Plant Health Care Report April 20, 2012
Tagged as: Elm flea weevil, leaf rollers, rust on buckthorn, heavy seed crops, Cankerworm, European elm scale, Hemlock rust mites, honeylocust plant bug, Linden looper, Oak apple gall, Spruce spider mite, Taphrina deformans
April 20, 2012 Issue 2012.2
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.
If you prefer a PDF version of the PHCR, please click here to download and print.
Quick View
What indicator plants are in bloom at the Arboretum?
Horsechestnut (Aeculus hippocastanum -figure 1) is at the very beginning of its bloom
Wayfaring tree viburnum (Viburnum lantana), our indicator plant last issue, is in late stage bloom
Accumulated Growing Degree Days (Base 50): 266 (as of April 19)
Accumulated Growing Degree Days (Base 30): 1231 (as of April 19)
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| Figure 1 Aesculus hippocastanum |
Insects:
- Honeylocust plant bug
- Linden looper
- Cankerworm on elm
- Elm flea weevil
- Hemlock rust mite
- Oak apple gall
- Spruce spider mites
- Leafrollers
- European elm scale
Diseases:
- Rust on buckthorn
- Taphrina fungus
Miscellaneous:
- Heavy seed crops
Events of Interest
- Behind the Blooms: Magnolias
- Monitoring and Managing Insect Pests
Degree Days and Weather Information
As of April 5, we are at 266 base-50 growing degree days (GDD50), which is 216 GDD 50 ahead of 2011 at this time and ahead of the historical average (1937-2011) by 257 GDD50. (There was a typo in the last issue. We were actually ahead of the historic average by 225 GDD, not 145). Since April 6, it has rained 2.27 in., which brings us to 6.91 in. this month and 6.91 in. for the year.
| Location | B50 Growing Degree Days Through April 19, 2012 |
Precipitation (in) |
| Aurora, IL* | 304 | -- |
| Carbondale, IL* | 578 | -- |
|
Champaign, IL* |
416 | -- |
| Crystal Lake, IL* | 265 | -- |
| Harvard, IL* | 251 | -- |
| Kankakee, IL* | 358 | -- |
| The Morton Arboretum | 266 | 2.27 |
| Northbrook** | 302.5 (as of 4/18/12) | 1.46 (April 11-17) |
| O'Hare* | 282 | -- |
| Peoria, IL* | 441 | -- |
| Quincy, IL* | 485 | -- |
| Rockford, IL* | 281 | -- |
| Springfield, IL* | 489 | -- |
| Waukegan, IL* | 215 | -- |
**Thank you to Mike Brouillard, Northbrook Park District 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 Updates: Insects
Honeylocust plant bug
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| Figure 2 Honeylocust plant bug nymph |
Honeylocust plant bug (Diaphnocoris chlorionis) nymphs have been feeding on newly-emerging honeylocust leaves (Gleditsia triacanthos). This insect has appeared about six weeks earlier than last year. The easiest way to find these and other plant bugs is to shake a branch over a white piece of paper. When you see a tiny green insect crawling on the paper, look at it through your hand lens. Honeylocust plant bugs have a pair of four-segmented antennae, although the antennae are probably too small at this stage to see without a microscope. Older nymphs also have yellow spots on their backs (Figure 2). This plant bug overwinters as an egg under the bark of two- and three-year-old twigs. The eggs hatch soon after bud break, and the nymphs crawl to unfolding leaves to feed. About a month from now, the nymphs will become adults. Both nymphs and adults feed on foliage until early summer and can cause severe leaf distortion, dwarfed leaflets, chlorosis, and yellow-brown leaf spots. A heavy plant bug infestation may cause a failure to leaf out or premature leaf drop. In the past, our heavily infested trees were able to leaf out again.
Management: Young nymphs can be knocked off leaves of small trees by spraying them with a strong stream of water. In severe infestations, insecticidal soaps, summer oils, and insecticides may be warranted. For chemical recommendations, refer to the Commercial Landscape and Turfgrass Pest Management Handbook 2010 (CPM) for commercial applicators, or the Home, Yard and Garden Pest Guide 2008 (HYG) for homeowners. Resistant cultivars can be planted such as 'Skyline' and 'Shademaster'. In general, yellow-leaved cultivars are more susceptible to this pest.
