No. 7/July 15, 2019

Fall Webworm
Fall webworms (Hyphantria cunea) are communal caterpillars that spin silk into a tent-like structure at the tip of branches.  These webs are often built around leaves they feed on.  Fall webworms feeds on a wide range of deciduous trees and shrubs including, but not limited to, crabapple, redbud, sweet gum, maple and oak.  As the caterpillars consume the leaves within the web, they will expand the web every week or so to include more leaves.  The web of mature caterpillars can be 2 to 3 feet long.

Marestail (Horseweed) in the Landscape
Horseweed or marestail (Conyza canadensis) is a fairly common weed in Illinois for corn and soybean fields, nurseries, and orchards.  It is less commonly thought of as a landscape weed, but it can find its way to ornamental plantings and control can be challenging. Both names are commonly used throughout the state.
This plant is a bit peculiar considering its life cycle.  It can grow as a winter annual or sometimes as a summer annual. Seeds germinate in late summer or early spring.  The seedlings develop a rosette of leaves, which can be easily confused with other rosette forming weeds such as Virginia pepperweed and shepherd’s-purse. From the rosette of leaves, plants will then “bolt” and produce a flowering stem.  Stems can reach 7 feet in height.  The rosette may go entirely unnoticed in a bed due to it’s low stature or the fact that once the stem forms, the basal leaves will deteriorate and disappear altogether with time.  The tall stems make marestail more noticeable.
An easy way to describe a mature marestail or horseweed is that of a single stem with many dark green, whorled leaves.  The stems is hairy with many small, flowering branches toward the top. I suppose it could look like the tail of a horse, given the name.  This weed should not be confused with horsetail (Equisetum arvense), which is an entirely different weed.

Ramorum Blight Confirmed on Samples Taken from Illinois Garden Centers
The Illinois Department of Agriculture published a press release July 2 announcing that Phytophthora ramorum, causal agent of Ramorum blight and Sudden Oak Death, had been confirmed on samples taken from garden centers in the state.:

Powdery Mildews on Ornamentals
Much of Illinois has settled into a hot and humid weather pattern conducive to powdery mildew infections. The six common genera of powdery mildew fungi in the Midwest all prefer warm, humid days. The spores germinate on foliage when the relative humidity is 23% to 99% but not in free moisture (rain). Powdery mildew is a common fungal disease of many annuals, perennials,  shrubs, and trees. Some species of powdery mildew fungi infect only a few closely related host plants, while others attack many genera of plants. This disease can spread quickly over a host plant; it doesn’t normally kill one.

Slime Molds
Warm, wet weather has stimulated the development of slime molds on mulch, turfgrass, and other surfaces. Slime molds can appear in many colors including white, gray, yellow, violet, blue, green, purple, brown and black. Although they can be quite alarming, slime molds do not infect or harm plants. They mostly feed on microorganisms and other decaying organic matter, while utilizing plants and other structures for support.