Diseases and Weeds

Weed of the Month: Ragweed Disease of the Month: Aster Yellows

Weed of the Month: Ragweed

It is the time of year again when both common ragweed (Ambrosia artemisiifolia) and giant ragweed (A. trifida) bloom. Usually confused with goldenrod, ragweed sends thousands of pollen grains into the wind, hoping to fertilize neighboring plants and causing misery to humans with allergies. This is a common weed of the roadside and ditch, but can easily sneak into planting beds, lawns, and natural areas.

Common and giant ragweed both grow as erect, summer annuals with primarily fibrous roots. Common ragweed typically grows one to four feet tall. Giant ragweed grows four to eight feet tall, but in fertile, moist soils, it can reach 12 to 18 feet. The leaves of common ragweed are mostly alternate, nearly smooth, fernlike, and deeply cut into a number of lobes. The leaves of giant ragweed are opposite, rough, hairy, and large with three to five lobes.Both plants have small, green, inconspicuous flowers that produce an abundance of pollen that is hazardous to hay fever sufferers. Treatment:

For lawns, ragweed can be prevented with proper practices including mowing high to shade out weeds as well as fertilizing and watering at appropriate times. For landscape beds, a thick mulch can be used to prevent seeds from germinating. Preemergent herbicides such as isoxaben can be used in the spring. Plants can be removed by hand or with a postemergent broadleaf herbicide such as 2,4-D or a 3-way product in lawns. For best success, apply to young plants. For spot applications, glyphosate could be used, however, it is suspected there are glyphosate resistant populations in Illinois so adequate control may not be achieved.

Pest of the Month: Aster Yellows

Aster yellows are commonly found on members of the aster family, Asteraceae, especially coneflowers (Echinacea spp.). It does pop up on other plants like carrots and onions, as well. The classic symptoms are distorted flowers, most often with green petals instead of the typical bright colors, although plants may also be stunted. Eriophyid mites may also cause distorted flowers, however, the petals do not tend to be different colors. Aster yellows is caused by a phytoplasma disease spread by leafhoppers, a sucking insect that hops from plant to plant.

Treatment:

There is no cure or treatment for aster yellows. Infected plants should be removed and discarded to keep the disease from spreading. Deadheading the flowers will not stop the spread. You may also try weed control near planting beds as the leafhoppers may feed on many weeds such as dandelions and plantain. You may also spray leafhoppers regularly with an insecticidal soap or horticultural oil in a commercial planting, but they are tricky and difficult to control.

Additional resources:

University of Illinois Extension Service https://web.extension.illinois.edu/state/ horticulture/index.php 217-333-0519 The Morton Arboretum http://www.mortonarb.org/Plant Clinic: http://www.mortonarb.org/trees-plants/ tree-and-plant-advice/ 630-719-2424 Chicago Botanic Garden Plant Information Service: https://www.chicagobotanic.org/ plantinfoservice 847-835-0972