Plant Health Care Report May 25, 2012
May 25, 2012 Issue 2012.6
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.
What indicator plants are in bloom at the Arboretum?
Japanese tree lilac (Syringa reticulata) (figure 1) began blooming this week.
Accumulated Growing Degree Days (Base 50): 537.5 (as of May 24)
Accumulated Growing Degree Days (Base 30): 2149.5 (as of May 24)
Figure 1 Japanese tree lilac (Syringa reticulata)
- Pesticide recommendations
- Insect populations building
- Still more galls showing up
- More leafminers
- Black vine weevil
- Two-marked tree hopper
- Virus on hosta
- Other viruses
- Downy leaf spot
- Bedstraw or cleavers
Degree Days and Weather Information
As of May 17, we are at 434 base-50 growing degree days (GDD), which is 238.5 GDD 50 ahead of 2011 at this time. So far, in May it has rained 2.42 in., which brings us to 9.35 in. for the year (compared to 10.95 in. last year at this time.
|Location||B50 Growing Degree Days
Through May 24, 2012
|Chicago Botanic Garden**||460.5 (as of 5/21)||.06 (5/14-21)|
|Crystal Lake, IL*||589||--|
|The Morton Arboretum||537.5||
**Thank you to 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/
Don't jump to conclusions
If you've been working in the world of plants for any length of time, you have seen the same
Figure 2 Damage from backhoe exhaust
disease and insect problems year in and year out. The danger in this is jumping to conclusions when you one see particular symptom, instead of looking at the whole picture. As an example, when the picture in figure 2 was shown to me, my mind went immediately to Zimmerman pine moth. I was wrong. Apparently a backhoe was being used extensively in the immediate vicinity of the pine tree and the exhaust from the backhoe is the cause of the brown branches. It may be tedious to keep asking questions about a problem, but it does pay off. When you know the full story, you can arrive at a very different answer than the one that came off the top of your head.
We rely on pesticide recommendations from University of Illinois Extension. They have recently updated the pesticide publication for homeowners. The 2012 edition is named "Pest Management for the Home Landscape", and it replaces the "Home, Yard and Garden Pest Guide". These recommendations are for homeowners. Commercial horticulturists can still use the Commercial Landscape & Turfgrass Pest Management Handbook (2010 edition). Both publications are available by calling 1-800-345-6087 or online at pubsplus.illinois.edu
Pest Update: Insects
Insect populations building
Here at The Morton Arboretum, we utilize pheromone traps to monitor insect emergence and to track insect populations. Recent checks of the traps show us that the elm bark beetle population is at its peak. These are the beetles that can carry the fungus that causes Dutch elm disease. Ash-lilac borer (this is the native borer, not Emerald ash borer) is on the rise as well. A recent check of the traps yielded 11 adults (an increase of three over the last check). Two Viburnum crown borers were found in the traps this week. Insecticide treatment for this pest is usually administered when we see mockorange in bloom, which is occurring now.
Still more galls showing up
|Figure 3 Hackberry nipple gall|
This seems to be the year of the gall. We have reports of a number of different galls making an appearance this week. Remember that galls are mostly harmless and there is no need for chemical control. We're giving you some pictures here so when you see these things on the plant you know that you don't really have a problem. Hackberry nipple gall (figure 3) on common hackberry (Celtis occidentalis) has been reported this week. This gall shows up virtually every year. It is so common that some people think it is a normal part of the tree.
Vein pocket gall (figure 4) is present on oak trees (Quercus species). This gall takes the form of an enlarged area that follows along the veins on the underside of the leaves. The Plant Clinic received another oak gall sample this week. This one was an oak apple gall (figure 5) in the early stage of development. Right now the gall is about the size of a small super-ball (remember that great toy?). It is developing on the leaf and may eventually get up to two inches in diameter. There is a larva developing in the middle of the gall.
|Figure 4 Vein pocket gall on oak|
|Figure 5 Oak apple gall|
|Figure 6 Hawthorn leafminer|
There are numerous species of minute insects known as leafminers that live and feed inside leaves of many different plant species. Earlier this year we reported on several leafminers (issue 4) including Arborvitae leafminer, elm leafminer and alder leafminer. Leafminer larvae eat leaf tissue between the upper and lower epidermis, leaving the leaf intact. Leafminers and their black frass can be seen within the mined leaf by holding a damaged leaf up to sunlight. This week, the following leafminers were seen:
• Shot hole leafminer on Bur Oak (Quercus macrocarpa) found in about 5% of the canopy.
