Plant Health Care Report May 18, 2012
May 18, 2012 Issue 2012.5
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?
Weigela (Weigela florida), (Figure 1) is in bloom.
Arrowwood Viburnum (Viburnum dentatum) is also blooming.
Accumulated Growing Degree Days (Base 50): 434 (as of May 17)
Accumulated Growing Degree Days (Base 30): 1906 (as of May 17)
• More galls
|Figure 1 Weigela florida|
• Rose slug sawfly
• Balsam twig aphid
• Dutch elm disease
• Seiridium canker
• Phytophthora and other root rots
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 17, 2012
|Chicago Botanic Garden**||357 (as of 5/14)||.16" (5/8-14)|
|Crystal Lake, IL*||455||--|
|The Morton Arboretum||434||.2" (5/11-17)|
|Northbrook**||490 (as of 5/16)||.06" (5/9-15)|
**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/
Seeds: too few or too many?
This strange spring is still yielding oddities. Some trees have set heavy crops of fruits and seeds while others are producing very little. Why is this happening? It all ties in to flowering time and frost. Many plants bloomed early and some even bloomed out of their normal sequence. Then we had three or four nights of cold temperatures and frost. Damage to the flowers depended on how far along they were in their development. Some trees (silver maple, redbud) flowered very early and were already producing seeds when the cold temperatures came. A side effect of heavy seed crops is slow emergence of leaves. The redbuds at the Arboretum, bloomed beautifully this year and have set a huge crop of seeds. As a result, they are leafing out more slowly than normal.
Pest Updates: Insects
|Figure 2 Hedgehog gall|
In earlier issues, we reported a number of galls on a variety of plants. Here are a few more that are showing up now. Galls generally are harmless and no control is needed. We are supplying these pictures and names for identification purposes, so you can recognize them and know that you are dealing with something harmless.
Recently we have seen hedge hog galls (figure 2) on chinkapin oak (Quercus muehlenbergii). This one, as the name implies, looks a bit like a hedgehog. Hedgehog galls are produced by the cynipid wasp, Acraspis erinacei, and are usually attached to the leaf midvein. They range in size from 1/4 to 1/2 inch in diameter and are yellow with red and pink 'bristles'. They are absolutely adorable and quite soft.
|Figure 3 Wool sower gall|
Wooly sower gall (figure 3) has been found on Quercus alba. These galls are also caused by a wasp, Callirhytis seminator. The gall looks like a wooly ball on the stem.
|Figure 4 Ash flower gall|
Ash flower galls (figure 4) have been reported on ash trees (Fraxinus sp.). These galls are caused by ash flower gall mites, an eriophyid mite (Aceria fraxiniflora), that feed on male flowers before buds are fully expanded. Feeding induces formation of round, greenish galls that become dry and turn brown in late summer and remain on the tree over the winter. Normally male flowers fall off after disseminating pollen, but when infested with ash flower galls mites, the galls may stay on the tree as long as two years. Seedless green ashes are most commonly attacked. Since many people are closely watching their ash trees, it should be noted that this gall has nothing to do with the emerald ash borer.
Rose slug sawfly
|Figure 5 Rose slug sawfly larva and damage|
Samples of rose slug sawfly (Endelomyia aethiops) larvae (figure 5) feeding on rose leaves were brought into the Plant Clinic this week. Larvae feed on the upper layers of the leaf, leaving behind the lower epidermal layer and creating a "window pane" effect. The larvae are greenish yellow with orange heads and are about 13 mm (1/2 inch) long when fully grown. They are covered in slime that helps protect them from predators. When larvae mature, they lose their slimy coverings.
Management: Minor infestations can be controlled by using a forceful jet of water to dislodge the sawfly larvae or by handpicking. Insecticidal soap can also be used for low populations of young larvae. More severe infestations can be controlled with neem oil or insecticidal sprays. Controls should be implemented now.
Balsam twig aphid
We are finding balsam twig aphid adults (Mindarus abietinus) on concolor fir (Abies concolor).
|Figure 6 Balsam twig aphid damage|
They are about 1/12th of an inch long, pale blue, and are beginning to cover themselves with white waxy wool. Due to the presence of the wool, the balsam twig aphids can be mistaken for balsam woolly adelgids. The aphids, however, are needle feeders (under the microscope you can actually see their mouth part pierced into the needle) whereas the adelgids feed on stems, branches, and twigs. Also, adult adelgids are purplish-black while the adult aphids are pale blue to pale green. Usually the colors of both are masked beneath their white waxy wool.
