F L O O D I N G and its effect on TREES

Insect and Disease Concerns of Flood Stressed Trees

Natural forests in floodplains have evolved to handle periodic flooding and, therefore, many trees in the flooded areas will indeed survive. However, the 1993 flooding has been prolonged and has occurred during the growing season, adding severe stress. The flooding has included not only areas forested by well adapted swamp hardwoods, but has also flooded many urban areas forested by trees that are not well adapted to flooding. In addition, many of the forests now flooded are located behind broken dikes. These forests developed after the dikes were installed in the 1950's and have not previously been flooded, did not evolve to handle periodic flooding and have a different species composition than typical bottomland floodplain forests. These forests, along with urban trees, could be highly susceptible to additional tree mortality from insect and disease attacks due to their weakened state.

Trees that survive flooding are in varying stages of health or vigor. Vigor is influenced by stress. Prolonged flooding is a major stress, especially during the growing season. Low vigor trees often die quickly from a combination of physical injury and rapid invasion from insects or diseases. High vigor trees may recover very quickly and could be healthy as early as the next growing season. Many trees are somewhere in the middle, struggling to survive and slowly trying to regain a higher level of vigor. Until trees regain an adequate level of vigor, they are susceptible to attack by insects and/or diseases. If an insect or disease is successful in invading a tree, the survival of that tree becomes less likely. The battle to regain vigor and ward off attacks from insects and diseases may continue for several years, causing tree death over a period as long as 3-5 years. Which trees become and continue to be stressed will depend on many factors including a species tolerance to flooding, length of inundation, sediment levels, etc. (see Flood Tolerance of Trees section).

Stress Symptoms

Flood stressed trees exhibit a range of symptoms including leaf chlorosis (yellowing), defoliation, reduced leaf size and shoot growth, sprouting, and crown dieback. Early fall coloration and leaf drop often occur. It also is common for stressed trees to produce large seed crops in years following a stressing event such as flooding. Again, it may not be unusual for symptoms to occur over several years. The symptoms may progress and eventually lead to tree death or they may subside indicating the tree has recovered.

A critical factor in determining the survival of flood stressed trees is whether they become invaded by insects and/or diseases. Flood stressed trees are prime targets for attack by "secondary organisms." Secondary organisms include a wide variety of opportunistic fungi and insects that selectively invade hosts only after they are weakened or predisposed by stress. It is believed that predisposing stresses such as flooding, drought, and defoliation impair host resistance mechanisms, and trigger biochemical responses which release carbohydrates, glucose, and other nutrients which stimulate secondary insects and diseases.

Further, certain root and collar rot diseases are favored by waterlogged, oxygen- deficient soil conditions, most notably those caused by the water mold fungi, Phytophthora spp. and Pythium spp.. Flooded soil conditions not only promote reproduction and dispersal of these fungi but also promote the susceptibility of plant roots to infection.

Finally, the wood of trees that have died as a result of the flooding will also be quickly attacked and utilized by wood boring insects and blue staining and decay fungi. Where landowners wish to salvage and sell dead or severely declining trees, they will need to be aware of the decline in wood quality that can occur quickly from insect and disease attack. This can significantly reduce the value of wood products. (See discussion of salvage considerations in Management Implications section).

The following is a list of insects and diseases to watch for:

Insects

Stem boring insects are the major group of "secondary" insects of concern. The most common stem borers are beetles, either adults or immatures (larvae) depending on the species. Other stem borers, which may cause problems on trees, will be a few moth or woodwasp larvae. Stem boring insects can be further divided into phloem borers and wood borers. Phloem borers include bark beetles and many of the metallic wood-boring beetles. They are serious pests because the damage they cause occurs in the phloem and outer sapwood layers. These two layers are important in food and water transport and if significantly damaged, trees are severely altered. Wood borers may spend some time in the phloem layer, but generally tunnel and feed within the wood of tree stems or branches. This tunneling is often not a serious impediment to tree survival though it can significantly reduce the "quality" of any eventual wood products. A major concern with wood borers is weakening of stems, which may lead to breakage during ice, wind, or snow storms.