• Elm leafminer on Scots elm (Ulmus glabra). About 65% of the canopy affected at this time.
• Hawthorn leafminer (figure 6) on bigfruit hawthorn (Crataegus macrosperma), 2% of leaves infested.
Management: Leafminer injury is generally an aesthetic problem so control is rarely justified. The occasional severe infestation can be controlled with systemic insecticides and should be applied when mines first appear.
Black vine weevil
Black vine weevil (Otiorhynchus sulcatus) damage has been seen on the leaves of Catawba
|Figure 7 Black vine weevil damage|
rhododendron (Rhododendron catawbiense). Adult black vine weevils are nocturnal creatures that feed along leaf margins, producing crescent-shaped notches (figure 7). Moderate feeding is not damaging to plant health. The more serious damage is done by the larvae which consume tender feeder roots, causing foliage of infested plants to turn yellow or brown. When young roots become scarce or the soil becomes excessively moist, the larvae will move to larger roots at the base of the plant. Severe larval infestations can ultimately kill the host plant.
Adult female weevils emerge from the soil in late May through early July and feed for three to four weeks at night before laying eggs in the soil beneath the host plant. Eggs hatch in two to three weeks and the larvae feed on roots until late fall. With the onset of colder temperatures, larvae burrow deeper in the ground to overwinter. Black vine weevils feed on a wide range of herbaceous and woody ornamentals. Preferred hosts are yew, hemlock, and various rhododendrons.
Management: If you place boards down in infested areas, the weevils will hide under the boards during the day. You can them pick them up and destroy them. Insecticidal sprays are effective in controlling adult weevils. Insecticides should be applied now before egg-laying occurs and repeated twice at 2-week intervals. Parasitic nematodes, Heterorhabditis bacteriophora, have been found to be effective in controlling larvae. They should be applied when larvae are present (in about five to seven weeks). Moderate to high soil moisture in July and August will help egg and larva survival. Remove excessive mulch layers to reduce soil moisture levels, and do not water plants unless necessary. Excessively damp soils in the fall also force larvae to move up the base of the plant where girdling can occur.
Good web sites: http://ohioline.ag.ohio-state.edu/hyg-fact/2000/2016.html
Two-marked tree hopper
|Figure 8 Two marked tree hopper nymphs|
Two-marked treehopper (Enchenopa binotata) nymphs were found on black walnut (Juglansnigra) shoots. They're about 1/8 inch long, dark gray to brown, and have spines sticking out of their abdomens (figure 8). The nymphs look quite different from the adults. Adults (figure 9) are dusky brown with two yellow spots on their backs (thus the name), have high, curved horns that point forward coming out of their thorax, and are less than ½ inch long. Both stages can, as you may imagine by their name, jump! Everyone should see two-marked treehoppers at least once in their lives, because they're so amusing the way they march along on twigs. Nymphs and adults suck plant juices, but don't do much damage. The damage appears as pale yellow stippling on the leaves. Treehoppers do, however, produce honeydew which encourages sooty mold. Female adults can injure twigs by laying eggs in slits made in the bark. Black locust, bittersweet,
|Figure 9 Two marked tree hopper|
wafer-ash, redbud, and Viburnum are also hosts for this insect.
Managent: Control is usually not necessary.
