Plant Health Care Report July 27, 2012

Tagged as: twig girdler, tree selection, smooth patch, leaf spots, twig pruner

July 27, 2012                                                          Issue 2012.15

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.

This is the last weekly issue of PHCR. We go to bi-weekly in August. The next issue will be August 10.

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?

 We have run out of ‘official’ indicator plants as listed in Don Orton’s Coincide.  Cardinal flower (Lobelia cardinalis) (figure 1) is beginning to bloom around Meadow Lake.

Accumulated Growing Degree Days (Base 50): 2039.5 (as of July 26)

Accumulated Growing Degree Days (Base 30): 4911.5 (as of July 26)

Insects:

Figure 1 Cardinal flower (Lobelia cardinalis)

• Twig pruners and girdlers

Diseases:

• Leaf spots
• Smooth patch

 Miscellaneous:

• Tree selection

Degree Days and Weather Information

As of July 26, we are at 2039.5 base-50 growing degree days (GDD), which is 473 GDD 50 ahead of 2011 at this time. We have had 2.74 inches of rain so far in July and 14.09" for the year (compared to 25.26" last year at this time).

 **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

 Twig pruners and twig girdlers

Figure 2 Twig pruner damage

Earlier this season, we reported on squirrels cutting branches from trees and dropping them in the yard.  At this time of year we may see similar damage from twig pruners (Elaphidionoides villosus).  Twig pruner larvae will cut the twig from the inside (figure 2), leaving a smooth circle inside.  The first sign of infestation are branch tips with fading foliage that eventually turn brown. The damage then turns to wilted and browned branches hanging from trees and scattered about the ground.

In spring, as oak leaves are beginning to form, adult twig pruners emerge and deposit eggs near the tips of twigs.  Larvae move to the center of the branch and begin to feed, tunneling down to the base of the twig.   In late summer they move to the sapwood, making circular cuts, weakening the stems. The weakened stems may hang on the tree and eventually fall to the ground on windy days or during storms. Larvae remain in the fallen branches, spend the winter as pupae, and emerge as adults the following spring.

The adult is a gray-brown beetle 1/2 to 3/4 inch long. Larvae are creamy white, legless, segmented, and reach ¾ of an inch at maturity. Oak twig pruner larvae feed on many tree species, including maple, oak, hickory, elm, walnut, and a number of fruit trees.

Another insect, the twig girdler (Oncideres cingulata), does similar damage, but the damage is caused by the adult, not the larva.  In late summer, in order to lay eggs, the adult beetle will chew a groove around the twig.  The eggs are laid in the part of the branch that will fall off the tree.  The larva will develop in the fallen twig where it will spend the winter and then pupate inside the twig in spring.  Adults will emerge in late summer.  The damage on the twig is rough in the center and smooth on the outside, opposite of the damage done by the twig pruner.

Management:   This insect will not kill or severely damage trees. To reduce populations, collect and destroy fallen branches and prune out wilted and damaged branches.

Good web site:  http://extension.missouri.edu/p/g7276

http://pubs.ext.vt.edu/2911/2911-1423/2911-1423_pdf.pdf

Pest update: Diseases

 Leaf spots

 This is the time of year when it seems like every plant has a fungal leaf spot.  This usually raises concerns for gardeners, but it often does not have to.  Many of the leaf spots that show up in mid- to late summer are not much of a problem and usually not worth treating.  Most leaf spots lead to little damage.

How do you know when to worry?  There are some situations that merit more attention.  If the leaf spots appear early in the season and lead to defoliation, the problem may be serious.  If the leaf spots are accompanied by other symptoms like dieback of branches, again the problem may need more attention.

Management:  Rake up and dispose of infected leaves to reduce inoculum for next year.  Avoid wetting the leaves when watering the plant.  If you determine that the leaf spot is serious enough to warrant treatment with a fungicide, remember that most fungicides are applied early in the season before the symptoms appear

Good websites:  http://ipm.illinois.edu/diseases/rpds/648.pdf

http://www.hort.uconn.edu/ipm/homegrnd/htms/27leafsp.htm

Smooth patch

Figure 3 Smooth patch

Smooth patch is one of those diseases that doesn’t really seem to be a disease.  Technically, smooth patch is a “disease” in that it changes the appearance of bark, but does not harm the tree. It is caused by the fungus Aleurodiscus oakesii. The fungus decomposes and sloughs off the outer, dead bark of the tree, usually of American elm (Ulmus americana), American hornbeam (Carpinus caroliniana), sugar maple (Acer saccharum), bur oak (Quercus macrocarpa) and white oak (Quercus alba). The result of the sloughing is patches of bark that are grayish, slightly sunken, and smoother than the original bark (figure 3). Patches range from a few inches to more than a foot across. Smaller patches may coalesce into large patches, and this is what we usually notice. Because this “disease” of the dead bark does not harm the tree, no control is necessary.

Management:  Management is not needed since the fungus does not invade living tissue and does not cause cankers or internal decay.

Good websites:  http://www.forestryimages.org/browse/subthumb.cfm?sub=813

Miscellaneous

Tree selection

With so many trees being lost to the emerald ash borer and to the storms of this summer, it’s time to think about tree selection.  Fall can be a good time to plant many species of trees.  A tree is an investment so it is worthwhile to take some time with the selection process.  There are several things to take into consideration when deciding on a new tree for the landscape.

First and foremost, the tree should be cold hardy to our area.  Northern Illinois is in USDA hardiness zone 5, so we need to look for trees that are hardy to zone 5 or smaller.  This is usually not a problem when buying locally.  If you are buying from the internet or via mail order, check the cold hardiness of the tree.

Consider the conditions of the planting site.  What type of drainage and soil moisture are present at the site?  Is compaction a problem?  What is the soil pH?  Usually in this area we are dealing with alkaline soils.  How much sun or shade does the location receive?  Try to find a tree species that will be able to grow well within the conditions at your planting site.

Also consider the physical dimensions of the tree at maturity.  How tall and wide will the tree be when it matures?  Even though it will take many years for a tree to reach its mature size, plan ahead and be sure that the planting site can handle the mature size of the tree.  Proper selection coupled with porper training will minimize the need for excessive pruning later in the tree’s life.

Maintenance needs to be considered as well.   No tree is perfect; they all have some issue to think about.  Some trees though have more problems than others or have more serious or chronic problems.  Disease and insect problems should be considered.  If a tree has the same insect or disease problem every year, the maintenance level can be higher than desired.  Many homeowners express a desire for fast growing trees.  Care needs to be taken here as many fast growing species have weak wood and are more prone to storm damage.  If the tree is to be planted near a deck or sidewalk, avoid messy trees that drop seeds, fruit or twigs.

As we look to replace trees, we should focus on planting a diversity of trees.  In the past we have seen serious problems arise when we have had too many of the same tree in the landscape.  Dutch elm disease was able to spread more rapidly because elms were so widely planted.  Emerald ash borer has become a major problem quickly since ash trees are abundant in the landscape.  Several years ago, the U.S. National Arboretum proposed a 10-20-30 formula which suggests planting no more than 10% of a single tree species, no more than 20% of species within any tree genus and no more than 30% of the species within any tree family within a given area.  A simple rule for tree diversity that any homeowner can follow:  see what trees your neighbors have in their yard and then plant something else.

Good websites: http://www.mortonarb.org/tree-plant-advice.html

http://urbanext.illinois.edu/treeselector/

________________________________________

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.

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

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