Plant Health Care Report June 1, 2012
Tagged as: Black spot on rose, Cercospora leaf spot, larch sawfly, Oak Wilt, Powdery mildew, Rose rust, rust, sycamore anthracnose, tarnished plant bug, thrips
June 1, 2012 Issue 2012.7
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?
Mockorange (Philadelphus coronarius) (figure 1) is in late bloom.
Accumulated Growing Degree Days (Base 50): 692 (as of May 31)
Accumulated Growing Degree Days (Base 30): 2444 (as of May 31)
Insects:
- Thrips on roses
Figure 1 Mockorange (Philadelphus coronarius) - Tarnished plant bug
- Larch sawfly
Diseases:
- Rust on roses…
- …..and other plants
- Oak wilt
- Cercospora
- Powdery mildew
- Anthracnose on sycamore
- Black spot on rose
Degree Days and Weather Information
As of May 31, we are at 692 base-50 growing degree days (GDD), which is 334.5 GDD 50 ahead of 2011 at this time. In May it rained 2.64 in., which brings us to 9.57 in. for the year (compared to 15.4 last year at this time).
| Location | B50 Growing Degree Days Through May 24, 2012 |
Precipitation (in) |
| Aurora, IL* | 789 | -- |
| Carbondale, IL* | 1410 | -- |
|
Champaign, IL* |
1086 | -- |
| Chicago Botanic Garden** | 593 (as of (5/28) | .62 (5/22-28) |
| Crystal Lake, IL* | 721 | -- |
| Harvard, IL* | 699 | -- |
| Kankakee, IL* | 918 | -- |
| The Morton Arboretum | 692 |
.03 (5/25-31) |
| Northbrook** | 746.5 (as of 5/30) | .87 (5/23-29) |
| O'Hare* | 762 | -- |
| Peoria, IL* | 1095 | -- |
| Quincy, IL* | 1197 | -- |
| Rockford, IL* |
773 |
-- |
| Springfield, IL* | 1223 | -- |
| Waukegan, IL* | 606 | -- |
**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 updates: insects
Thrips on roses
 |
| Figure 2 Thrips damage on dogwood |
Several cases of thrips on roses (Rosa spp.) have been reported to the Plant Clinic. There are numerous species of thrips that attack a wide range of ornamental plants. These insects feed by rasping soft plant tissue and sucking the juices of leaves and flowers. Feeding injury appears as leaf distortion (figure 2), silvering (tiny silver streaks) of the leaf surface, and slight necrosis of petals and petal browning. The roses that have been brought into the Plant Clinic have shown deformed flower buds with distorted petals, while leaves have been unaffected. Infested flowers may fall apart quickly.
Adult thrips (figure 3) are tiny black-to-pale-yellow insects about 1/20 of an inch long with fringed wings. They look like small splinters until they start walking. They are spread long distances by floating with the wind or being transported on infested plants. Healthy woody plants in the landscape normally outgrow thrips damage.
 |
| Figure 3 Thrips adult |
Management: Though damage is often minor, management is often warranted since thrips are notorious disease vectors. The western flower thrips (Frankliniella occidentalis) wreaks havoc in greenhouses by spreading tomato spotted wilt disease to many ornamental plants. Here are a few cultural tips for controlling your thrips:
• Remove weeds in plant beds.
• Keep plants well watered during periods of drought.
• Clean up the debris from plant beds and dispose of old, spent flowers.
• Remove infested plant material as soon as it is noticed.
• Avoid excessive applications of nitrogen fertilizer.
• Avoid shearing plants as this stimulates new growth that is susceptible to attack.
The above cultural practices will help manage but not eliminate thrips. If your problem is severe, chemical controls can be used. Apply insecticidal sprays when thrips are numerous. Blue sticky cards are useful to assess thrip population levels.