Symptoms of stem borers may include small holes in the bark. Entrance holes may have pitch, sap, or sawdust exuding from them. Exit holes are generally very clean and may be round, oval or D-shaped. Removal of the bark should expose larval tunneling. (Bark removal is an additional wound and should only be done on trees already dead or those which are not of high value).

Special management practices for stem borers: 1) Prevent additional wounding or root damage to trees. Wounds create stress and act as attractants to many insects. 2) Sanitize areas by removing and destroying large broken limbs and dead trees. This material may act as breeding sites for stem-boring insects which may later infest surrounding live trees. 3) Increase tree vigor through light fertilization treatments and watering if soil conditions become excessively dry. (This may be required for 2-3 years). 4) Insecticides rarely help; they should only be used for high value trees and following the recommendation of a professional entomologist, arborist or forester. Stem borer insecticides are applied to the tree bark as a protectant; therefore, application must be made prior to infestation.

It is unknown if leaf-feeding (caterpillars) or sucking insects (scales and aphids) will become more of a problem following flooding. Plant stress can alter the biochemistry of trees making nutrients and sugars more available to insects feeding on leaves or sap. This could increase survival of these insects and increase their population size. Outbreaks of caterpillars or scales and aphids could further increase stress levels on trees severely weakened by the flood. Therefore, control of these insects should be a priority on high value trees for the next 1-3 years. This may require insecticide application(s). Insecticide recommendations should follow label guidelines.

There are too many tree species and associated insects involved to make an individualized list of potential insect pests. The following trees, however, are notorious for attacks by insects following stressful periods and often require close watch: all pines, oaks, hybrid poplars, birches (especially ornamental white birches), and hickories. Also, resource managers should be especially aware of pine bark beetles, Ips spp.; twolined chestnut borer, Agrilus bilineatus, attacking oaks; bronze birch borer, Agrilus anxius; and hickory bark beetle, Scolytus quadrispinosus.

Diseases

Armillaria Root Disease
This disease, also called shoestring root rot, can attack hundreds of species of forest, shade, and ornamental trees and shrubs. Although some Armillaria species are aggressive pathogens, others are opportunistic and work singly or in conjunction with other secondary action pests. In the Northeast for example, oak trees weakened by stress are often attacked by both Armillaria root rot and the two-lined chestnut borer. Armillaria root rot is commonly associated with drought-stressed trees, an association that is well documented by research. Excess soil moisture may be as stressful as drought to trees because it can cause "physiological drought" by interfering with water uptake in oxygen-deprived roots. Excess soil moisture and increased severity of Armillaria root rot have been observed in oak and chestnut species in Germany and Austria (Bazzigher 1956), in larch in Japan (Kawada et al. 1962), and in rubber trees in Nigeria (Fox 1964).

Symptoms and Management
Diagnostic symptoms include white colored mycelial fans under the bark, shoestring-like rhizomorphs, and "honey" mushrooms which are present only in the late summer or fall. Nonspecific symptoms include leaf chlorosis and defoliation, reduced leaf size and shoot growth, crown dieback, and death. (Refer to the enclosed USFS Forest Insect and Disease Leaflet entitled Armillaria Root Disease for color photographs of symptoms and information on management practices).

Canker Diseases
A wide range of fungi incite canker diseases in both hardwood and conifer hosts by invading the bark, cambium, and outer sapwood of branches and stems weakened by mechanical injuries, insect feeding, water extremes, or other diseases. Branches and main trunks of trees submerged in flood waters or injured by floating debris will be prime targets for invasion by canker fungi. Some of the most common canker diseases include Nectria, Cytospora, Botryosphaeria, and Botryodiplodia.