Good website:
http://woodypests.cas.psu.edu/FactSheets/InsectFactSheets/html/Honeylocust.html
Linden looper
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| Figure 3 Linden looper |
Linden looper (Erannis tiliaria) larvae (Figure 3) began feeding on American linden (Tilia americana) last week. Feeding by young larvae at this time will cause small holes in expanding leaves. Older larvae consume the entire leaf, except midribs and major veins. Serious infestations may result in defoliation. The larvae have rusty-brown heads and yellow bodies, with thin, wavy black longitudinal lines on their backs. They reach 1.5 inches at maturity. Preferred hosts of the linden looper include maple, linden, oak, apple, birch, elm, hickory, crabapple, and hawthorn.
Management: Infestations are rarely severe so control is generally not warranted. For severe infestations, Bacillus thuringiensis var. kurstaki (Btk) is effective against young larvae and should be applied now.
Good web site: http://www.forestpests.org/hardwood/lindenlooper.html
Cankerworm on elm
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| Figure 4 Cankerworm |
Spring (Paleacrita vernata) and fall (Alsophila pometaria) cankerworms have been found on elm. Commonly known as the 'inchworm', cankerworms, (Figure 4) are in the same family as loopers and have a characteristic 'looping' form of movement.
The fall cankerworm caterpillar eggs are laid in late fall and winter. The spring cankerworm caterpillar eggs are laid in early spring. Both cankerworm eggs hatch at their host's budbreak. Full-grown cankerworms are about 2.54 cm (1 in) long and range in color from yellow-green to black. Cankerworms feed on the buds and new leaves of host trees in spring, eventually devouring all but the midrib of a leaf, and often defoliating an entire tree. Currently, small holes are being seen in the leaves. Trees suffering from a heavy defoliation will usually produce a second crop of leaves, but their overall vitality may be diminished. Cankerworms infest many deciduous trees and shrubs, but prefer elms (Ulmus) and apples (Malus).
Management: Light infestations are not harmful to tree health, and natural enemies such as flies, wasps, and birds help to control the cankerworm populations. Heavy infestations can be controlled with Bacillus thuringiensis var. kurstaki (Btk) or other insecticides. To obtain good results, Btk or insecticides should be applied when larvae or feeding damage is first noticed in the spring.
Good website:
Elm flea weevil
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| Figure 5 elm flea weevil adult |
Damage from the European elm flea weevil (Orchestes alni) (Figure 5) adults feeding on the undersides of leaves of Ulmus 'Morton' have been sighted at the Arboretum. This pest first appeared in Northern Illinois in 2003 and has caused significant foliage damage to Siberian elm and its hybrids, elm hybrids containing U. carpinifolia and Chinese elm (U. parvifolia) since then.
Adult feeding results in tiny shot holes in the leaves, and heavy feeding can cause newly expanding leaves to wither and turn brown. After feeding, the female weevil cuts a cavity into the leaf mid-vein and inserts an egg. The hatching larvae create blotch mines at the leaf tips. Larvae feed for about 2-3 weeks, and then pupate within the mined leaf. The significant feeding can reduce photosynthetic capacity of the tree, thereby impacting overall tree vitality.
Management: Insecticides are effective in controlling adults and could be applied now. Depending on how long the insecticide is effective, several applications may be needed. However, spraying a large elm may not be practical.
Good website: http://www.ipm.iastate.edu/ipm/hortnews/2011/7-13/elmfleaweevil.html
http://hyg.ipm.illinois.edu/article.php?id=184
Hemlock rust mite
Hemlock rust mites (Nalepella tsugifolia) (Figure 6), an eriophyid mite, was found last week on Eastern
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| Figure 6 Hemlock |
hemlock (Tsuga canadensis 'Von Helms Dwarf). Both adults and nymphs were identified. Rust mites suck the juices from conifer needles, causing the infested needles to turn yellow then brown. If the infestation is severe, mite-ridden needles will drop off the tree. The mites themselves are cigar-shaped, pale yellow, about the size of dust, and can only be viewed using a strong hand lens or dissecting scope. These cool season mites may also attack fir, yew, and spruce.