Pest Update: Diseases:
Virus on hosta
Figure 10 Virus on 'Blue Cadet' hosta
The Illinois Department of Agriculture informed our plant clinic of virus-infected specimens of 'Blue Cadet' hosta (figure 10 and 11). While no laboratory confirmation has been made, it is being treated as though it is hosta virus X. Hosta virus X was first identified in 1996 and has now reached epidemic proportions on hostas. Since symptoms vary by cultivar, it is important to know what the cultivar normally looks like. The most dramatic symptoms include line patterns, especially along veins, mottling or mosaic patterns, puckering, blotches, or ringspots. Sometimes only small brown dead spots are seen. It can take a year or more for symptoms to appear.
|Figure 11 Virus on 'Blue Cadet' hosta|
Before the virus was identified as a problem, some hosta sports with unusual mottling and coloration were propagated and put on the market. Scientists later found the cause of the unusual look was hosta virus X.
Hosta virus X is not spread by insects, but can be spread on hands, pruning tools, and by propagation. Tools should be sterilized when pruning or dividing hostas. There is no control, so infected hostas should be dug up and destroyed.
A number of plant samples that are exhibiting viral symptoms have been submitted to the
Figure 12 Virus on geranium
Plant Clinic this spring. The Plant Clinic is not a culture lab, so we are diagnosing based on symptoms. We have seen viral symptoms on euonymus (Euonymus fortunei), Rozanne geranium (Geranium 'Rozanne) (figure 12), bleeding heart (Lamprocapnos spectabilis) (figure 13) and possibly on roses (Rosa hybrids). Honeysuckle (Lonicera sp.) has been found with two viruses, one causing bunchy-top symptoms and another causing mottling.
Unlike the Hosta virus X, many viruses are spread by sucking insects. They can also be spread on hands, pruning tools and by propagation. Symptoms vary but include mottled colors, contorted leaves, excessive shoots, puckering and ringspots.
Management: Viruses cannot be controlled with any chemicals. Management of viral
|figure 13 Viruses on bleeding heart|
diseases includes removal of the plant, management of sap sucking insects that may carry the pathogen and sterilization of pruning tools. Viruses are systemic within the plant, so pruning off parts of the plant will not solve the problem. Viruses need a live host to survive, so they do not kill the host. Viruses do not survive in the soil, so replanting is possible after complete removal of the infected plant.
Good web site: http://www.ipm.ucdavis.edu/PMG/r280101411.html
Downy leaf spot
|Figure 14 Downy leaf spot on upper leaf surface|
Downy leaf spot (figure 14 and 15), also known as white mold or white leaf spot, caused by the fungus Microstroma juglandis, has been found on hickory (Carya sp.). Powdery, white, fuzzy spots that are more concentrated near the leaf veins are forming on the underside of the leaf surface. Corresponding chlorotic spots appear on the upper leaf surface. These spots vary in size and may coalesce to form large angular lesions. The fungus may also cause witches' brooms near the ends of branches with stunted and yellowish leaves that may drop in early summer.
Management: Downy leaf spot attacks hickories and walnuts but is not a significant threat to the trees. Brooms can be pruned to improve the appearance of the tree. Chemical
|Figure 15 Downy leaf spot on lower leaf surface|
control is not recommended.
Good websites: http://web.aces.uiuc.edu/vista/pdf_pubs/600.pdf
Pest updates: weeds
Bedstraw or cleavers
This has been a weedy spring, most likely a side effect of the mild winter we enjoyed.
|Figure 16 Bedstraw (photo credit Sharon Yiesla)|
Garlic mustard has been prolific, and dandelions look like they are on steroids. Now another weed is running rampant. Samples of bedstraw or cleavers (Galium aparine) (figure 16) are coming into the plant clinic almost daily. This annual weed has tiny white flowers and a slightly sticky surface. It is related to the ground cover sweet woodruff (Galium odoratum) but not nearly as welcome as that plant. Bedstraw has tiny white flowers and is presently flowering and starting to go to seed. Because the plant is slightly sticky and the stem breaks off very readily, it is easy for this plant to get stuck to animals (and gardeners), and this helps to spread the seeds.
Management: Since this is an annual weed, chemical control is not warranted. The stem breaks easily, so pulling the plant out of the garden may be the best bet. You won't get the root, but with an annual you don't really need to. This plant needs to be pulled now before the seeds are fully ripe.
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.
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.
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