Good web site: http://entnemdept.ufl.edu/fasulo/woodypest/thrips.htm
http://www.ipm.ucdavis.edu/PMG/PESTNOTES/pn7429.html
Tarnished plant bug
Tarnished plant bugs (Lygus lineolaris) were found on black walnut (Juglans nigra) recently. These true bugs (Order: Hemiptera) can be found nationwide on a variety of herbaceous plants and some agro-economic crops. These 1/4" long insects overwinter as adults in leaf debris, in bark, or in other protected areas. They emerge in the spring and begin feeding on buds and young leaves. In early to mid-summer the females deposit their eggs in plant organs (stem, leaves, buds, etc.), which hatch after a few weeks. Depending on their location, there can be 2-5 generations per year.
Although these plant bugs can damage important plants, they do have some redeeming qualities. Tarnished plant bugs are omnivores, and they also feed on aphids and other small soft-bodied insects.
Good website: http://entoplp.okstate.edu/ddd/insects/tarnishedbug.htm
http://ipm.illinois.edu/fieldcrops/insects/tarnished_plant_bug/index.html
Larch sawfly
 |
| Figure 4 Larch sawfly larvae |
Larch sawfly (Pristiphora erichsonii) (figure 4) larvae have been found on the needles of larches on The Morton Arboretum grounds. The larvae have black heads, gray-green bodies and are 16 mm (about 1/2 inch) long when fully grown. They feed in colonies on tufts of needles and are very obvious when present. Defoliation is rarely complete. Repeated heavy feeding can cause thin foliage and reduced growth. All species of larch are susceptible to attack.
Larch sawfly overwinters as pupae in the ground and adults emerge in mid-spring. The female cuts a double row of slits along one side of an elongating shoot and deposits eggs. The egg slits damage the shoot causing it to curl. While these slit needles will continue to grow, they develop a curliness that is characteristic of larch sawfly damage.
Management: Minor infestations can be controlled by using a forceful jet of water to dislodge the sawfly larvae or by handpicking. These mechanical controls should be done now. Severe infestations can be controlled with insecticides. Insecticides are most effective when used on young sawfly larvae.
Good website: http://www.extension.umn.edu/distribution/horticulture/DG6703.html
Pest update: disease
Rust on roses....
 |
| Figure 5 Rust on rose leaf |
We're finding rust on rose (Rosa spp.) leaves. Bright orange "powder" (figure 5) appears initially as spots on the leaves and later may coalesce as the disease worsens. This powder is actually a cluster of aeciospores of the fungus (Phragmidium sp.). These spores reinfect other roses and cause orange red spots on the leaves and long, narrow lesions on the stems. Leaves may wither and fall off, and shoots may become distorted and reddish. Plants infected by this obligate parasite will gradually decline in vigor.
Management: Infected plant parts should be pruned out and destroyed immediately. Do not work with the plants in wet weather and provide ample air circulation in plantings. When buying new roses, select disease-resistant roses.
Good websites: http://ohioline.osu.edu/hyg-fact/3000/3063.html
.....and other plants
 |
| Figure 6 Rust on hollyhock |
When we say rust, we are really talking about a number of different fungi, affecting different plants, but causing similar symptoms (orange-yellow spots). In addition to the rust we have been seeing on roses, we are also seeing rust on some other ornamentals. Rust on hollyhock has been showing up for a few weeks. This disease is caused by the fungus Pucciniamalvacearum. This is a different fungus than the one that causes rose rust, but we will still see orange-yellow spots or pustules on the leaves (figure 6).
Management: Remove infected leaves and clean up plant debris around hollyhocks. Increase air circulation around the plants. Fungicides may be needed.
 |
| Figure 7 Rust on Aesculus |
The Plant Clinic has also seen a case of rust on buckeye (Aesculus sp.). This is not a common occurrence. This particular rust is caused by the fungus Puccinia andropogonis. The symptom on buckeye is orange spots on the leaves (figure 7). Big bluestem (Andropogon gerardii) is an alternate host for this disease. Although some defoliation may occur on buckeye, the damage is not usually serious, so no management is needed.
