Sweet William: Pests and Diseases

By Julie Christensen

Sweet William flowers (Dianthus barbatus) are an old favorite, beloved for clusters of small white, violet, pink or red blossoms and fragrant leaves. Sweet William is technically a perennial, but unless you live in a mild climate, it will grow more as a self-sowing annual or biennial. Always leave a few seed heads on the plants so new plants will appear the following spring.

The Sweet William flower is fairly simple to grow, but it does have a few pests and diseases. The best way to keep it healthy is to give it good growing conditions, including fertile, well-draining soil and moderate moisture. Sweet William thrives in full sun, unless you live in a hot climate. Then, plant it in light shade for better blooms. Well-tended Sweet William blooms from late spring to fall. The plant is at home in containers, as well as mixed beds or borders where it grows 1 to 3 feet high and 2 feet wide.

Common Sweet William Flower Diseases

Fusarium Wilt is caused by a soil fungus that can live in the soil for many years. Plants infected with fusarium wilt slowly turn yellow. The leaves turn downward instead of growing upward and the roots and lower portion of the plant turn brown. To treat fusarium wilt, fumigate the soil with methyl bromide. Fusarium wilt is more common in warm, moist regions.

Gray mold, also called botrytis, causes brown spots on the flowers and leaves and can kill an entire plant if left untreated. Remove all affected foliage and spray with an appropriate anti-fungal spray. It overwinters in dead plant debris, so cut back Sweet William in the fall. Allow the soil to dry out between watering and avoid overhead sprinklers to keep leaves dry.

Leaf Spot is spread by air-borne spores. The first sign of leaf spot is yellowish, withered spots with a purple margin on the leaves. As the disease progresses, entire leaves die and drop. Eventually the stems die, followed by the entire plant. To prevent leaf spot, plant Sweet William so air circulates freely and avoid overcrowding. Use drip systems or soaker hoses, rather than overhead sprinklers, which encourage the spread of the disease. Don’t work in the garden while it’s wet. Treat infected plants with a fungicide labeled safe for use on Sweet William.

Rust, also caused by air-borne spores, causes orange or yellow spots on the leaves of the plants. The problem is more likely in wet, moist conditions, so use drip systems or soaker hoses instead of overhead sprinklers. Remove any infected leaves and discard, rather than compost. Treat infected plants with a fungicide, such as Mancozeb, labeled safe for use on Sweet William.

Root Rot, is a soil-borne disease, which as the name implies, causes the roots to rot and turn black. In well-draining soils, a few roots may turn black, but the plant can produce more roots and recover. In soggy, heavy soils, though, the plants often succumb to the disease. Amend heavy soils with compost or manure to improve drainage, or grow Sweet William in a raised bed or container. Allow the soil to dry out between waterings.

Common Sweet William Pests

Slugs, those tiny snails without shells, are voracious eaters and can quickly decimate your Sweet William. They thrive in moist conditions, the very conditions that breed disease, so one of the best ways to control them is to allow the soil to dry out. Remove mulch, especially if you live in a cool, moist climate, and try planting Sweet William in sun instead of shade. Use slug traps or baits with care because these products are toxic to animals and humans.

Grasshoppers cause the most damage at the end of summer when they’re typically found in high numbers. Unfortunately, grasshoppers are hard to control. Several insecticides, such as Carbaryl or Acephate kill grasshoppers effectively, but because grasshoppers can fly long distances, new grasshoppers may quickly take their place. Grasshoppers usually don’t kill Sweet William, although they may leave the plants looking ragged and untidy.

For Further Reading on the Sweet William Flower:

Sweet William from Washington State University Extension.

Sweet William from the University of California IPM Online.

When she’s not writing about gardening, food and canning, Julie Christensen enjoys spending time in her gardens, which includes perennials, vegetables and fruit trees. She’s written hundreds of gardening articles for the Gardening Channel, Garden Guides and San Francisco Gate, as well as several e-books.