Rust in the flower garden (2024)

Rust is a common fungal disease of many different plants in the flower garden. The bright red, orange, yellow, and brown colors of this disease often draw attention and can detract from a plant’s appearance.

The disease rarely does significant damage to the health of the plant. On rare occasions, a highly susceptible plant may suffer severe damage due to rust. These plants should be replaced with less susceptible plants.

Managing rust in the flower garden

Plant resistant varieties

• Rust resistant plants are only available for some perennial and annual flowers.
• Look for rust resistant varieties in seed catalogs, on seed packages, and on plant labels.
• Russian hollyhock (Alcea rugosa) and Figleaf hollyhock (Alcea ficifolia) are more resistant to hollyhock rust than common hollyhock (Alcea rosea).
• Ivy geranium (Pelargonium peltatum) is more resistant to geranium rust than the more common florist's geranium (Pelargonium x hortorum).
• Some rust resistant varieties of snapdragon and daylilies are available.
• A widely used resistant variety can ‘lose’ its resistance as the population of rust fungi changes to overcome this resistance. If a resistant variety becomes infected with rust, switch to a different variety.

Prevent rust from starting or spreading

• Inspect all plants prior to purchase. Look closely at the inner and lower leaves where humidity is high. Reject any plants that have yellow, orange, or red spots.
• Tolerate low levels of rust in the flower garden. Some spotting on the leaves will not significantly affect the health and growth of the plant.
• Inspect plants regularly. Examine the inner and lower leaves where humidity is high.
• Pinch off rust infected leaves and remove them from the garden. Do not remove more than 1/3 of the plant’s foliage at any one time.
• At the end of the growing season, cut infected plants at the soil line and remove all plant debris from the garden.
• Severely infected plants should be removed and replaced with a resistant variety or a plant from a different family.
• Infected leaves, stems, or plants should be buried in a compost pile that heats up to 148° F or taken to a municipal compost facility.
• Reduce moisture on plant leaves.
  • Space plants to allow air movement between plants.
  • Thin or divide overgrown perennials.
  • Use drip irrigation or soaker hose to apply water to the soil, not the leaves.
  • If sprinkler irrigation is the only option, water early in the day on a sunny day so leaves dry quickly.
  • Water until the soil is moist to a depth of 6 to 9 inches. Do not water again until the top 3 to 6 inches of soil are dry.
  • Mulch the soil around plants with a 2 to 3 inch even layer of woodchips or other organic mulch.

Using pesticides

Fungicides are rarely necessary to manage rust in a home garden and should only be used to protect high-value plants that cannot be replaced and have a history of severe infection.

• Fungicides must be applied to healthy green tissue early in the growing season before infection begins.
• Examine the lower leaves of plants with history of severe rust once a week. When the first leaf spot is observed, pinch off the infected leaves and begin fungicide sprays to protect healthy tissues.
• Repeat applications are often necessary throughout the growing season and should be applied according to label instructions.
• Fungicides will not cure or remove existing rust infections. Once the majority of leaves have leaf spots, it is too late to treat.
• Fungicides with the active ingredient Myclobutanil or Azoxystrobin are most effective against rust fungi.
• Fungicides with the active ingredient Chlorothalonil or Thiophanate methyl provide some protection against rust fungi but not complete protection.

Reviewed in 2024