Plant Health Care Report May 11, 2012
Tagged as: Carpenter bees, Tremella mesenterica, Anthracnose, Azalea bark scale, Cedar rusts, Currant spanworm, Emerald Ash Borer, frogeye leafspot, oak leaf blister, oblique-banded leafroller, Slugs, spiny witch hazel aphid
May 11, 2012 Issue 2012.4
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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?
Cranberrybush Viburnum (Viburnum opulus var. americanum), our indicator plant in issue 3, is in late bloom.
Black Locust (Robinia pseudoacacia), (Figure 1) is blooming.
Accumulated Growing Degree Days (Base 50): 382 (as of May 3)
Accumulated Growing Degree Days (Base 30): 1714 (as of May 3)
Insects:
- Emerald ash borer (EAB)
Figure 1 Black Locust (Robinia pseudoacacia) - Azalea bark scale
- Currant spanworm
- Oblique banded leafroller
- Spiny witch-hazel aphid
- Carpenter bees
Diseases:
- Anthracnose
- Oak leaf blister
- Cedar rust update
- Frogeye leaf spot
- Tremella mesenterica
Other pests:
- Slugs
Degree Days and Weather Information
As of May 10, we are at 382 base-50 growing degree days (GDD), which is 271 GDD 50 ahead of 2011 at this time. So far, in May it has rained 2.22 in., which brings us to 9.15 in. for the year (compared to 10.33 in. last year at this time.
| Location | B50 Growing Degree Days Through May 10, 2012 |
Precipitation (in) |
| Aurora, IL* | 403 | -- |
| Carbondale, IL* | 934 | -- |
|
Champaign, IL* |
650 | -- |
| Crystal Lake, IL* | 374 | -- |
| Harvard, IL* | 356 | -- |
| Kankakee, IL* | 509 | -- |
| The Morton Arboretum | 382 | 1.86" |
| Northbrook** | 417.5 | .57" (5/2-5/8) |
| O'Hare* | 401 | -- |
| Peoria, IL* | 667 | -- |
| Quincy, IL* | 757 | -- |
| Rockford, IL* | 408 | -- |
| Springfield, IL* | 768 | -- |
| Waukegan, IL* | 300 | -- |
**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/
Editor's note: I would like to clarify that the photo of alder leafminer used in the May 4th issue, labeled as Figure 15, is the property of Donna Danielson. Thank you, Donna, for allowing us to use this photo.
Pest Updates: Insects
Emerald ash borer (EAB)
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| Figure 2 Emerald ash borer adult |
We have had reports of emerald ash borer (Agrilus planipennis) adults (figure 2) emerging at the end of last week. This is an ongoing and serious problem in many parts of northern Illinois. EAB is an exotic beetle that was discovered in southeastern Michigan near Detroit in the summer of 2002. The larvae feed on the inner bark of ash trees, disrupting the tree's ability to transport water and nutrients. The larval feeding causes the destructive damage. Adults leave a D-shaped exit hole in the bark when they emerge in May - August (400-450 GDD50.) The adult beetles are metallic green and about 1.27 cm (0.5 in) long. They nibble on ash foliage but cause little damage.
Symptoms of an EAB infestation include upper crown dieback, yellow foliage, or small branches (known as epicormic shoots) that come directly out of the lower trunk. Woodpeckers like to eat EAB larvae; so heavy woodpecker damage on the trees may be a sign of infestation. When bark is removed, serpentine (S-shaped) galleries are visible (figure 3). Besides Michigan and Illinois, the emerald ash borer is also established in Windsor, Ontario, and several states in the mid to eastern United States.