The aphids overwinter as eggs in bark crevices of the host plant. In early spring, a few weeks before balsam fir bud break, small blue-green nymphs emerge and feed primarily on the old growth, causing little damage. In late spring, the second and third generations feed on new needles, causing curling and permanent deformity of new shoots (figure 6). Copious amounts of honeydew are also present.
Balsam twig aphids also infest Fraser fir, Siberian fir, subalpine fir, white spruce, Colorado blue spruce, and juniper.
Management: Damage to trees is primarily aesthetic so control is not recommended.
Pest Updates: Diseases
Dutch elm disease
|Figure 7 Flagging symptom of DED|
We often see Dutch elm disease (DED) pop up in early June, so it seems like a good time to review the problem, especially since the beetles got an early start this year. So far we have not had any reports of active DED, but that could change soon. DED is caused by two closely related species of fungi: Ophiostoma ulmi (formerly known as Ceratocystis ulmi) and O. novo-ulmi. The American elm is extremely susceptible and the disease has killed hundreds of thousands of them across the U.S. All native elms are susceptible, Asian elms are resistant, and European elms are more or less in between.
Symptoms of new DED infections are yellowing, curling, and wilting leaves on outer branches in the canopy. This is called flagging (figure 7). When the bark is removed, brown streaks can be found in the outer wood (figure 8). The fungus grows beyond the visible streaks and can rapidly spread to the trunk and kill the entire tree.
There are two insect vectors responsible for transmitting DED: the native elm bark beetle (Hylurgopinus rufipes) and the European elm bark
|Figure 8 Streaking symptom of DED|
beetle (Scolytus multistriatus). The beetles carry the fungus to healthy trees as they feed on twigs and upper branches. Spores can enter the tree through the feeding wounds. The fungus causes the xylem to plug up and the tree to wilt and die. Beetles eventually lay their eggs in the bark of infected trees and tunneling larvae become coated with the fungus. The larvae continue the cycle by emerging as adults to feed on the healthy elms, with the fungus on their bodies. The beetles typically have multiple generations per year in the Midwest and are present from late April through September.
DED can also be transmitted through root grafts. A network of roots allows the disease to move freely from one elm tree to the next and can result in a whole stand or parkway of elms becoming infected. Root grafts between trees are especially prevalent in cramped urban and suburban parkways. The best way to prevent DED transmission via root graft is to have the roots trenched. DED can also be transmitted on infested pruning tools. Remember to sanitize your pruning tools when working around any disease problem.
Management: Monitoring and sanitation are crucial to controlling DED. Elms should be inspected for flags weekly from now through July and every few weeks through September. If a tree is newly infected, pruning may successfully eradicate the disease if no more than 5%–10% of the tree shows symptoms or at least seven to ten feet of clear wood occurs between the streaking and the main trunk. A final pruning cut, 7–10 feet beyond the streaks, is necessary to ensure the fungus is removed. Sterilize pruning tools between cuts with 70% alcohol or a similar disinfectant. If a tree shows many flags or completely wilts, it must be removed quickly so that beetles and root grafts do not transmit the disease further. Root grafts should be severed before removal of a diseased tree. Girdling the tree by removing the bark/cambium in a strip near the base of the tree can be done temporarily before removal. Potential elm bark breeding material, such as elm logs and stumps with intact bark, should be chipped and destroyed or, at the very least, stripped of bark.
There are several options for preventing Dutch elm disease. Valuable elms can be injected with a fungicide. A biocontrol tool, Dutch Trig®, is also available. Neither Dutch Trig® nor fungicides are 100% effective. Arbortect is highly effective for DED when it is transmitted by beetles, but not as effective when DED is root graft transmitted.
Plant resistant elm varieties: the Asian elms, lace bark elm (U. parvifolia), and Siberian elm (U. pumila) are highly resistant to the disease, though Siberian elm is considered to be a weedy, weak-branched tree. The Morton Arboretum has bred several excellent elms named 'Triumph', 'Accolade', and 'Commendation', available through Chicagoland Grows®. Since they are hybrids of resistant Asian elms, they are also resistant to DED. There are some commercially available American elms that show resistance to DED. They include 'Princeton', 'New Harmony', 'Valley Forge', 'Jefferson' and 'Independence'.
|Figure 9 Seridium canker|
Seiridium canker (figure 9) has been found on the Arboretum grounds on Port Orford Cedar (Chamaecyparis lawsoniana). Seiridium canker, caused by Seiridium unicorne, is a fungal disease of bald and Leyland cypress, Oriental arborvitae, and occasionally junipers. Elongate, sunken cankers appear on twigs and branches often accompanied by extensive resin flow. Spore-producing structures of the fungus appear on the bark surface of the cankers as small circular black dots. Cankers restrict water flow and may ultimately lead to branch dieback and tree death. This fungus usually attacks trees that are suffering from an environmental problem such as drought or winter damage.
