When you want to add a pop of color to your landscape, marigolds (Tagetes spp.) are an ideal choice for the garden. Hardy as perennials in U.S. Department of Agriculture zones 9 to 11, though widely grown as annuals in far colder zones, these plants produce vibrant gold, orange and bronze flowers that can brighten any landscape.
Marigolds are fairly low-maintenance plants too, adapting to most soil types and requiring only moderate watering. However, even the low-maintenance marigold may run into problems on occasion, so there are certain pests and diseases that you should keep an eye out for if you want to maintain your plants' health and appearance.
Caterpillars on Marigold Foliage
If you notice holes or chewed edges on your marigold's leaves, you may have a problem with foliage-eating caterpillars. Several species of caterpillars, including tobacco budworm, orange tortrix, cabbage looper, beet armyworm and sunflower moth, are attracted to marigolds and will chew holes in the foliage and blossoms of the plant. The damage can eventually stunt the plant's growth or kill it entirely.
The larval stage of butterflies and moths, caterpillars have a long, soft tube-like body that varies in color, depending on the species. While they cannot fly, they do have three sets of legs on their thorax that allow them to move along the marigold's leaves. If you see caterpillars on your plants, removing them by hand is usually the easiest solution.
Aster yellows is a viral disease caused by a minute organism even smaller than bacteria that can stunt or deform a marigold. You can identify an affected marigold by a mass of frail, upright shoots that grow around the base of the plant and are usually yellow in color. The plant's flowers are usually deformed as well, taking on a stunted, leaf-like appearance. Eventually, the marigold may wilt and die.
Aster yellows is spread by leafhoppers and aphids, which carry the organism and transmit it to the marigold when they suck cellular material from the leaves. To control the spread of the disease, remove infected plants immediately and disinfect any gardening tools that you used to tend to the plants. If you notice nearby weeds that attract leafhoppers or aphids, remove them as well.
During hot, dry spells, your marigolds may experience a problem with two-spotted spider mites. These insects are extremely small, so they only look like moving dots on the leaves of the plant to the naked eye. Belonging to the arachnid class of insects with spiders and ticks, spider mites form colonies, usually found on the underside of leaves, where they extract the plant's cellular material. While a few mites aren't usually a problem, a large infestation can do visible damage to the marigold. Stippling or light dotting on the leaves is usually the first sign.
However, as the spider mites continue to feed on the plant, the leaves may turn red or yellow and fall off. Groups of spider mites may also spin thin, delicate web on marigolds, which you may notice around the marigold's leaves. Since the insects are usually drawn to dry plants, keep your marigolds properly watered to avoid infestations. If you notice spider mites on your marigolds, spraying the plants with water usually washes the pests away.
Marigold Diseases: Rots and Blights
Marigolds may also be affected by root, crown and stem rot, which are caused by fungi that infect the crowns and roots of the plant. Phytophthora is one of the more common fungi to attack marigold, and usually affects the roots of plants in poorly drained soil.
Marigold foliage may take on a dull color and may eventually yellow and wilt. Because the fungus attacks the roots, plant growth is usually stunted as well, and the marigold may eventually die. If you remove the plant from the ground, the roots are typically dark in color with brown tips and are soft enough to break. If you find infected plants, remove them from your garden immediately. To prevent root, stem and crown rot in your marigolds, avoid overwatering so excess moisture doesn't build up. Make sure that the soil has proper drainage as well.
- University of California Agriculture and Natural Resources: Pests in Gardens and Landscapes – Marigold
- University of California Agriculture and Natural Resources: Pests in Gardens and Landscapes – Foliage-feeding caterpillars
- University of California Agriculture and Natural Resources: Pests in Gardens and Landscapes – Asters yellows
- University of California Agriculture and Natural Resources: Pests in Gardens and Landscapes - Root, stem, and crown rots—Fusarium, Pythium, Phytophthora, and Rhizoctonia spp.
- University of Minnesota: Spider Mites
- Pennsylvania State University: Marigold Diseases
- Missouri Botanical Garden: Aster Yellows
- Hemera Technologies/AbleStock.com/Getty Images