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Rust in the flower garden

M. Grabowski

Top five things to know about rust on plants

severly infected plant with dead leaves at the bottom, spotted leaves in the middle, andunharmed leaves on top

M. Grabowski, UMN Extension

Rust infection of asters often results in death of the lowest leaves and yellow to orange spots on higher leaves

Importance

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.

Monarda plants with missing petals and yellow rust spots

M. Grabowski, UMN Extension

Rust-infected Monarda plants have yellow leaf spots on the lower leaves

Close up of a Monarda leaf with orange spores on it

M. Grabowski, UMN Extension

Powdery orange spores of the rust fungi can be seen on the lower leaf surface of a Monarda plant

Susceptible plants

Many commonly grown annual and perennial flowering plants, as well as ornamental grasses, can be infected by rust fungi. Each species of rust fungi is only able to infect a limited group of plants. Some infect only one species of plant. Others are able to infect several plants within the same family.

Some rust fungi have an ‘alternate host’. These rust fungi spend half of their life on one group of plants and the other half of their life on a completely different group of plants. A common example is cedar apple rust, which spends half of its lifecycle infecting Junipers (Juniperus spp.) and the other half infecting apple or crabapple trees (Malus spp.).

Other rust fungi have no alternate host and their entire lifecycle occurs on one plant or plant family. Hollyhock rust infects only hollyhocks and weeds in Malvaceae, the hollyhock family.

Identification

sunflower plant infected with rust spots on leaves

M. Grabowski, UMN Extension

Rust on sunflower can be seen on leaves, sepals and other green plant parts.

small, brown rust spots on a leaf

M. Grabowski, UMN Extension

Powdery brown spores of sunflower rust can easily be seen by rubbing the leaf with a tissue

Biology

Rust is caused by a group of related fungi in the order Uredinales.

Rust fungi have complicated lifecycles that include five different types of spores and sometimes two completely different host plants. Rust fungi can only survive by feeding off of live plant tissue. Rust fungi do not intentionally kill a plant, but infected plants may have reduced vigor due to the rust fungi’s ability to divert plant resources for its own needs. Spores of rust fungi are easily spread by the wind and occasionally splashing water. They need a film of water on the foliage from rain, dew, or irrigation to germinate and start a new infection.

Rust fungi can be introduced to the garden on infected plants or as windblown spores from nearby infected plants. Weeds from the same family as the garden plant can become infected and serve as a location for the rust fungi to survive and reproduce. At the end of the growing season, rust fungi can produce a thick-walled resting spore that allows it to survive through the winter. These are formed on the edges or within leaf spots or stem infections and persist from one season to the next in plant debris. Some rust fungi survive by infecting perennial plant parts like branches or crowns.

Management

Caution: Read all label directions completely before buying and applying fungicides. Follow all instructions. The label is the final authority for use of the product.

2017

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