Plant Health Care Report August 10, 2012
Aug 10, 2012 Issue 2012.16
Our report includes up-to-date disease and insect pest reports, as well as color images, for northeastern Illinois. You'll also find a table of accumulated growing degree days throughout Illinois, precipitation, and plant phenology indicators to help predict pest emergence. The report is published bi-weekly on Fridays in April and August, and weekly May-July.
Arboretum staff and volunteers will be scouting for insects and diseases throughout the season. We will also be including information about other pest and disease problems based on samples brought into the Arboretum's Plant Clinic from homeowners and professionals.
Our next issue will be August 24 and
it will be the last issue of the season.
What’s in bloom at the Arboretum?
Swamp rose mallow (Hibiscus palustris) (figure 1) is in full bloom.
Accumulated Growing Degree Days (Base 50): 2365.5 (as of Aug 9)
Accumulated Growing Degree Days (Base 30): 5497.5 (as of Aug 9)
- Anthracnose on lily-of-the-valley
- Verticillium wilt
- Rhizosphaera needle cast revisited
- Bacterial leaf scorch
- Beneficial nematodes for Japanese beetles
- Webbing on shrubs
Degree Days and Weather Information
As of Aug 9, we are at 2365.5 base-50 growing degree days (GDD), which is 395.5 GDD 50 ahead of 2011 at this time. We have had .6 inches of rain so far in Aug and 14.81" for the year (compared to 29.19" last year at this time).
|Location||B50 Growing Degree Days
Through Aug 9, 2012
|Chicago Botanic Garden**||2384 (as of 8/6)||.39 (7/31-8/6)|
|Crystal Lake, IL*||2568||--|
|The Morton Arboretum||2365.5||
|Northbrook**||2578 (as of 8/8)||.07 (8/1-7)|
**Thank you to Mike Brouillard, Northbrook Park District and Mike Annes, Chicago Botanic Garden, for supplying us with this information.
*We obtain most of our degree day information from the GDD Tracker from Michigan State University web site. For additional locations and daily degree days, go to http://www.gddtracker.net/
Pest update: insects
|Figure 2 Fall webworm larva|
Fall webworm (Hyphantria cunea) caterpillars have been found on the Arboretum grounds this week. This caterpillar is known to feed on more than 100 species of deciduous trees. Preferred hosts include hickory, ash, birch, black walnut, crabapple, elm, maple, oak, and pecan. The caterpillars are pale green to yellow, with black spots, and covered with long, silky white hairs (figure 2) . There are two races, black-headed and red-headed. The black-headed webworms are supposed to appear about a month earlier than the red-headed race. Full-grown caterpillars reach about one inch in length.
Fall webworms overwinter in the pupal stage in the ground, under loose bark, and in leaf litter. Adult moths appear from late May through August, and females deposit eggs in hair-covered masses on the underside of host leaves. Eggs hatch into caterpillars in about one week and begin to spin a messy web over the foliage on which they feed (figure 3). The webs increase in size as caterpillars continue to feed. In about six weeks caterpillars will drop to the ground and pupate. Damage is generally aesthetic since this pest usually eats leaves late in the season and webs are typically concentrated to limited areas.
|Figure 3 Fall webworm nest|
Some people confuse fall webworm and eastern tent caterpillar. How can you tell the difference? Eastern tent caterpillars are spring caterpillars and form thick neat tents in the angles of branches. Fall webworm caterpillars are active much later in the season and make a messy web at the ends of the branches. Eastern tent caterpillars go outside the tent to feed and return to the tent at night. Fall webworm feeds in the nest and expands the nest to enclose more leaves to feed on.
Management: Insecticides generally are not warranted. The unsightly webs can be pruned out of small trees. Since these caterpillars stay in the web while feeding, pruning the webs at any time of day will eliminate the caterpillars. Webworms also have many natural enemies including birds, predaceous bugs, and parasitic wasps.
Good web sites:
The Plant Clinic has received some reports of an interesting 'bee' visiting local flowers. Turns out it's not a bee at all, but a tachinid fly. There are several species of tachinid fly, but the one that is being seen regularly has a gray to silver thorax (the front of the body) and a black abdomen covered with stiff hairs (figure 2). The eyes are large and prominent.
Tachinid flies are beneficial insects for two reasons. First, the adult flies are pollinators and we
|Figure 4 Tachinid fly (photo credit: Sharon Yiesla)|
can always use more pollinators. Second, the larvae are parasitic on other insects, attacking many that we consider garden pests. One species is even parasitic on Japanese beetles, though they don't do enough damage to control this annoying insect.
How does the tachinid fly parasitize another insect? Basically while predators hunt and consume their prey, parasites use their host as a place to raise their young. The adult fly may lay eggs on leaves where they will be consumed by the host insect. The eggs hatch inside the host and the larvae will feed on the host's body. Eggs may also be laid directly on or in the host. They may be 'glued' on, usually behind the head where the host can't get to them to knock them off, or the adult fly can insert them into the host body with a sharp appendage called an ovipositor. Again, the larvae will feed on the inside of the host, eventually killing it as the larvae develop and mature.
