Plant Health Care Report July 20, 2012
Tagged as: grubs, white grubs, polyphemus moth, tortoise beetle, aster yellows, head blight of Silphium, galls, galls on fragrant sumac, hackberry mosaic virus, milkweed tussock moth, Phomopsis canker on kerria, Septoria leaf spot
July 20, 2012 Issue 2012.14
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?
Panicled hydrangea (Hydrangea paniculata) (figure 1) is beginning to bloom.
Accumulated Growing Degree Days (Base 50): 1842.4 (as of July 19)
Accumulated Growing Degree Days (Base 30): 4574.5 (as of July 19)
- Grubs in lawns
Figure 1 Panicled hydrangea (Hydrangea paniculata)
- Polyphemus moth caterpillar
- Milkweed tussock moth
- Tortoise beetle
- Galls, yet again
- Aster yellows
- Head blight of Silphium
- Hackberry mosaic virus
- Fungal problems on Kerria
Degree Days and Weather Information
As of July 19, we are at 1842.5 base-50 growing degree days (GDD), which is 498.5 GDD 50 ahead of 2011 at this time. We have had 2.03 inches of rain so far in July and 13.38" for the year (compared to 21.68" last year at this time).
|Location||B50 Growing Degree Days
Through July 19, 2012
|Chicago Botanic Garden**||1783 (as of 7/16)||.58 (7/10-16)|
|Crystal Lake, IL*||2013||--|
|The Morton Arboretum||1842.5||
|Northbrook**||1975 (as of 7/18)||.71 (7/11-17)|
**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
Grubs in lawns
We are coming up on the time when we need to consider grub control in lawns. White grubs are the larvae of several beetles including Japanese beetles, chafers and June beetles. While eggs of these species will hatch into grubs at various times in late summer, most of the damage begins around July and early August, so this would be the time to apply grub control. The grubs will continue to feed on turf roots until the weather gets cold. Then they will go deeper into the soil to spend the winter. When spring returns, the grubs will come back to the surface, but by this time they are older and tougher and insecticides are less successful.
How do you know if your lawn needs grub control? Grubs eat the roots of the lawn and this will lead to brown areas in the lawn. Unfortunately, since we are having a drought, your lawn is already brown unless you are irrigating. If your lawn has grubs, you will be able to pull the lawn up like a carpet since the roots are gone. Homeowners who are irrigating the lawn should be watchful this year. The beetles have to bury their eggs in the soil. They are more likely to do this in soils that are moist and easy to dig. So those of you who are watering may be more likely to deal with grubs this year.
Is grub control a good idea for everyone this year? Not necessarily. Since we are in the middle of a drought, our lawns are stressed. Chemical insecticides may do more harm than good on these stressed lawns. Also we are having a lot of days with very high temperatures. Many insecticides should not be used above 85 degrees F. Check the label of the product you are planning to use for specific application information. Some homeowners want to apply grub control "just in case". If your lawn has never had grubs before and you are not irrigating this year, it would be best to skip the grub control. The survival rate of eggs and grubs is not good in hot, dry soils, so the drought may limit the grub population in many lawns. Treating dry lawns would be a waste of insecticide and money.
What about Milky Spore? Milky Spore is a bacterium that specifically targets the grubs of Japanese beetles. It will not affect the white grubs of other beetle species. Most universities in northern states are reporting poor results with this product. Even in warmer southern states where the results are better, it takes a long time for the product to be effective.
Polyphemus moth caterpillar
|Figure 2 Caterpillar of polyphemus moth|
A polyphemus moth caterpillar (Antheraea polyphemus) (figure 2) has been found on an oak (Quercus species) on the Arboretum grounds. What a treat – these are caterpillars with quite a bit of heft to them. These huge, fat caterpillars will ultimately grow to three inches long. The caterpillar is green with yellow knobs called tubercles with fine hairs growing out of the knobs. There are thin, yellow, vertical stripes along the sides of the caterpillars and tiny red spiracles that are actually breathing pores. The caterpillars prefer to eat oak leaves, but will eat leaves of hickory, elm, maple, birch, linden, willow and chestnut. They will pupate before winter and turn into a beautiful, large moth next year. The polyphemus moth (figure 3) is one of the largest moths in Illinois, with a four to six inch wing span and large eyespots on its hind wings.
|Figure 3 Polyphemus moth|
Management: The caterpillars are easily eaten by birds because they are so large and slow-moving. These are such splendid moths that it's worthwhile losing some leaves to the caterpillars just to see the adult moths!
Milkweed tussock moth
Milkweed tussock moth caterpillars (Euchaetes egle) (figure 4) were found devouring the
|Figure 4 Milkweed tussock moth caterpillar|
leaves of the common milkweed plant (Asclepias syriaca). They have black and white "hair pencils" along their front, back, and sides and six pairs of thick yellow and black tufts of hair along their middle and grow up to an inch long. The caterpillars feed in colonies and roll into balls, dropping to the ground when disturbed. They are late season feeders on all kinds of milkweeds, so check your butterfly weed (Aesclepias tuberosa). They may defoliate patches of milkweed. Adult males sing to attract females by making clicking sounds.
Management: Bacillus thuringiensis var. kurstaki (Btk) will kill young caterpillars, but is not as effective against older larvae.
|Figure 5 Larva of tortoise beetle|
Off and on this season we have seen an interesting insect, the tortoise beetle (there is morethan one species). Earlier this season, a sample of an adult beetle came in to the Plant Clinic. It is about ¼ of an inch in diameter and has a round body that is a beautiful shiny gold color. Apparently its outer shell is transparent and it is able to change colors. It chews holes in leaves of sweet potato vine and bindweed, one of the most annoying weeds in our yard. This week the larval stage (figure 5) was found on our grounds feeding on bindweed. It carries fecal material on its back to deter predators from eating it. The larvae and adult both chew holes in leaves (figure 6).