Management: We are unaware of any cultural controls. For severe infestations, insecticidal soaps, summer oils, or miticides should be applied after eggs hatch, which is usually when saucer magnolia is in the pink bud stage.
Good website: http://www.entomology.umn.edu/cues/Web/146HemlockRustMite.pdf
Oak apple gall
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| Figure 7 Oak apple gall |
There are numerous types and forms of oak leaf galls. We are now seeing 'spring' galls because they develop while leaves are expanding. Fall galls will begin to appear on oaks in mid-summer when leaves are fully expanded.
Oak apple galls (Figure 7), caused by cynipid wasps, were found last week on pin oak. When fully developed, the galls are globe shaped, 1-2 inches in diameter, and filled with a spongy mass, and they are usually found on the midribs of leaves. The adult cynipid wasp lays eggs in developing leaves which causes adjacent plant cells to grow and engulf the egg, thereby providing it with food and shelter.
Management: Leaf galls rarely affect tree health so control is not required.
Good web sites:
http://www.fs.fed.us/r8/foresthealth/pubs/oakpests/p34.html
http://www.maine.gov/agriculture/pesticides/gotpests/diseases/factsheets/galls-cornell.pdf
Spruce spider mites
Spruce spider mites (Oligonychus ununguis) were found on black spruce (Picea mariana)
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| Figure 8 Spruce spider mite damage |
about 10 days ago. Spider mites are very tiny (you need a hand lens to see them clearly) and have eight legs. Spider mites have needle-like mouth parts which they use to suck up cells. They can cause severe stippling of spruce needles (Figure 8). Badly infested needles appear bronze and fall off the tree. Spruce spider mites prefer cool temperatures in the 60s to low 70s F and become inactive during the hot summer months. This is unlike two-spotted spider mites that prefer warm weather. Damage from spruce spider mites often becomes visible later in the season after the mites are gone. In addition to spruce, arborvitae is a frequent host. Juniper, hemlock, pine, Douglas fir, Fraser fir, and larch can also be attacked by this pest.
Remember that not all spider mites are pests. Some mites are predacious mites, that is, they eat the bad spider mites. So, how can you tell the difference between the pests and the predators? Shake a branch vigorously over a blank, white piece of paper. If the tree has mites, you will see tiny dots running around on the paper. If you crush them with your finger, they will be either green or yellowish-orange. The green ones have been eating plants, but the yellowish orange ones have been eating other spider mites. Predaceous mites also move faster and generally have longer legs. Having a lot of predaceous mites reduces your need to use chemicals.
Management: There are many predators of spruce spider mites, including lady beetles (ladybugs). Sometimes a strong spray of water can blast spider mites off the tree. Applying insecticidal soap can be effective. Horticultural oils also kill mites, but will remove the blue color on blue spruce. Other chemicals are not warranted unless you have severe outbreaks.
Good Websites:
http://woodypests.cas.psu.edu/FactSheets/InsectFactSheets/html/Spruce_Spider_Mite.html
http://extension.psu.edu/ipm/program/christmas-tree/pest-fact-sheets/needle-discoloration-and-injury/spruce-spider-mite.pdf
Leafrollers
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| Figure 9 Leafroller damage |
There are about 200 species of leafrollers that attack ornamental plants. So far this year, we have found Malus leafroller on Malus coronaria. These green to yellow green caterpillars roll up leaves and feed from within the shelter of the rolled-up leaf (Figure 9).
Management: None required as leafrollers usually cause minimal damage.
Good website: http://urbanext.illinois.edu/hortanswers/detailproblem.cfm?PathogenID=89
European elm scale
A sample of European elm scale adults (Gossyparia spuria) on elm came into the Plant Clinic
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| Figure 10 European elm scale |
this week. These are soft scales, and since they are sap feeders, they produce honeydew, which attracts sooty mold. Very heavy infestations can cause leaf and twig dieback. Severely injured leaves remain on the tree all winter. Mature females prefer feeding in the forks of twigs.
