Michelle Grabowski, University of Minnesota Extension Educator
Cool temperatures this spring seem to be encouraging rust fungi on roses. Several different species of Phragmidium (the rust fungus) can infect both wild and cultivated roses. Gardeners should keep an eye out for two different forms of this fungus.
Bright powdery orange spores, known as uredinia, are likely to catch a gardener’s eye. These spores form in raised pustules on the underside of infected leaves, stems, or petioles (central portion of the leaf that the individual leaflets are connected to). Yellow to brown leaf spots may be noticeable from the top surface of the leaf but may not form on all rose cultivars. Infected petioles and young green stems may actually become twisted and distorted around the site of the infection. Rust fungi can infect all plant parts except the roots and gardeners may notice bright orange pustules in unusual places!
Photo 1: Uredinia on the lower surface of a rose leaf. Michelle Grabowski
If the bright orange uredinia are found on a rose plant, carefully inspect all stems that are one year or older for dark brown to black raised bumps forcing their way through the epidermis. These dark brown to black pustules are the same rust fungus in a different form. Dark brown to black teliospores fill these pustules and are tough enough to survive Minnesota’s harsh winter weather.
Photo 2: Uredinia on a young rose stem. Michelle Grabowski
Rust fungi have a complicated lifecycle that involve 5 different unique spore stages. The two described above are those most easily seen by gardeners. The bright orange uredinia can spread on the wind to infect more leaves within the plant or on nearby roses. These spores need cool temperatures, preferably 65-70F, and 2-4 hours of continuous moisture on the plant surface in order to start a new infection. If weather conditions become hot and dry, spread of the rust fungi is dramatically reduced.
Teliospores typically do not form until late summer or early fall. They are often found in the same area as the bright orange uredinia and therefore can also occur on leaves, stems and petioles. If gardeners are seeing the dark brown to black teliospore pustules now, these were formed last year and are likely the source of this year’s infections.
Photo 3: Rust fungi can infect all above ground parts of the rose including rose hips. Michelle Grabowski
The amount of damage that rust fungi can do to a rose plant varies greatly depending on several factors. Most roses are somewhat susceptible to the disease, but a few cultivars are particularly sensitive and will lose leaves in response to just a few bright orange pustules. For most roses, disease severity depends on weather conditions. If cool damp weather persists the rust fungi can grow and spread dramatically. If the summer turns hot and dry, the rust may dry up as well. A few leaf spots will not seriously affect the health of the plant but if infections on stems grow large enough to girdle the stem or otherwise restrict the flow of water and nutrients, that stem may be killed. The fungi that cause rust on roses will not infect other plants in the garden.
Managing Rust on Rose
Damage from rust on roses can be minimized by slowing the spread of the disease.
- Examine the underside of the rose leaves and young green shoots for bright orange pustules.
- Pinch off infected leaves and shoots. Collect diseased plant parts in a bag and remove them from the garden. Never remove more than 1/3rd of a plant’s foliage even if it means leaving a few leaf spots on the plant!
- Carefully inspect last year’s stems for black to dark brown raised pustules. If present, prune out and destroy all infected stems.
- If last year’s leaves are still present around the base of the plant, rake them up and remove them from the garden.
- Prune roses to promote good air movement in and around the plant.
- When planting new roses allow enough space for the mature plant. Avoid crowding plants together which increases humidity in the canopy.
- Use drip irrigation or a soaker hose instead of sprinkler irrigation to keep leaves dry.
- Several fungicides including those with the active ingredient triforine and propicanazole will protect healthy rose leaves from infection by the rust fungi. Always read and follow all label instructions when using a pesticide! In Minnesota fungicides are rarely necessary.
Remember a little bit of rose rust will not seriously affect the health of the plant. Roses will tolerate leaf spots as long as green leaf tissue remains. Stem infections should be taken more seriously as they can affect a larger portion of the plant. In both cases use of good sanitation and moisture reducing practices are often enough to successfully manage the disease.
Photo 4: Black pustules of teliospores on last year's infected stems plus newly infected leaves. Michelle Grabowski