A note of caution: while these flies will parasitize many garden pests, they do attack many caterpillars and have been known to parasitize some of the good guys, like monarch caterpillars.
Good websites: http://www.entomology.wisc.edu/mbcn/kyf409.html
|Figure 5 Ailanthus webworm larvas|
Ailanthus webworm (Atteva aurea) caterpillars (figure 3) were found by our scouts feeding on tree-of-heaven (Ailanthus altissima). An adult moth (figure 4) was also submitted to the Plant Clinic last week. The caterpillars have sparse light hairs, a broad stripe down their backs that has been described as olive-green, and alternating black and white stripes along their sides. They cluster together in a loose web and feed on leaves from within the web. This insect is usually seen on tree-of-heaven often considered a weedy tree. The adult moth has bright orange bands that alternate with bands of black with white dots. It rolls its wings around its body, giving it a long, slender
|Figure 6 Ailanthus webworm moth|
shape. The moth is actually a good pollinator that may be found on a variety of flowers.
Management: Considering that this insect feeds on a tree that has become pest, management really isn't needed. If, by some chance, you own a tree-of-heaven and really love it(?), you can cut the webs out of the tree.
Good websites: http://www.uark.edu/ua/arthmuse/ailamoth.html
Pest updates: diseases
Anthracnose on lily-of-the-valley
Anthracnose (Ascochyta majalis) is a common fungal disease on lily-of-the-valley
|Figure 7 Anthracnose on lily-of-the-valley|
(Convallaria majalis). It is showing up on plants in our ground cover garden. On lily-of-the-valley, it causes circular to oval, brown spots with purplish red margins (figure 5). The spots are one-half inch in diameter or larger. Diseased tissue drops out and the foliage dies prematurely. It does not kill the plants but does weaken them. As a result of infection, there may be fewer flowers next year.
Management: Destroy diseased foliage in the fall and remove diseased plants when seen.
|Figure 8 Streaking under bark|
Verticillium wilt has been confirmed on smoke bush (Cotinus coggygria) in the last two weeks. Verticillium wilt is a fungal disease that affects over 300 herbaceous and woody plants. The disease is caused by many host-specific strains of two soil-borne fungi, Verticilliumalbo-atrum and V. dahliae. Verticillium dahliae is believed to be the predominant species attacking trees in the Midwest. The disease attacks many herbaceous plants as well as woody plants and has both acute and chronic (long-term) modes of action.
This is a soil-borne disease in which overwintering microsclerotia (tiny masses of fungus that are very good at surviving under unfavorable conditions) remain in the soil in a type of dormancy until unsuspecting roots cross their path. The fungus enters the root through wounds or direct penetration. Once inside a root, the fungus colonizes water-conducting tissue (xylem) and gradually spreads upward through the plant. The fungus produces toxins that cause the plant to block off the xylem in an attempt to limit the growth of the fungus. This cuts off the flow of water which results in leaf wilting, yellowing and browning, branch dieback, and often plant death. The wood beneath the bark is streaked in many species because of the "plugging" response. Typically when a wilting branch is cut in cross section, or the bark peeled back, brown streaks can be seen in the outer ring of sapwood (figure 6). Some plants, like ash and Japanese tree lilac, will not show streaking.
Verticillium can be a chronic problem, that is, killing a branch or two annually, or it can kill the whole plant in one season. Chronic (long-term) symptoms may also appear such as: stunted, chlorotic, and deformed foliage; slow growth; and abnormal seed production.
Verticillium can be spread to new plants and soils by seeds, tools, irrigation furrows, and in the soil and roots of new transplants and nursery stock. Once the fungus is introduced into soil it can survive for several years as microscopic microsclerotia, even in really bad conditions. This is one tough fungus.
Symptoms are not enough to determine that a plant is infected with Verticillium. A culture lab should be used to verify the diagnosis. In case the tree dies and needs to be replaced, you want to replace it with a tree resistant to the Verticillium fungus.
Management: Verticillium wilt is difficult to control because of the pathogen's ability to hunker down and survive in the soil with or without a host plant. Fungicides are ineffective in controlling Verticillium. The best course of action is sanitation and prevention. Dead branches should be pruned out as they occur to help overall plant vigor. Because the disease can be transmitted via sap, sterilize pruning tools between cuts. Remove chronically infected trees.
Other control measures:
• Start with clean plant materials and soil.
• Plant trees in sites that are appropriate for the plant.
• Water during dry periods, but do not overwater.
• Use a three to four inch layer of organic mulch to retain moisture and prevent soil temperature fluctuation.
• Do not over-fertilize. Maintain a balanced fertility. Unbalanced nitrogen (too high or too low), too low potassium and too low phosphorous can lead to more disease.
• Avoid injuries to the roots, trunk, and branches.
• Plant resistant varieties.
• Remove severely infected trees and replace with plants that are not susceptible to Verticillium.