Female scales are oval, red-brown, and surrounded by white, cottony fringe (Figure 10). They're only about 1/8 of an inch in diameter. They produce one generation per year. They lay eggs near the end of June into July. The eggs hatch within a few hours into bright yellow crawlers. Crawlers migrate to feeding sites along the midrib of the underside of leaves, where they will remain until the end of summer. In fall, crawlers return to limb or bark crevices to overwinter as immature females. They mature in late May, mate, and begin depositing eggs.
Management: Horticultural oil or insecticidal soap can be sprayed on crawlers. You can wrap double-sided tape on a branch to check for crawlers. Insecticidal soaps, summer oils, or insecticides should be applied when crawlers are present, which would normally be late June. With the weather being odd this year, be looking for crawlers 3-4 weeks earlier than normal.
Good website:
http://www.entomology.umn.edu/cues/Web/126EuropeanElmScale.pdf
Pest Updates: Diseases
Rust on buckthorn
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| Figure 11 Rust on buckthorn |
Let's start the season with a disease we can all love, crown rust on buckthorn (Rhamnuscathartica) caused by the fungus Puccinia coronata. In general, buckthorn is considered an invasive weed. The State of Illinois officially added it to the list of exotic weeds regulated by the Illinois Exotic Weed Act a few years ago. The act states that "it shall be unlawful for any person . . . to buy, sell, offer for sale, distribute or plant . . . exotic weeds without a permit issued by the Department of Natural Resources". So we can be happy to see that this plant is diseased. Unfortunately, rust is not fatal.
Symptoms of crown rust are bright orange swollen spots (aecia) on leaves and petioles (Figure 11). A number of susceptible grasses, including oats and rye, are the alternate hosts for this rust.
Management: None required as buckthorn is not a desirable plant in the landscape.
Good website:
http://www.extension.umn.edu/projects/yardandgarden/ygbriefs/p418buckthornrust.html
Taphrina fungus
Taphrina bullata was found recently on (Chinese pear) Pyrus pyrifolia (Figure 12).
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| Figure 12 Taphrina on pear |
Our pathologist, Stephanie Adams, tells us this is a little unusual, but probably not of major concern. It does, however, make us think about some other diseases caused by different species of Taphrina, peach leaf curl, plum pockets and oak leaf blister. These diseases tend to develop when we see cool, wet weather around the time of leaf expansion (leaves are expanding now and we are seeing cooler temperatures and some increase in moisture, so be watchful).
With peach leaf curl (caused by Taphrina deformans), young, succulent leaves become
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| Figure 13 Peach leaf curl |
puckered and deformed as they develop (Figure 13). The puckered areas turn yellow and then red. A white bloom appears on the deformed part of the leaf. Shortly after, the leaves turn yellow and fall off. Diseased twigs become swollen and stunted. Diseased fruits also become distorted and swollen with discolored areas on the skin. Peach leaf curl generally does not kill the tree, but annual infections may weaken a tree and predispose it to other problems.
Management: The fungus overwinters in buds. Fungicides are only effective when applied in fall after leaf drop or in spring before buds swell. Once the leaves have emerged, fungicides are no longer effective.
Good web sites:
http://www.agcom.purdue.edu/AgCom/Pubs/BP/BP-54.pdf
With oak leaf blister, caused by Taphrina caerulescens leaves develop wrinkled, raised,
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| Figure 14 Oak leaf blister |
pale whitish-yellow blisters on their upper surface (Figure 14) and corresponding pinkish-gray depressions on the lower leaf surface in spring and early summer. Blisters range from 1/10th of an inch to an inch in diameter. As they age and merge, the blisters turn reddish brown with pale yellow margins and the leaf may become distorted. Red oak (Quercus rubra) is the most susceptible species. Oak leaf blister, like other Taphrina diseases, usually develops only during cool, wet springs and is more homely than harmful to the oaks. Infected leaves become distorted and may prematurely drop. The disease usually dissipates during the summer.
Management: The fungus survives the winter on twigs and bud scales. On oak, leaf blister is more unsightly than harmful, so control is not a high priority.
