Sauk County Gardener: Aster yellows can devastate a garden
“Plant diseases are shifty enemies.” ~E.C. Stakman
A couple weeks ago, as I as working in my garden, I noticed an unusual looking coneflower. I suspected it might be aster yellows, but I didn’t have time to deal with it at the time, as was it right before we were heading to Missouri for a family reunion. After I returned and was working in my flower gardens to prepare the yard for our daughter’s bridal shower, I became very concerned as I found a couple more deformed coneflowers on some other plants in my garden. I also discovered some of my tall garden phlox had powdery mildew. These are just a couple of the diseases that can affect your summer plants and distract from the beauty of your garden.
Powdery mildew is primarily a cosmetic problem that can be managed, but aster yellows is a much more serious problem. Aster yellows is a viral-like disease caused by a phytoplasma (a bacterial parasite) that is passed from plant to plant by insects, such as the aster leafhopper when it sucks the sap from an infected plant and then passes it to a healthy plant. It can affect a large variety of vegetables, annuals, and perennials - over 300 plant species. Some vegetables that can be affected include carrots, lettuce, celery and tomatoes. Annuals include common favorites - zinnia, petunias, marigolds and snapdragons. Some perennials that can be affected are asters, chrysanthemums, black-eyed Susan and coneflowers. To make matters worse, common weeds like plantain and dandelion can serve as host plants and who doesn’t have those weeds in their garden? Now for the really bad news - once a plant gets aster yellows, there is no cure for it. The plant needs to be removed and disposed of in your trash, burned or buried deep in your compost so that insects can’t feed on the infected plants.
Aster yellows presents itself in a variety of ways depending on the plant species. The major symptom of aster yellows is chlorosis or the yellowing of the leaves while the veins remain green. Plant foliage may become curled. Flowers can be deformed and have unusual or deformed turfs of leaves from inside the center of the flower. They may also be small, remain green and not develop color on their petals. The flowers may fail to produce seeds as well. You may also see many thin, spindly stems growing closely together to form what looks like a witches’ broom.
To manage aster yellows, it’s important to completely remove the infected plants since there is no cure.
Early removal can help prevent aster yellows from spreading throughout your garden. Remove weeds that can be hosts for this disease, so it doesn’t survive from one season to the next. Light colored or reflective mulch can disorient insects and minimize feeding on plants. In the vegetable garden, you can use floating row covers to help prevent leafhoppers from feeding on your carrots, potatoes, and other plants. Also consider planting less susceptible plants such as verbena, geranium and impatiens.
Aster yellows on coneflowers can also resemble the damage caused by eriophyid mites. In the case of the eriophyid mites, the damage can sometimes just be cosmetic and do little damage to the health of plant. I also know that the spread of aster yellows is worse when we have a cool, wet summer and I don’t feel we’ve had that kind of weather. Finally, my coneflowers are not showing any signs of chlorosis. With these things in mind, I’m not able to make an accurate diagnosis, so I’m going to send a sample into the University of Wisconsin-Madison/Extension Plant Disease Diagnostics Clinic (PDDC). In some cases, the PDDC can make a diagnosis based on a picture. As the damage from aster yellows and eriophyid mites looks very similar on coneflowers, I’m going to send an actual plant sample as one diagnosis means. Depending on the diagnosis, I will have to remove and destroy a lot of plants and the other diagnosis means I might be able to control the mites with a miticide.
If you have a plant that you cannot diagnosis what the problem is, first consult your local Extension office horticulture expert. Your local rep may be able to diagnosis it after talking with you and/or by looking at a picture or plant sample. If they are unable to diagnosis the problem, they may direct you to submit a sample to the PDDC. It may mean the difference between living with a cosmetic problem or tearing out a large number of beautiful plants.
Jeannie Manis is a Wisconsin-certified Sauk County Master Gardener volunteer. If you have any gardening questions, please contact the Extension Sauk County by emailing to tim.ripp@saukcountywi.gov or calling the University of Wisconsin Madison Division of Extension Sauk County office at 608-355-3250.