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| Figure 3 EAB galleries under bark |
Here in Illinois, several counties are quarantined, due to the infestation. State and federal quarantines have been established to limit the artificial (or human-assisted) spread of the pest. The federal quarantine of the entire state of Illinois restricts the movement of all EAB-host material (ash nursery stock, ash wood and all non-coniferous firewood) across the state borders to other non-quarantined states. For up-to-date quarantine information go to http://www.agr.state.il.us/eab/Quarantine_Compliance.htm
Management: Currently management includes prevention of the infestation for all ash trees and removal of infested trees. For detailed information on the chemicals available for EAB control, download Insecticide Options for Protecting Ash Trees From Emerald Ash Borer, which is the comprehensive work of several collaborating researchers.
Arboretum articles on EAB:
http://www.mortonarb.org/tree-plant-advice/article/600.html
http://www.mortonarb.org/tree-plant-advice/article/593/eab-advice-to-homeowners.html
Good websites:
http://www.emeraldashborer.info
http://www.na.fs.fed.us/fhp/eab/
http://www.agr.state.il.us/eab
Azalea bark scale
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| Figure 4 Azalea bark scale adults |
Adults of azalea bark scale (Erlococcus azaleae) (figure 4) have been found on the Arboretum grounds. The adult females are approximately 1/8 inch long and covered with a white waxy protective coating. They resemble small mealybugs but are nearly always found at branch crotches. If you squash them, you can see that they are actually red. Females feed on twigs and stems. The feeding sometimes causes chlorosis (yellowing) of the leaves. Branch dieback may occur in heavy infestations. This scale creates honeydew (a sugary liquid insect excrement). Leaves and twigs are often covered with sooty mold, which is a dark saprophytic fungus (fungi that live on dead stuff, not on living organisms) that grows on the honeydew.
Azalea bark scale overwinters as immature females. As the females mature in spring, they secrete white, waxy threads which become matted into a thick covering over their entire body. The crawlers, which are reddish pink and extremely tiny, emerge predictably at approximately 850 to 1,100 GDD and tend to settle in bark crevices and branch crotches. They use their piercing, sucking mouthparts to feed on plant sap. There is only one generation a year in this area. Common hosts are rhododendron, hawthorn poplar, and willow.
Management: Prune out dead or dying infested plant parts. Hand picking method can be applied to adult scales before the crawlers hatch. Severely infested branches and twigs can be pruned out. Apply dormant oil during winter or use an insecticidal spray in summer after all the crawlers have hatched. Beneficial insects (e.g., ladybird beetles and parasitic wasps) help control these pests, so use insecticides sparingly and only if less toxic means seem to be ineffective. Note that some summer and dormant oils may be toxic to azaleas. Also, oils should not be used on plants under drought stress or during excessive heat and humidity conditions.
Good web sites: http://ipm.ncsu.edu/AG189/html/azalea_bark_scale.HTML
http://www.hort.uconn.edu/Ipm/homegrnd/htms/43rhodo.htm
Currant spanworm
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| Figure 5 Currant spanworm larva |
The currant spanworm (Itame ribearia) (figure 5) has been found on alpine currant (Ribes alpinum). This lemon yellow, frosty white, black spotted caterpillar is quite beautiful. The currant spanworm is a fairly uncommon pest. When disturbed, the caterpillars drop down from the plant on a strand of silk. The spanworm pupates in the ground and the moth emerges in late June. The moth is light tan with a row of parallel gray dashes across each wing. The eggs are laid on the bark in July and hatch the following spring.
Management: Treatment is seldom needed. In a severe infestation Bacillus thuringiensis kurstaki (Btk) can be used.
Good website: http://www.extension.umn.edu/gardeninfo/diagnostics/deciduous/ribes/leaveschewed.html
Oblique banded leafroller
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Figure 6 Oblique banded |
Dotted hawthorn (Crataegus punctata) is being attacked by an oblique banded leaf roller caterpillar (Choristoneura rosaceana) (figure 6). The insect primarily attacks members of the rose family such as roses, hawthorns, cotoneaster, apples, and Prunus species. It is a pale yellow green caterpillar with thin hairs on its body. Leaf rollers choose growing terminals and roll the leaves together, fastening the edges with strands of silk. The insect feeds within the rolled up leaves.