Management: This is not usually needed as the beetle has some natural enemies, and it seldom is present in numbers large enough to do serious damage. Also if it happens to be feeding on bindweed, why would you stop it?
|Figure 6 Damage from tortoise beetle|
Good website: http://insects.tamu.edu/fieldguide/bimg195.html
Galls, yet again
It certainly has been a year for galls, and July offers us a selection. Remember that most galls really don't harm the plant and there is generally no cause for alarm. We put this information in the newsletter so when you come across one of these you can say "Oh, that's just another gall."
This week our scouts have found galls (figure 7) on fragrant sumac (Rhus aromatica 'Gro-low'). These are caused by eriophyid mites. Buttonbush galls (Aceria cephalanthi) (figure 8), caused also by a mite have shown up on the leaves of buttonbush (Cephalanthus occidentalis). A gall caused by a psyllid (Pachypsylla venusta) (no photo) is showing up on hackberry (Celtis sp.). This gall is a petiole gall, and it causes a rounded gall to form on the petiole of the leaf.
|Figure 7 Galls on fragrant sumac|
|Figure 8 Buttonbush gall|
Pest update: diseases
|Figure 9 Aster yellows on purple coneflower
Photo credit: Mary Spiewak
We are getting reports of aster yellows showing up in flower gardens. This disease was once thought to be caused by a virus, but the causal organism has been reclassified as a phytoplasma. It can affect a wide range of flowers and vegetables, around 300 species. It is common in members of the aster (daisy) family, like marigolds, zinnias and mums. Reports this year seem to mostly be on purple coneflower (Echinacea purpurea). Aster yellows causes strange, deformed growth of the flowers, foliage, and sometimes roots (seen in carrots). Purple coneflowers are showing deformed flower heads in the form of stunted petals, completely deformed flower heads (figure 9), green petals (figure 10) or deformed flower heads poking out of other flower heads. The disease organism is transmitted by leafhoppers, which are sap feeding insects. They spread the organism when they feed on the host.
Management: There is no cure or treatment for aster yellows. Infected plants should be removed from the garden to prevent spread to other plants by the leafhoppers. Do not compost the plants.
Fiogure 10 Aster yellows on purple coneflower
Head blight of Silphium
|Figure 11 Head blight of Silphium|
Flower head blighting has been found on Compass plant (Silphium laciniatum) at The Arboretum. The flower heads die (figure 11 and turn black and form a shepherd's crook before they bloom. If the flowers bloom, they do not blight. About one-inch below the base of the flower the stem is pinched (figure 12) and sometimes white fungal growth (mycellium) is present. This disease can affect other species of Silphium (rosinweed, prairie dock and cup plant). Mycosphaerella (fungus) seems to be the cause.
Management: Sanitation is the best way to prevent spread or infection for next year. Clip off all affected tissue and rake up fallen debris and remove it from the site. The Volunteer Scout that reported
|Figure 12 Stem damage due to head blight|
this said that the blighting doesn't seem to affect the overall health of the plant. It just causes a loss of the flowers.
Hackberry mosaic virus
Mosaic virus symptoms have been seen on hackberry (Celtis occidentalis) at The Arboretum.
|Figure 13 Hackberry mosaic virus|
The symptoms of the viral infection are like they sound, a mosaic pattern of spots of light green or tan tissue between veins (figure 13) and some marginal necrosis. The interesting thing about viral infections is that the virus does not want to kill the host, because it needs the host in order to reproduce, but it does make the host weaker and more susceptible to other infections. Since viruses are unable to move on their own, they rely on vectors to move them around. The vectors may be insects, humans, or anything else that can carry the viral particle and create a piercing wound into the host. The only way to prevent a viral disease is to control the vector. Once the tree has a viral disease, it will have it for life.
Management: The best way to manage a viral disease is to improve the health and vigor of the host. Running soil and foliar nutrient tests and adding the needed nutrients to the soil, mulching, and watering during dry periods are the best ways to manage.
Fungal problems on Kerria
Septoria leaf spot was diagnosed on Japanese kerria (Kerria japonica). This disease can be
|Figure 14 Septoria leaf spot on Kerria|
fairly damaging to kerria leaves. Leaves become covered with small brown and yellow spots (figure 14) and may fall prematurely. Kerria has also been very susceptible to a canker disease caused by the fungus Phomopsis japonica for the last few years. This has become such a common problem on kerria that it has resulted in the demise of a lot of kerria in the Chicago area. Cankers on branches vary in size and appear as discolored areas(figure 15). Branches girdled by the cankers wilt and die. The cankered areas may crack exposing black fruiting bodies. Microscopic spores (conidia) are produced in large numbers during extended periods of wet weather. Phomopsis is spread by splashing water, by insects, and mechanically (pruning, wounds). This fungus overwinters in cankers as mycelia and pycnidia.
Management: For the canker, prune stems four to six inches below diseased tissue. Clean
|Figurw 15 Phomopsis blight on Kerria|
pruning tools between cuts. For the leaf spot, remove and eradicate diseased leaves to reduce inoculum. Give plants adequate space so leaves dry thoroughly and air movement is unimpeded.
Good web site for pictures:
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: 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.