Rhizosphaera needle cast revisited
Way back in issue 1 we talked about Rhizosphaera needle cast because we often see
|Figure 9 Fruiting bodies erupting on spruce needle|
symptoms in the spring from infections that occurred the previous spring. Sometimes those symptoms will show up in late summer and we need to be looking carefully to see if symptoms seen now are those of Rhizosphaera or are due to drought. With Rhizosphaera, infected needles initially turn yellow, and small dot-like fruiting bodies (pycnidia) can be seen (with a hand lens) erupting through the stomata (figure 7). Later, the needles turn purple to brown and begin to drop. Although trees are not usually killed by this pathogen, branches which lose needles for three to four consecutive years may die. If left unchecked, the disease can turn the tree into an undesirable landscape specimen in two to three years.
Bacterial leaf scorch
Bacterial leaf scorch is a disease that occurs on many tree species including elm, oak, maple, mulberry, and sycamore caused by the bacterium Xylella fastidiosa. Trees infected with X. fastidiosa exhibit marginal leaf browning, often bordered by a pale halo. Leaf discoloration begins at the leaf margin and moves toward the midrib. Trees typically begin to show bacterial leaf scorch symptoms during early to mid summer. These symptoms will progressively worsen until leaf drop in the fall. Symptoms recur each year and spread over the tree's crown; thus, reduction in growth and dieback are common in affected trees. Bacterial leaf scorch spreads systemically and causes slow decline and death of a tree.
Management: Bacterial leaf scorch is transmitted by xylem-sap feeding insects such as spittlebugs and leafhoppers. Avoid wounding susceptible trees to prevent infection by these vectors. Infected trees should be removed to prevent spread to healthy trees. There is no known effective preventative treatment or cure for bacterial leaf scorch. Trunk injections with antibiotics have been shown to suppress symptoms and delay tree death. Pruning infected branches has met with limited success.
Beneficial nematodes for Japanese beetles
Beneficial nematodes are one of the tools in the battle of the Japanese beetles. These microscopic roundworm-like organisms are targeted to attack the grub stage of the Japanese beetle. This is a legitimate technique, but our weather this year may make it hard to implement. The nematodes are applied to the soil from July through September when the grubs are actively feeding on the roots of the lawn. The soil needs to be moist, which is not easy this year. The lawn should be irrigated before application and should be kept moist after the application. These are living organisms and they require moisture to live. Do not apply in high heat as the nematodes may die (soil temperatures should be between 70 and 86 degrees.) Some mail order suppliers will not ship the nematodes during times of high temperatures. The weather forecast is for more moderate temperatures and some rainfall, so we may be able to use this product soon.
Nematodes are not an insecticide, but a biological control. The nematodes will enter the body of the grub, carrying bacteria. The bacteria kill the grub and the nematodes feed on the bacteria and the grub body, which liquefies. More nematodes are produced and when the grub body decays, they are released into the soil to attack more grubs.
Webbing on shrubs
This goes under the "oddity" category. The webbing that is showing up on shrubs (and sometimes lawns as well) is simply spiders at work. We tend to notice this webbing in times of drought, because there is no rain to wash the webbing away. When we are having normal rainfall, the spiders often have to rebuild webs on a regular basis. The lack of rain just makes the webs last longer, so they get noticed more. Most spiders are harmless to plants, but will eat plant pests, so leave them alone.
The Plant Health Care Report is prepared by Sharon Yiesla, M.S., Plant Clinic Assistant and edited by Fredric Miller, Ph.D., research entomologist at The Morton Arboretum and professor at Joliet Junior College; Doris Taylor, Plant Information Specialist, and Carol Belshaw, an Arboretum Volunteer. The information presented is believed to be accurate, but the authors provide no guarantee and will not be held liable for consequences of actions taken based on the information.
Thank you...I would like to thank the volunteers that scout for us and who found most of the insects and diseases that are in this report. The Scouting Volunteers include: Davida Kalina, Ann Klinglele, LeeAnn Cosper, Bill Sheahan, Kathy Stephens, and Loraine Miranda. Your hard work is appreciated. Thanks also to Donna Danielson, who teaches at The Morton Arboretum and shares her findings when she scouts for her classes.
Indicator plants are chosen because of work done by Donald A. Orton, which is published in the book Coincide, The Orton System of Pest and Disease Management. This book may be purchased through the publisher at: http://www.laborofloveconservatory.com/
The 2010 Commercial Landscape & Turfgrass Pest Management Handbook, for commercial applicators, and the 2012 Pest Management for the Home Landscape for homeowners from the University of Illinois, are available by calling (800-345-6087).
This report is available on-line at The Morton Arboretum website at http://www.mortonarb.org/tree-plant-advice.html
Copyright © 2012 The Morton Arboretum
Not printed on recycled paper, or any paper for that matter.
- Verticillium Wilt 11%
Verticillium wilt is a serious fungal disease that causes injury or death to many plants, including trees, shrubs, ground covers, vines, fruits and vegetables, and herbaceous ornamentals. It is a disease of the xylem, or water-conducting tissues, in...
- Anthracnose of Shade Trees 10%
One of the most common and unsightly diseases of shade trees is anthracnose. Anthracnose is a foliar disease caused by several species of fungi whose spores, when released, infect newly emerging leaves. The disease, which becomes active in spring,...