Common geranium (Pelargonium hortorum) is easy to grow as a houseplant or outdoors in U.S. Department of Agriculture hardiness zone 10. A healthy geranium will have erect leaves without any discolorations or curling edges. The appearance of brown leaves signals a variety of potential problems.
The leaves of common geranium are round or heart-shaped and medium green in color. A distinguishing characteristic of geranium leaves is a band of darker green that runs around the perimeter of the leaf near the edge. This is referred to as a zonal band and explains why common geranium is also called zonal geranium.
While deer and rabbits won’t usually much on geraniums, caterpillars find the leaves tasty. Aphids and whiteflies may also snack on geranium leaves, especially on plants grown indoors. Evidence of these pests includes the appearance of small holes in the leaves encircled by a ring of necrotic leaf tissue. Usually this dead tissue is brown or black, although it may also look dark red or purple.
Forgetting to water your geraniums will eventually cause leaves to turn brown and drop. But over-watering can make plants susceptible to Pseudomona, Erwinia and Xanthomona bacteria and lead to leaf spot, tipburn or blight. Symptoms of bacterial leaf disease include brownish-black lesions. As the disease advances, these lesions develop a yellow outer edge and the older infected portions at the center turn brown. Without intervention, the leaves will die and fall off.
Brown leaves on a geranium plant is often a sign of fungal problem. Root rot, also known as water mold, is caused by an attack of Pythium fungi at the roots, likely the result of poor soil drainage. This disease also causes roots to turn from white to black or gray. Browning on geranium leaves that begins with a v-shaped section of discoloration is a symptom of a Botrytis fungal infection, which eventually gives way to the development of fuzzy spores and total discoloration of leaves as the disease progresses.
- International Geranium Society: Plant Genera Within the Geranium Family
- University of Minnesota Cooperative Extension: Outdoor-Indoor Geranium Culture
- Missouri Botanical Garden: Pelargonium x Hortorum
- University of Minnesota Extension: Bacterial Leaf Diseases of Foliage Plants
- University of California Statewide Integrated Pest Management Program: Pythium Root Rot
- North Carolina State University: Pictorial Guide to Geranium Wilt Disorders
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