Symptoms
Cankers appear as localized dead areas on branches or stems and are commonly associated with wounds or dead branch stubs. They often appear discolored or sunken, and the bark may or may not remain attached to the face of the canker. Some canker diseases such as Nectria produce zonate or target-like cankers in response to successive layers of callus tissue forming at the progressing edge of the canker. Cankers can girdle branches or small stems and result in wilting or dieback. Canker diseases are rarely fatal to their hosts unless large or multiple cankers girdle the main stem.

Management
Because wounding and predisposition play a role in the development of canker diseases, the best approach to management is to minimize tree stress and injuries. See Management of Flood Stressed Thees below for more information.

Phytophthora and Pythium Root Diseases
Phytophthora spp. and Pythium spp., commonly known as water mold fungi, are ideally suited for waterlogged soil conditions. Plant roots stressed by reduced oxygen in waterlogged soils exude more amino acids and ethanol which attract infective spores to root surfaces. Infective spores are dispersed in surface water such as flood, runoff and irrigation waters. Survival rates of these spores in water has not been widely studied; however, one study demonstrated that spores of Pythium aphanidermatum survived for 185 days after submersion in pond water (Stolzy 1984). This suggests that other species of water mold fungi may have similar survival rates in water. If this is true, an increase can be expected in root and collar rot diseases caused by species of Phytophthora and Pythium.

Symptoms of Phytophthora and Pythium Root Diseases
Symptoms include stunting, leaf chlorosis, reduced leaf size, basal stem cankers which often ooze sap, root and collar decay, crown dieback and death. Pythium spp. cause damping off and root rot disease on young seedlings in nurseries and can infect nearly all conifers and hardwoods. Phytophthora spp. incite root and collar rot diseases on a wide range of nursery and forest tree hosts including:

Management of Phytophthora and Pythium Root Diseases
Management strategies for these two diseases must be targeted at nursery operations since little can be done to control these diseases after trees are outplanted. Nursery management practices include:

  1. Avoiding planting tree species highly susceptible to Phytophthora spp. in poorly drained fields.
  2. Improving soil drainage in poorly drained fields.
  3. Employing alternative cropping regimes in fields with a previous history of root rot disease. Consider bare fallow for 1 to 2 years to reduce disease inoculum.
  4. Using chemical fumigation.

Outplanted trees exhibiting symptoms of root disease may benefit from efforts to enhance tree vigor. (See Management of Flood Stressed Trees below for more information).

Management of Flood Stressed Trees

Urban and Landscape Sites
The best approach to managing flood stressed trees is to enhance tree vigor by proper tree maintenance and protection from additional stresses. Tree vigor can be enhanced by fertilizing with a low nitrogen fertilizer, aerating the soil, mulching, and watering if soil conditions become excessively dry. Dead or severely cankered branches should be removed. Prune only when bark surfaces are dry or during the dormant season.

Newly transplanted or mature, high value trees may need protection from leafspot diseases such as anthracnose and from insect defoliators and various sucking insects such as aphids or scales. It should be noted that leafspot diseases are not severe every year. Trees need protection during spring seasons that have frequent rainfall at budbreak and during leaf expansion. Refer to the enclosed USDA Forest Service brochure entitled, How To Identify and Control Dogwood Anthracnose, for more information. Although the above mentioned publication deals specifically with dogwood anthracnose, the cultural control recommendations are applicable for leafspot diseases of trees and shrubs in general. A fungicide not mentioned, thiophanate methyl, is labeled for control of anthracnose on shade trees and woody ornamentals. Several tradenames of fungicides that contain thiophante methyl include Cleary 3336, Topsin M, Domain, Fungo and Zyban.1 Fungicides should be used only to supplement a cultural control program. Read fungicide labels carefully to determine registered uses and application rates.

Forest Stands
Any harvest or salvage activities should create a minimum of damage to remaining or regenerating trees or disturbance to the site itself. Soil compaction, rutting, bark removal, and branch breakage can all act as additional stress on other trees in the stand.

Salvage activities such as "sanitation cuts" can be beneficial by removing breeding material of stem boring insects. (See Management Implications section for additional salvage considerations).