Management: Leaf rollers generally don't do sufficient damage to warrant control.
Good website: http://www.ipm.msu.edu/fruitpests/leafroller.htm
Spiny witch-hazel aphid
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| Figure 7 Spiny witch-hazel aphid damage |
Spiny witch-hazel gall aphids (Hamamelistes spinosus) are feeding on the underside of river birch (Betula nigra) leaves. Their feeding causes leaves to appear corrugated, gradually curl, turn brown, and drop prematurely (figure 7).
The insect overwinters in two ways – either as an egg on witch-hazel twigs or as a hibernating female on birches. Eggs hatch in spring, and the nymphs remain on the twigs and feed. The feeding on witch-hazel is what causes spiny galls to form. Each gall is hollow and contains numerous young aphids. As the aphids mature, they exit the gall and fly to their alternate host, the river birch. Meanwhile, the overwintering aphids on river birch move to new leaves in spring and give birth to young aphids that feed and live on leaves. These aphids eventually migrate to witch-hazel to feed on the flower buds and complete their life cycle.
Management: Leaf damage is a cosmetic problem and trees are not severely harmed. Aphid populations can be reduced by spraying the underside of the leaves with a hard stream of water.
Good websites: http://www.entomology.umn.edu/cues/Web/063Aphids.pdf
http://www.hgic.umd.edu/_media/documents/IPMSeriesBirchHG58pfv.pdf
Carpenter bees
The Plant Clinic has received several calls regarding carpenter bees. Carpenter bees bore into wood trim, outdoor wooden furniture, porch ceilings, dead tree limbs, and any weathered wood. They excavate tunnels and can cause considerable damage. They look like bumble bees, but are larger and have a shiny black abdomen. They do not nest in living wood. Carpenter bees have strong jaws and chew 1/2 inch round entrance holes on the underside of wood. Then they chew horizontal tunnels up to seven inches long. The bees are not actually eating the wood. They only create tunnels for nesting sites. We don't have any good pictures of this insect, so visit the websites listed below to see what carpenter bees look like.
Management: Insecticides can be applied to the tunnel entrances on cool evenings when bees are less active. Do not plug the tunnel entrance.
Good web sites: http://www.ces.ncsu.edu/depts/ent/notes/Urban/carpenterbees.htm
http://ento.psu.edu/extension/factsheets/carpenter-bees
Pest Updates: Diseases
Anthracnose
We have had enough rain that we are now seeing anthracnose pop up. Our scouts found anthracnose on oak. Samples of anthracnose on ash and maple have come into the Plant Clinic this week. Anthracnose is primarily a foliar disease affecting many deciduous trees including ash, sycamore, elm, oak, and maple. In the case of sycamore anthracnose, the disease can affect small branches and twigs, causing dieback. The disease is caused by several different fungi, including Apiognomonia errabunda, A. veneta, Discula fraxinea, Glomerella sp., Gnomonia sp., and Stegophora ulmea, depending on host species. Symptoms vary with the plant host, weather, and time of year when infection occurs. Infection is more severe when prolonged spring rains occur after new growth is produced. Although the symptoms appear in late spring into the summer, the initial infection took place in the early spring at bud break and before the leaves hardened off. Once the symptoms show up, it is too late for any chemical applications to be effective.
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| Figure 8 Ash anthracnose |
With ash anthracnose, (figure 8) initial symptoms are small irregular, dark brown, necrotic patches, often accompanied by leaf curl and distortion. Premature leaf drop may occur on highly susceptible species. Damage is usually minor on ash in our region, and symptoms dissipate as rains diminish and temperatures increase in early summer.
Management: Cultural methods are usually sufficient to reduce the severity of ash anthracnose in our region, these include:
• Prune trees to remove diseased twigs and branches and to open up the canopy for better air circulation and light penetration.
