If you find a plant with strange growth in your garden, it may well have aster yellows disease. This disease was first identified on aster flowers many years ago. The responsible organism is a minute, virus-like particle of genetic material called a phytoplasma, primarily transmitted by leafhoppers. When a leafhopper feeds on an infected plant, it sucks up phytoplasma particles and its salivary glands become “infected”. When feeding on healthy plants, 10-14 days after initial infection of the insect, the leafhopper injects phytoplasma into the phloem and disease symptoms will appear in the plant 10-40 days later. There is no cure for aster yellows and infected plants should be removed as they become reservoirs of infection, capable of increasing the amount of disease present.
The disease has an extremely wide host range including many ornamentals such as aster, delphinium, coneflower, calendula, zinnia, chrysanthemum, petunia and snapdragon. Vegetables and herbs such as lettuce, carrot, pumpkin, tomato, celery and coriander can also become infected.
Disease symptoms can be very variable depending on the host plant. In many cases, plants become stunted and very yellow-looking (chlorotic), but occasionally the plants are taller than normal and stand out from the rest of a crop of healthy plants, as is the case in canola and coriander fields. Another obvious symptom is flowers do not develop normal petals. Instead, the flowers typically look like a cluster of abnormal, yellowish-green, bladder-like leaves. This can be seen in delphinium, echinacea, calendula, aster, and even dandelion.
Carrots are a common host if the leafhopper population is high. The first sign of carrot infection is bronzing of the leaves followed by a dense growth of small, chlorotic leaves at the top of the carrot. When the carrots are pulled up, a mass of small roots has usually developed on the main carrot root, and the carrot will have a bitter flavour. Disease symptoms may differ depending upon what host species is infected. For example, while carrot roots may be bitter and hairy, lettuce may show pink or tan spots and have twisted inner leaves. Several years ago, there was a very high population of leafhoppers, and a pumpkin patch at the University of Saskatchewan was affected. The infected pumpkin plants were very obvious as they were extremely chlorotic, the leaves were abnormally shaped and no flowers developed. Obviously, this had a severe impact on yield, as none of the infected plants could produce pumpkins.
Occasionally, similar symptoms can be caused by infection with other viruses, or accidental exposure to herbicide. In the latter case, plants sometimes recover and symptoms disappear. This does not happen with aster yellows infection.
Management strategies to help control the disease:
1. Remove isolated specimens of infected plants. Early diagnosis and prompt removal of infected specimens help reduce the spread of the disease. Also, infected vegetables can be harvested early, which is helpful in reducing the reservoir of infection.
2. Reduce leafhopper numbers. This can be difficult, however. Susceptible crops, or individual plants, can be covered with fine mesh cloth to keep leafhoppers away from the plants.
3. Remove weeds that may provide an early source of infection for feeding leafhoppers. Dandelions can survive our winters even when infected, and leafhoppers feeding in the spring can pick up the phytoplasma and transmit it to healthy garden plants.
4. Plant less susceptible species such as geranium, impatiens, nicotiana, salvia and verbena. This is helpful if you are having difficulty with aster yellows infection in the flower bed.
Jill Thomson is a plant disease specialist (retired) who enjoys gardening in Saskatoon with her family, including the grandchildren and dogs.
This column is provided courtesy of the Saskatchewan Perennial Society (SPS; saskperennial@hotmail.com). Check our website (www.saskperennial.ca) or Facebook page (www.facebook.com/saskperennial) for a list of upcoming gardening events.
