No. 7/July 22, 2022

Twospotted spider mites
Twospotted spider mites (Tetranychus urticae) are herbivorous mites that feed on a broad range of deciduous trees and broadleaf evergreens. In cool or damp weather their populations are controlled naturally by fungi and other pathogens. However, their populations flourish in hot, dry conditions.

Common Burdock
Common burdock (Arctium minus) is a biennial, broadleaf weed that reproduces by seed. It forms a rosette with a large, fleshy taproot in its first year. In its second year, the stems grow erect reaching 3 to 5 feet in height. They can be reddish in color. The stems are much branched and hairy. The basal leaves are large at 15 by 20 inches. They are hairy and roughly heart-shaped with a hollow petiole. The underside of the leaf is light green and woolly, while the upper side is darker and smoother. The stem leaves are smaller, alternately arranged, and egg-shaped. While considered a biennial, blooming may not occur until the third or fourth year. The flower heads are produced from July to October and look like that of some thistles. Flowers are composed of small red-violet disk flowers, which dry down to 1/2-inch burs. The outer bracts have velcro-like hooks that readily catch on clothing and animal fur. This allows for widespread seed dispersal. This plant only reproduces by seed, and one can produce about 15,000 seeds. Dead stems will persist into the following spring.

Botryosphaeria Twig Canker and Dieback on Oak
The U of I Plant Clinic receives numerous oaks samples each summer. Many of these samples were sent to the clinic by clients concerned about oak wilt. Fortunately, only a small portion of the samples turn out to be infected with the disease. The remaining samples often have other pest or disease problems with symptoms resembling oak wilt.

Effect of Water pH on Pesticides
Resistance is often blamed for the failure of an insecticide or miticide to manage a given pest or pests. However, another possible reason for lack of control may have to do with the pH of the spray solution. pH is a measurement of the concentration of hydrogen ions [H+] in a solution. It is a scale indicating the degree of acidic and basic properties of water. The pH scale ranges from 0 to 14. A pH value below 7 is acidic, whereas a pH value above 7 is basic or alkaline. A pH of 7 is considered neutral.

Weekly Plant Clinic Sample Summaries
While not a true pest survey, University of Illinois Plant Clinic sample summaries can provide a glimpse of pest problems occurring around the state. Diane Plewa and Chelsea Harbach recently started publishing weekly sample summaries on the pant clinic’s website. They’re available here: Each weekly summary categorizes the samples by ornamentals, fruit and vegetables, and field crops. Sample information includes host, diagnosis, pathogen/pest, and the county the sample originated from.