• Maintain tree vigor with proper watering and fertilization.
• Mulch around the base of the tree (always keep mulch about 5-10 cm (2 to 4 in) from the trunk).
• In the fall, clean up and destroy fallen leaves to reduce the source of inoculum.
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| Figure 9 Oak anthracnose |
Oak anthracnose, (Discula spp.), (figure 9) has been found on chinkapin oak (Quercus muehlenbergii). Symptoms on trees in the white oak group follow one of three patterns: 1) early infection in which young leaves turn brown and shrivel during leaf expansion; 2) a later infection in which large, irregular blotches develop and distort leaves. The lesions then dry, become papery, and may turn tan to white; and 3) mature leaves are infected and develop small necrotic spots. All three patterns typically start at the bottom of the tree because of high moisture and rainfall flow.
Management: In most years, control of anthracnose on oaks is unnecessary because the disease does not affect the long-term health of oak trees. Collecting and destroying fallen leaves and twigs, and pruning dead twigs, will help reduce the overwintering population of the pathogen. Pruning during the dormant season will also increase air circulation and lower humidity within the canopy. Mulching and watering (not overhead) during dry periods will help keep trees healthy. Chemical sprays to control anthracnose are rarely justified except when the disease occurs in stressed or recently transplanted trees, or when the disease causes repeated defoliations.
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| Figure 10 Maple anthracnose |
Maple anthracnose, (figure 10) also caused by a Discula species, has been found on sugar maple (Acer saccharum). There are three types of foliar symptoms that are caused by two or more different fungi: (1) elongate lesions that form initially along the vein and extend into interveinal areas; (2) irregularly shaped necrotic spots that are often centered on veins; and (3) necrotic blotches that cover large areas of the leaf. We are now seeing necrotic spots. The disease is typically more severe during cool, wet weather, sometimes even causing defoliation.
Management: Maple anthracnose is primarily an aesthetic problem. Proper fertilization and irrigation of trees that have considerable disease may help trees refoliate and maintain their vigor. Because spores overwinter in infested fallen leaves, remove these leaves from the property to prevent next year's infection.
Good websites: http://urbanext.illinois.edu/hortanswers/detailproblem.cfm?PathogenID=5
http://ohioline.osu.edu/hyg-fact/3000/3048.html
http://www.extension.umn.edu/yardandgarden/ygbriefs/p413ashanthrac.html
http://www.na.fs.fed.us/spfo/pubs/fidls/anthracnose_east/fidl-ae.htm
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| Figure 11 Oak leaf blister |
Oak leaf blister
Oak leaf blister, (figure 11) caused by Taphrina caerulescens has been found on bur oak (Quercus macrocarpa). Refer to the April 20 issue (issue #2) for more information on this disease.
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| Figure 12 Cedar apple rust |
Cedar rust update
In issue #1, we reported that the cedar rust galls were becoming active on the junipers. Now we are seeing cedar apple rust spots (figure 12) developing on the upper surfaces of crabapple leaves. These rust spots contain the spermagonia. Once the spermagonia are sexually fertilized, hyphae grow through the leaf to produce the spores that will infect the junipers.
Frogeye leaf spot
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| Figure 13 Frogeye leaf spot |
Crabapples are showing symptoms of frogeye leaf spot (figure 13). Frogeye is caused by the fungus Botryosphaeria obtusa, which also infects the fruit and bark. Right now, the disease appears as round, purple leaf spots with tan centers. The spots turn gray-brown as they age and can develop concentric circles; hence the name frogeye.
Early frogeye is sometimes mistaken for apple scab, and the two may occur together, but the symptoms are quite different. Frogeye spots also contain pepper-like fruiting structures (pycnidia) that are visible with a hand lens in mature lesions.
