyne spp.) that harm basil are called "root-knot nematodes." They are tiny worms that attack the roots, causing swellings, or knots, on which they feed. Basil plants become stunted, wilt and eventually die. Adding compost to soil may help, because compost contains beneficial microbes and fungi that can kill these nematodes.
Basil can be affected by the fusarium wilt fungus (Fusarium oxysporum), which causes yellowing of leaves, sudden wilting, leaf drop and dark streaks on stems. Plants suffering from fusarium wilt need to be destroyed or discarded to avoid spreading the disease. No organic treatments are available. Try growing fusarium-resistant basil varieties, if you experience a fusarium wilt infection.
Gray mold (Botrytis cinerea) can grow on stems of basil. High humidity, combined with cool temperatures and poor air circulation, promotes the spread of the disease. The best method to avoid gray mold is to increase air circulation around your plants.
Downy mildew (Peronospora spp.) is a grayish downy mold or fungus that has recently become a scourge of basil in some areas of the United States. It covers the underside of leaves and causes yellowing of leaves.
Brown or black spots can develop on basil leaves and are mainly caused by three fungi: Alternaria spp., Cercospora spp. and Colletotrichum spp. Poor air circulation and overwatering promote leaf spot. Dispose of affected basil plants, increase air circulation, and water basil near the soil -- avoid wetting the leaves. A fungicide containing potassium bicarbonate may help get rid of leaf spot.
- Clemson University Extension: Basil
- Cornell University Vegetable MD Online: Expect and Prepare for Downy Mildew in Basil
- Insect, Disease & Weed I.D. Guide; J. J. Cebenko and D.L. Martin; 2001
Based in Connecticut, Marie-Luise Blue writes a local gardening column and has been published in "Organic Gardening" and "Back Home." Blue has a Ph.D. in biological sciences from the State University of New York at Stony Brook and wrote scientific articles for almost 20 years before starting to write gardening articles in 2004.