Management: Prune infected branches at least one inch below the canker and sterilize pruning tools between cuts. Avoid water stress and tree wounding.
Phytophthora and other root rots
Recent rains have brought out a number of root rots. Phytophthora species are common
|Figure 10 Phytophthora root rot on yew|
culprits for landscape problems. So far this season, we have found Phytophthora root rot on yew (figure 10) and pine. Depending on the host and the Phytophthora species the infections can result in root rots, stem rots, and foliar blighting. There are many different microbes that can cause these plant problems. Many of them are fungal and some are bacterial. Rhizoctonia and Fusarium root rots have been found on ginkgo. Bacterial soft root rot has been found on arborvitae.
Symptoms of any root rot infection include wilting, dieback, and blighting over the entire canopy. This is most easily recognized by standing back and looking at the entire plant. Wilting symptoms are usually expressed by foliar dieback on the outer tips of the branches, while the other leaves wilt. If there are only a few branches showing symptoms of a problem, it is unlikely to be a root rot, but should probably be examined anyway.
Next, grab a shovel and begin to dig near the stem of the tree. This can be tricky because you don't want to damage the large-woody structural roots. Find the fine roots that form a fine webbed-network in the upper few inches of the soil. Cut a few off and take a knife or fingernail and scrape away the outer surface of the root. If the inside of the root is white or creamy white, the root is healthy. If the outer surface sloughs off in your hand, or the inside of the root is tan, brown, or red, there is a problem.
If there is excess water in the area, and the roots and soil smell acrid, like the soil has become anaerobic, try to dry up the area. If it's a bacterial problem, this should take care of it . If you dry up the area and the plant continues to decline it's likely a fungal problem. Being able to positively identify which fungus is causing the problem is difficult and can only be done by looking at the roots under a microscope, or by growing the fungus out of the roots. For this, contact the University of Illinois Extension Plant Clinic (go to http://web.extension.illinois.edu/plantclinic) to help you. There are also commercial labs that diagnose samples for homeowners and some tree care companies that diagnose these problems.
Knowing which fungus is causing the problem is important when it's time to treat the problem, because there is no universal fungicide. And most fungicides used to treat root rots are regulated and require a Pesticide Applicator's License.
Once a root rot fungus has been introduced into the landscape, eradicating them is nearly impossible. The best cultural way to manage root rots is to prevent them. Make sure purchased plants are healthy and disease free. Check the roots while at the garden center by pulling them out of the containers. Also, prevent excess stress by planting the correct plant in the location.
We spend a lot of time talking about diseases and insects in the Plant Health Care Report. Let's look at the flipside: preventing or minimizing problems through proper horticultural techniques. Watering is one of the most important things we can do for our plants (even more important than fertilizing). This is a good time to look at watering because we have had such a strange year. We had very little snow this winter, so very little snow melt. Spring, in general, has been drier than normal. In recent weeks the weather report has called for rain almost every day, but many areas got very little, whiles others got too much. This can be deceptive. The daily call for rain makes us think it really did rain.
Buy a rain gauge. It is a small investment but a very useful one. A rain gauge can help us know how much rain really fell in a given location. This can be important in this year when rainfall has been scarce in certain areas. Many of us may need to get out and water our plants, especially since we are now running into periods of hot temperatures for several days at a time.
All of our plants (trees, vegetables, flowers) can benefit from proper watering in dry times. Note the emphasis on proper. Newly planted trees may need to be watered 2-3 times per week. Established trees may need water every 10-14 days (we are trying to keep the top 8-12 inches of soil moist). It is best to water trees with a long, slow soaking. Soaker hoses work great for this, but remember that they deliver water very slowly so they will be on for a while. You can also move a trickling hose every 20 minutes or so to different areas under the canopy. A good rule of thumb for herbaceous plants is to apply one inch of water per week and to do it all at one time, so the water penetrates deeply into the soil, encouraging deep rooting. That means we need to get away from running a sprinkler for a few minutes each day. That is shallow watering that never delivers enough water to the roots, but keeps the leaves wet. Water on the leaves can lead to fungal growth. So proper watering serves our plants in two ways; by relieving stress and reducing the incidence of fungal infection.
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 (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
Copyright © 2012 The Morton Arboretum
Not printed on recycled paper, or any paper for that matter.
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