As the summer winds down, Minnesota gardeners look to fall blooming asters like goldenrod and New England aster to bring color to the garden. In addition to colorful blossoms, less desirable colorful rust fungi can commonly be found infecting the leaves of these perennials. Many gardeners first notice rust infection when the lower leaves of an aster plant turn brown and die. In severe cases, over 50% of the leaves can be killed, often from the bottom up. Upon closer examination, a gardener will notice bright orange or chocolate brown bumps on the lower surface of green leaves and along green stems. These rust pustules are filled with hundreds of fungal spores.
There are several different rust fungi that infect asters in Minnesota. Infection by Coleosporium asterum results in yellow leaf spots on the upper leaf surface and raised orange spore filled pustules on the lower leaf surface of New England aster and golden rod. Like many rust fungi, C. asterum needs two different host plants to complete it's life cycle. In addition to infecting asters, C. asterum also infects the needles of red, Scots and jack pines. Infected pine needles have small white column-like spore producing structures that release powdery orange spores in early summer. These spores are carried by wind and infect near by asters. Spores produced within aster leaf spots throughout the summer reinfect the aster plant and any nearby asters. In fall, however a different spore type is formed on the aster that is carried by wind to infect nearby pine trees.
If leaf death is not severe, rust can be tolerated on asters. Infection by rust fungi often results in little to no affect on plant growth or blossom production. To reduce the severity of the disease, gardeners should take steps to reduce moisture on the foliage. Dense beds should be thinned and overgrown plants divided. Water with drip irrigation or use sprinklers early on a sunny day so that leaves dry quickly. Mulch the soil with woods chips or other organic matter to keep soil moisture from evaporating and increasing humidity in the plant canopy. Plants with a history of infection can be scouted regularly throughout the summer. As rust infection develops on a few leaves, theses leaves can be pinched off and buried to reduce spread of the pathogen.