The Arboretum has evaluated more than seventy crabapple cultivars for resistance to both frogeye leaf spot and scab diseases during the last two years. The frogeye disease level varies from year to year and is worse after very cold winters. Below is a list of cultivars from University of Illinois Extension that are considered good for Illinois and without other major disease problems.
Crabapple (Malus sp.) that show good resistance to frogeye leaf spot in Illinois
Cultivar Form (H x W) Flower Fruit
Malus baccata var. jackii 20'x 20' white red-purple
M. 'Lancelot' 10'x 8' white gold
M. 'Prairie Maid' 20'x 25' pink orange- red
M. 'Prairifire' 20'x 20' red-purple purple-red
M. 'Red Jewel' 18'x 12' white red
M. sargentii 8'x 15' white red-purple
M. 'Silver Moon' 20'x 15' white red
Management: Remove dead or diseased branches and prune susceptible trees to open dense crowns. This will increase air flow and keep the leaves drier. The fungus needs periods of cool and wet condition in order to infect the leaves; increasing airflow will prevent these favorable conditions. Fruits that become spotted or dried (mummified) are another overwintering inoculum source that should be removed if possible. Choose resistant cultivars whenever possible for future plantings.
Good website: http://www.ca.uky.edu/agcollege/plantpathology/ext_files/ppfshtml/ppfs-fr-t-3.pdf
Tremella mesenterica
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| Figure 14 Tremella mesenterica |
A sample of this fungus (figure 14) was found on a Katsura tree by a member of the horticulture staff and confirmed by our pathologist. This fungus, with such a serious sounding scientific name, is commonly known as witch's butter and is classified as a jelly fungus. Apparently this is not much of a disease problem. This fungus is a saphophyte and is actually growing on dead wood.
Other pests
Slugs
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| Figure 15 Slug |
Guard your hostas; the slugs (figure 15) have arrived. Slugs are a common pest in wet weather or if landscapes are watered. They are mollusks, not insects, and are related to oysters, octopi, and clams. Slugs secrete a slimy substance to help them move about. They need moisture to create this slime, so they are highly dependent on soil moisture. Slugs feed at night when humidity is high, so the best time to see them feeding on our plants is to check the plants at night with a flashlight. They feed on many plants in the landscape, including annuals, perennials, bulbs, ground covers, trees and shrubs, preferring succulent foliage and fruit lying on the ground. Slug damage on leaves appears as irregularly shaped holes or tattered edges. Insects also eat leaf margins, but large holes in leaves are more indicative of slug feeding.
The gray garden slug is the most common slug in our area. They average about 1.9 cm (0.75 in) long, but may reach up to 3.81 cm (1.5 in). They have two pairs of tentacles on the front end of their body. Most slugs overwinter as eggs in debris. When they hatch in spring, the young slugs begin to feed immediately.
Management: Hostas with thick leaves are much less likely to suffer from slug damage. A combination of strategies is necessary to combat slugs. They can be handpicked and placed in a jar of soapy water. They are not strong swimmers and drown in the jar. Temporary traps of rolled, wet newspaper and boards placed near damaged plants provide shelter for the slugs during the day. Check the boards and papers in the morning. The slugs can then be collected and destroyed. The key to this is to collect and destroy the slugs. If you skip this step, you are aiding and abetting the slugs. Slug hideouts, such as excessive mulch piles and weeds, should be eliminated. Watering late in the day should be avoided because the moist conditions make slug movement easier.
Some gardeners place shallow pans of beer (cheap beer works fine, keep the good stuff for yourself) in slug-infested areas. The slugs are attracted to the yeast and drown in the beer. Thin strips of copper bands placed around the bases of shrubs and trees repel slugs by giving them an electric shock when their bodies touch the copper. Make sure the slugs are not inside the copper bands when setting them out.
Insecticides are not effective against slugs because they are not insects. Registered commercial slug baits are available.
Good website: http://www.ext.colostate.edu/PUBS/INSECT/05515.html
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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, 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 .
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