Good websites: http://www.coopext.colostate.edu/TRA/PLANTS/holyrust.shtml
Oak wilt
 |
| Figure 8 Leaf symptoms of oak wilt |
We have not yet had any reports of oak wilt for this year, but this is a good time to review the disease and be watching for symptoms. Both the red and white oak groups are susceptible to oak wilt; however, the former is most susceptible. Symptoms between the two groups are different. In red oaks, death is rapid with wilt symptoms starting at the top of the tree and progressing inward and downward on the lateral branches within a few weeks. Leaves wilt from the leaf tip and margins to the bases and typically turn an off-green before showing bronze coloration (figure 8). Near complete leaf drop usually occurs by the middle of summer, making infected trees stand out. Fallen leaves are often green at the base. There can be profuse suckering at the base of the tree. When an infected branch is cut in cross section, or bark peeled back, very light brown streaking or speckling can sometimes be seen in the outer ring of sapwood (figure 9). Symptoms of the disease on white oaks are similar, but infected white oaks die slowly, a branch at a time, over a period of one to many years. Leaf discoloration of affected white oaks usually resembles autumn colors (but appears much earlier than autumn), and brown streaking in the outer growth ring of sapwood is often apparent.
 |
| Figure 9 Streaking in oak |
The fungus invades the xylem and induces the tree to clog its own water-conducting vessels. Water flow is stopped and cells begin dying. Oak wilt can spread from infected trees to healthy trees through root grafts and by sap-feeding beetles that carry spores of the fungus from one tree to another as they feed and visit wounds.
Management: Monitoring and rapid removal (sanitation) is key to controlling oak wilt. Remove infected oaks as soon as you confirm the disease. Vector insects feed on fresh pruning wounds; therefore, oaks should not be pruned during the growing season when the nitidulid beetles are active. This disease can spread to other healthy oaks about 25 to 50 feet away (depending on tree size) via root grafts. To help halt the spread, dig a trench to a depth of approximately three feet between infected and healthy trees to break root grafting. Some systemic fungicides are labeled for preventing this disease. Trenching and fungicide injections must be done by trained professionals.
Good websites:
http://na.fs.fed.us/spfo/pubs/howtos/ht_oaklab/toc.htm
http://www.ag.uiuc.edu/~vista/abstracts/a618.html
Cercospora
 |
| Figure 10 Cercospora leaf spot on linden |
Cercospora leaf spot (figure 10) was found on linden (Tilia species). The spots are about 3 mm (1/8 inch) in diameter and light brown with distinct dark margins. Like many fungal leaf spots, Cercospora is more of an aesthetic problem with one or a few cycles per year. It should not damage a large tree.
Management: Reduce humidity levels around the tree by avoiding overhead watering and watering lawns in the morning so the area will dry out thoroughly. Also provide plenty of spacing around the tree. The inoculum of this disease overwinters in fallen leaves so rake up and discard or compost fallen leaves in the fall.
Powdery mildew
 |
| Figure 11 Powdery mildew on ninebark |
Numerous samples of powdery mildew have come into the Plant Clinic. This disease has been appearing on the leaves of Diablo ninebark (Physocarpus opulifolius 'Monlo') (figure 11), peony (Paeonia species), roses (Rosa species) and even on bluegrass lawns. Hundreds of plant species are susceptible to powdery mildew, but the disease is caused by many different fungal species and is host specific. This means that the powdery mildew on coralberry will not infect lilacs and so forth.
Powdery mildew appears as a superficial white to gray coating over leaf surfaces, stems, flowers, or fruits of affected plants. Initially, circular powdery white spots appear. These spots coalesce producing a continuous patch of "mildew." Later in the season, cleistothecia (fungal fruiting bodies that look like black pepper under a hand lens) will appear. Warm days and cool nights favor this fungal disease, and we have seen this weather pattern fairly consistently this season. This disease is one of the few that is deterred by free water since spores will not germinate in free water on leaves. However, the disease still needs high humidity to infect the plant. Leaf curling and twisting result, and in severe infestations you may see premature defoliation and deformed flower buds. Although unsightly, powdery mildew is usually not fatal in the landscape.
Management: Infected plant parts should be removed as soon as symptoms appear. Dispose of fallen leaves and do not handle plants when foliage is wet. Water plants during periods of drought to keep them healthy. High humidity can increase disease severity so avoid overhead watering in late afternoon or evening. Put plants in locations where there is good soil drainage and sufficient sunlight. Provide proper plant spacing for good air circulation. Powdery mildew on some plants can result in significant damage, and fungicides may be needed. To obtain optimum results, spray programs should begin as soon as mildew is detected. In the future, plant mildew-resistant cultivars and species.