Insecticide use is rarely practical or ecologically sound in forested situations.

Hazard Evaluation of Flood Damaged Trees

As noted above, flooding results in some trees being stressed, physically damaged, and/or insect and disease infested. These trees possess defects that decrease their structural integrity, making them more prone to windthrow and structural failure. Defective trees located in high use areas such as yards, parks, or other recreational areas are hazardous and pose safety risks to people and property. (The Minnesota Department of Natural Resources, in cooperation with USDA Forest Service, has written a manual entitled, How To Detect, Assess and Correct Hazard Trees In Recreational Areas. Copies are available from: Minnesota DNR, 500 Lafayette Rd., St. Paul, MN 55155-4049, price: $6.00).

References

Bazzigher, G. 1956. Pilzschden an Kastanien nordlich der Alpen. [Fungal damage to chestnuts north of the Alps.] Schweizerische Zeitschrift fur Forstwesen. 107: 694- 695.

Fox, R.A. 1964. A report on a visit to Nigeria (9-30 May 1963) undertaken to make a preliminary study of root diseases of rubber. Document Research Archives, Rubber Research Institute of Malaya 27. [Review ofApplied Mycology 43:3003]. 34p.

Kawada, H.; Takami, M.; Hama, T. 1962. [A study of Armillaria root rot of larch. Effects of soil conditions on its occurrence and some information of field observa- tion.] Meguro:Bulletin of the Government Forest Experiment Station. 143: 39-98. In Japanese.

Stolzy, L.H. and Sojka, R.E. 1984. [Effects of flooding on plant disease.] In: Kozlowski, T.T., ed. Flooding and plant growth. Academic Press, New York. 356 p.

Recommended Books

Insects of Eastern Forests, USDA-Forest Service, Miscellaneous Publication 1426, 1985, 608 pages. (excellent reference for insect biology).

Diseases of Trees and Shrubs, WA. Sinclair, H.H. Lyon, and WT.Johnson authors, Cornell University Press, Ithaca, NY;, 1987, 574 pages. (Excellent color pictures, very comprehensive).

Insects That Feed on Thees and Shrubs, W.T. Johnson and H.H. Lyon authors, Cornell University Press, Ithaca, NY; 1988, 556 pages. (Excellent color pictures, very com- prehensive).

How to Detect, Assess and Correct Hazard Trees in Recreational Areas, Minnesota Department of Natural Resources, 68 pages.

Insect and Disease Contacts

Illinois
Jim Appleby
Forestry Department
University of Illinois
607 E. Peabody Drive
Champaign, IL 61820
217) 244-3431

Kansas
Les Pinkerton
State and Extension Forestry
Kansas State University
2610 Claflin Road
Manhattan, KS 66502-2798
(913) 537-7050

Iowa
John Walkowiak
Iowa DNR
Wallace State Office Building
Des Moines, IA 50319
(515) 242-5966

Nebraska
Mark Harrell Dept. of Forestry
101 Plant Industry Bldg.
University of Nebraska
Lincoln, NE 68583-0814
(402) 472-6635

Minnesota
Olin Phillips
Minnesota DNR
Forestry Division
500 Lafayette Road
St. Paul, MN 55155
(612) 296-5965
(701)228-2633

Ed Hayes
Minnesota DNR
P.O. Box 6247
Rochester, MN 55903
(507) 285-7428

Missouri
Susan Burks
Missouri Dept. of Conservation
1110 S. College Ave.
Columbia, MO 65201
(573) 882-9880
(573) 882-4517
burkss@mail.conservation.state.mo.us

North Dakota
Ardan Tagestad
North Dakota Forest Service
Box 21A, Forestry Dr.
Bottineau, ND 58318

South Dakota
Rich Dorset
South Dakota Div. of Forestry
1429 East Sioux
Pierre, SD 57501
(605) 773-4161

Wisconsin
Allen Prey
Wisconsin DNR
3911 Fish Hatchery Road
Madison, WI 53711
(608) 275-3274

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