“Rusty” Plants: How Fungi and Weather Cause Gardeners Problems
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Warm, wet weather is welcome in the summertime for gardeners. It helps plants grow more lush and means you don’t have to drag the hoses across the yard (today, anyway!). But, humid, warm weather also helps plant pathogens grow. Even the healthiest of plants can’t always fight off infection.
Why are my plants “rusty”?
Rust fungi are some of the most common and most striking pathogens, and they can also be devastating to a plant’s performance. Most rust fungi will not kill their host plants, as they need them to survive so they can survive, but they can weaken a plant to the point of it not being functional in the landscape anymore.
It is sometimes hard to know your plant is infected with the fungus until the rust shows. A plant will look healthy one day and within a couple days will look like someone dusted it with traffic-cone orange dust or will have a globe of orange strings attached to a branch or fruit. This orange “rust” is the fungus’ reproductive parts, similar to flowers on a plant! The appearance of the fungus corresponds with favorable weather for it to spread its spores!
Most species require two separate host plants to complete their life cycle, meaning they will live on or in one species as one stage of their life and then on another completely different species for another stage of their life. Most of these species pairs are made of an evergreen conifer, like a pine tree or a red cedar, and an herbaceous perennial or deciduous tree, like an apple.
Cedar-Apple or Hawthorn Rusts, Gymnosporangium ssp.
These are some of the most striking fungi of the rusts. We usually get frantic phone calls in the spring and summer as people observe the brown, golf-ball sized gauls covered in bright orange tentacles on their red cedar trees. These are actually fungal growths that the fungus uses to spread through the air. At this stage, it is the teliospores that are produced on the orange tentacles, called gelatinous telial horns. These spores fly through the air and germinate to form the next part of the fungus’ life cycles.
All together there are four different types of spores: basidiospores, teliospores, spermatia, and aeciospores. What folks might not realize is that this is the same
fungus that will infect and ruin apples, quince fruits and serviceberries. What looks like a beautiful, ripening fruit will eventually turn to a ball of orange tentacles as well, showing the fungus has already infected the fruit.
Removing cedar trees within 4-5 miles can break the cycle, and 2-3 miles can make fungicide sprays more effective. It is probably more feasible to use a fungicide spray program to treat fruit at critical times in the spring and summer.
Turf Rust, Puccinia spp.
Turf care takes intentional maintenance or you run the risk of stressed turf, and stressed turf means susceptible turf. Turfgrasses, especially broadleafed species, are susceptible to several different rusts in the genus Puccinia. The rust will not usually kill the turf, but will weaken the plants. The spores can travel on the wind, through irrigation splashing, and on shoes! The keys to reducing the pathogen are mowing at the appropriate height, making sure there is good airflow across the turf, and the appropriate amount of nitrogen is applied according to a soil test. There are effective fungicides for the fungi, as well, but applying them at the right time for control is critical.
Bluestar, or Amsonia, Rust, Coleosporium apocynaceum
Bluestars, or Amsonia ssp., are beautiful herbaceous perennials that are becoming more and more popular as native plants and pollinator gardening are coming in vogue. The stems emerge from the ground in the spring and emit a spray of light blue flowers. The green foliage comes in a variety of textures, and fades into a beautiful, golden yellow in the fall. Unfortunately, this fall color can seemingly come early as the leaves become infected with rust. The alternate host of most of the species that infect bluestars harbor over in pine trees, like the loblolly pine, unfortunately. Scouting for rust and applying fungicides at the appropriate time can keep this beautiful plant in your garden!
Please contact your local N.C. Cooperative Extension office if you would like to learn more about rusts and how to select appropriate fungicides for your plants.
Amanda Wilkins is the Horticulture Agent for North Carolina Cooperative Extension in Lee County.
Cedar Apple Rust (Gymnosporangium juniperi-virginianae Schwein.)- US Forest Service
Rust in Turf, NC State TurfFiles
First Report of Rust Disease Caused by Coleosporium apocynaceum on Amsonia ‘Blue Ice’ in Indiana- M. Abbasi, M. C. Aime, T. C. Creswell, and G. E. Ruhl
Amsonia Rust on Extension.org
Coleosporium Rusts, Angelica Werth
Science Direct’s Profile on Coleosporium