Good web site:
http://www.ag.uiuc.edu/~vista/abstracts/a663.html
Miscellaneous:
Heavy seed crops
Silver maples and various elms have set very heavy seed crops this year due to good conditions at pollination time. In the last few days, we have noticed many of these seeds ripening and turning brown. Since the trees are not fully leafed out, the brown seeds are obvious and give the tree an overall brown look which may lead home gardeners to assume their tree is sick. Many of the maples and elms at the Arboretum have this look right now. Trees that are reported as looking unhealthy may just need a closer look to determine if the 'problem' is just ripening seeds, rather than a disease.
Events of interest
BEHIND THE BLOOMS: MAGNOLIAS
Did you enjoy the spectacular early magnolia bloom this spring? Do you have a magnolia tree, but not sure what it is? Discover the amazing world of magnolias and find new trees for your landscape! Join Kunso Kim, Head of Collections and Curator at the Arboretum, and other Arboretum experts, for a morning of magnolia excitement.
Learn about the magnolia's rich history and deep evolutionary roots as you view a presentation on stunning magnolia blooms and hybrids for the Midwestern landscape. Bring a small branch of a magnolia from your garden, and we will identify it! Plus, examine rare books and botanical prints from the special collections of the Sterling Morton Library, including Georg Dionysius Ehret's spectacular Magnolia and the very dramatic Magnolia campbellii found in Illustrations of Himalayan Plants.
After a break for refreshments and coffee, venture out on a hike to explore the specimens and seldom-seen species in the Arboretum's extensive magnolia collections, recently designated a significant collection by the North American Plant Collections Consortium. Hone your tree identification skills by examining form and foliage morphology of different magnolia species learn how to grow a wide range of common and unusual magnolias, including new hybrids and Asian magnolias, and find out what is being done to protect magnolia diversity today. Walk away with an expanded appreciation of magnolias and new ideas for how to add their large flowers, large leaves, elegant colors and exotic scents to any garden.
Note: Coffee and light refreshments will be provided. Held indoors and outdoors.
Supplies: Please dress for walking, including sturdy shoes and weather appropriate attire, and bring materials for taking notes.
♦H138. Saturday, May 5; 9 am – 11:30. Sterling Morton Library. $25 (non-members $32). Limit 40.
**********************
MONITORING AND MANAGING INSECT PESTS
Learn the life cycles, signs and symptoms, and methods of control for the most common insects in our landscapes. Get up-to-date information on easy, less toxic strategies for managing insects and when treatment would do more harm than good. In the classroom portion of each section, you will become acquainted with current seasonal insect threats; then spend a morning identifying them in the field. Different insects will be covered in each section – register for both sections for a full picture of spring insect pests.
Recommended Text: A list of recommended references will be distributed in class.
Certificate Information: Home Landscape Gardening Certificate elective, Naturalist Certificate elective (6 hours); if both sections are completed, this class may be used to fill a Home Landscape Gardening Certificate Requirement.
♦H424. Wednesday, 6:30 – 9:30 pm & Saturday, 9 am – Noon. Thornhill Education Center. $69 (non-members $82). Limit 16. Donna Danielson, Arboretum Instructor
Section a: May 2 & May 5 Section b: May 30 & June 2
The Plant Health Care Report is prepared by Sharon Yiesla, M.S., Plant Clinic Assistant and edited by Stephanie Adams, M.S. Research Specialist (Plant Heath Care); 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: Mary Carter Beary (my apologies for misspelling Mary Carter's name in the last issue), Davida Kalina, Ann Klinglele, LeeAnn Cosper, Kathy Stephens, and Loraine Miranda. Your hard work is appreciated.
Literature recommendation:
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 (CPM), for commercial applicators, and the Home, Yard & Garden Pest Guide (HYG) 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
For pest and disease questions, please contact the Plant Clinic at (630) 719-2424 between 10:00 and 4:00 Mondays through Saturdays or email This e-mail address is being protected from spambots. You need JavaScript enabled to view it . Inquiries or comments about the PHC reports should be directed to Sharon Yiesla at This e-mail address is being protected from spambots. You need JavaScript enabled to view it .
Copyright © 2012 The Morton Arboretum
Not printed on recycled paper, or any paper for that matter.
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