Good websites:
http://ohioline.osu.edu/hyg-fact/3000/3047.html
http://www.uri.edu/ce/factsheets/sheets/powderymildew.html
Anthracnose on sycamore
 |
| Figure 12 Anthracnose on sycamore |
About 4-6 weeks ago, we noticed damage on newly emerging sycamore leaves. At that time the damage was from cold temperatures. Now we are seeing early symptoms of anthracnose on sycamore (Platanus occidentalis) caused by the fungus Apiognomonia veneta. Leaf blight symptoms are brown foliar lesions that extend along the veins, often in V-shaped patterns (figure 12). The leaves turn brown and may drop prematurely. Sycamore anthracnose is normally expected when we have cool, wet weather during leaf development. Although rain has been minimal this season, it has been enough to get the disease started. The couple of days of rain we are getting now could enhance the problem in the near future. Considerable defoliation may occur in late spring, but trees normally bounce back and produce a second set of leaves that remain disease free. For trees that were hit with frost earlier, this may be their second set of leaves, so we may see some trees struggle with this round of anthracnose.
There are two other stages of this anthracnose: shoot and leaf blight and canker formation. Shoot and leaf blight results when the pathogen enters succulent shoots. It causes the rapid death of expanding shoots and leaves. The pathogen overwinters in twigs and is active whenever temperatures are high enough in the fall, winter, and spring. During winter, cankers form on infected shoots and kill the buds.
Repeated infection results in deformed shoots and witches brooms (dense clusters of twigs). Although this disease can weaken trees and increase their susceptibility to attack by other pathogens and pests, it is not lethal.
Management: Dead twigs should be pruned as they develop throughout the growing season. Fertilize trees that are defoliated to increase their vigor. In the fall, rake and discard fallen leaves to reduce the source of inoculum. It is impractical to spray fungicides on large trees, but for smaller, specimen trees, the disease can be controlled with fungicides applied in four intervals: 1) just before bud break, 2) during bud break, 3) when leaves are fully expanded, and 4) 10 to 20 days later. Systemic fungicide injections are also used in spring and fall.
In the future, plant disease-resistant varieties. Oriental plane tree (Platanus orientalis) and London plane tree (Platanus x acerifolia) are less susceptible than American sycamore. Unfortunately, they are less cold hardy so they must be sited carefully.
Good websites:
http://www.na.fs.fed.us/spfo/pubs/fidls/anthracnose_east/fidl-ae.htm
http://ohioline.osu.edu/hyg-fact/3000/3048.html
Black spot on rose
 |
| Figure 13 Black spot on rose |
Black spot of rose is showing up on a number of roses, including 'Knock Out' roses, which are normally considered resistant to this disease (remember that resistant does not mean immune). Black spot is caused by the fungus Diplocarpon rosae. Round to irregular black leaf spots with fringed margins appear on either leaf surface but primarily on the upper surface (figure 13). When infection is severe, the entire leaf will turn yellow and drop. Repeated defoliation will lead to reduction in flower quality and quantity, stunting and weakening of the plant, and increased susceptibility to other diseases.
The fungus overwinters on fallen leaves and diseased canes. Spores are splashed by water or wind-blown rain from fallen leaves and cane lesions to newly emerging leaves and succulent stems in the spring. Warm temperatures, combined with wet leaves and high humidity, will result in abundant spore germination and infection in about one day. Black spots become evident 3 to 16 days later.
Management: Remove infected leaves and canes to reduce inoculum. Plant roses in sunny locations with good air circulation and avoid overhead watering. Avoid planting them too densely. Fungicides should be applied as soon as leaves emerge and continued, at labeled intervals, until leaves drop in the fall. Lengthen spray intervals or skip applications during dry weather.
Good web site:
http://www.ag.uiuc.edu/~vista/abstracts/a610.html
http://www.ianrpubs.unl.edu/epublic/live/g1060/build/g1060.pdf
__________________________________________________________________________
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. Thanks also to Donna Danielson, who teaches at The Morton Arboretum and shares her findings when she scouts for her classes.
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, 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
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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