Forest Service

North Central

Research Station

Research Note NC-390

2005

ABSTRACT.—Oaks commercially treated with

propiconazole on 29 sites in Minnesota in 1998 were

evaluated for efficacy in controlling oak wilt. Root graft

spread occurred in 39 percent of preventively treated red

oaks over 5 years; spread in white oaks occurred only once.

Propiconazole generally prevented further disease symptom

development in white oaks.

KEY WORDS: Ceratocystis fagacearum, oak wilt,

propiconazole.

Evaluation of Propiconazole

Operational Treatments of Oaks

for Oak Wilt Control

Jordan Eggers, Jennifer Juzwik, Shawn Bernick,

and Lori Mordaunt

JORDAN EGGERS and JENNIFER JUZWIK are a biological

technician and research plant pathologist, respectively, with

the North Central Research Station, USDA Forest Service,

St. Paul, MN 55108; SHAWN BERNICK is a scientist with

Rainbow Treecare Scientific Advancements, 2239

Edgewood Ave. S., Minneapolis, MN 55426; and LORI

MORDAUNT is a section head with Rainbow Treecare,

2239 Edgewood Ave. S., Minneapolis, MN 55426. JJ is

corresponding author: to contact, call (651) 649-5114 or

e-mail at jjuzwik@fs.fed.us.

Oak wilt, caused by the fungus Ceratocystis fagacearum

(Bretz) Hunt, affects oaks (Quercus spp.) in both the red

(Lobatae) and white (Quercus) oak groups. Multiple

species of both groups are common in urban and rural

forested landscapes of the Midwestern United States.

Following infection, red oaks wilt within weeks to months

and die in the same or the next summer. In white oaks,

infection causes scattered wilting of crown branches or

major limb or fork dieback in one to several years. Death of

white oaks does not usually occur until several years later.

Oak wilt is a significant problem in both urban and

suburban landscapes where trees are of higher value than

those in forested settings. Increasing development of

wooded areas in major urban centers of the Upper Midwest

has led to increases in oak wilt (Juzwik and Schmidt

2000). Various oak wilt control tools and integrated oak

wilt management regimes are used in such settings (Juzwik

et al. 2004). One such tool is the fungicide propiconazole

(PPZL).

The first studies involving intravascular injection of oaks

with PPZL for oak wilt control were conducted in 1987 on

live oaks (Quercus virginiana Mill. and Q. fusiformis Small)

in Texas (Appel 2001). A few published studies exist on

effective use of PPZL on oaks in the Northern United States

(Johnson 2001, Nair 1995, Osterbauer and French 1992,

Osterbauer et al. 1994). In the Northern States, current

operational use of PPZL by commercial arborists is based

on observation and experience, as well as on these

published reports. Arborists desire documentation of their

operational treatments to either test or validate their

application practices for PPZL.

In 1998, evaluation studies of operational treatments of red

and white oaks in the Minneapolis-St. Paul, Minnesota,

metropolitan area began in a cooperative effort of the USDA

Forest Service and a local arboricultural research company.

The objectives of the 5-year project were to 1) evaluate

efficacy of operational PPZL treatments of oaks used in an

effort to prevent belowground spread of C. fagacearum from

wilting red oak(s) to healthy red oaks; 2) evaluate efficacy

of PPZL operational treatments of bur (Quercus macrocarpa

Michx.) and white oak (Quercus alba L.) for preventive

treatment as in 1; and 3) evaluate efficacy of therapeutic,

operational treatments of bur and white oaks with PPZL.

This observational study involved re-visiting treatment sites
2

three times over 5 years to determine long-term efficacy of

PPZL in controlling oak wilt. No controls were included in

this evaluation study.

MATERIALS AND METHODS

Study Area and Trees

The trees monitored in these studies were located on 29

sites in 20 suburbs surrounding the Minneapolis—St. Paul,

Minnesota, area. The trees were commercially treated by

the cooperating arboricultural company for clients

requesting the service. Treated species of the red oak group

were northern red oak (Quercus rubra L.) and northern pin

oak (Q. ellipsoidalis Hill). Treated species of the white oak

group were bur oak and white oak.

Study Design

Three related studies were defined based on the intended

purpose of the chemical treatment. In the red oak

preventive study, operational treatment was intended to

protect apparently healthy red oaks located within root

grafting distance of recently wilted red oaks from

developing oak wilt symptoms. The intent of operational

treatment in the white oak preventive study was similar,

except the treated trees and the nearby C. fagacearuminfected trees exhibiting active wilt were bur and/or white

oaks. In the white oak therapeutic study, all white and bur

oaks on a site diagnosed with oak wilt were treated with

PPZL. Diagnosis of oak wilt was based on visual presence

of symptomatic foliage or crown wilting and, in some cases,

laboratory isolation of the pathogen. The extent of wilting

in the crowns of affected oaks ranged from 5 to 50 percent

at the time of PPZL treatment.

Treatment

All study trees were injected with PPZL at the standard

lower rate stated on the product label (Novartis Crop

Protection, Greensboro, NC): 10 ml product (1.4 g a.i or

.045 oz.) per 2.5 cm (1 in.) diameter of main tree stem at

1.4 m (4 ft) height added to the appropriate amount of

water (1.0 liter water for each 10 ml of PPZL). Established

protocols for macro-infusion were followed (Prosser et al.

not dated). At least four crews, each consisting of two

technicians and one crew leader, performed the injections.

A total of 14 individuals were assigned to the various crews

during the treatment period. In the spring before treatment,

all crews received training in standard PPZL macro-infusion

techniques. The trees were treated between June 17 and

September 28, 1998, which corresponded to the period

between full leaf expansion (late spring) and onset of fall

coloration. In the therapeutic white oak study in

subsequent years, selective pruning was performed on the

treated trees during winter to remove dead wood from the

tree crowns.

Data Collection and Tree Condition Assessment

Diameter of the tree stem at 1.4 m height (diameter at

breast height – d.b.h.) was recorded for each treated tree at

the time of PPZL injection. In the two preventive studies,

the distance between each treated tree and the nearest oak

with wilt symptoms was also determined and recorded.

The crown condition of each tree was assessed at the time

of PPZL treatment in 1998 and once during the summer in

1999 (first year), in 2000 (second year), and in 2003 (fifth

year) after treatment. The arborist firm developed a rating

system, which determined a score for each of the following

categories: characteristics of crown development, trunk

quality, branch structure, twig growth rate, foliage color,

incidence of insect and disease organism damage or

presence, and evidence of visible root problems. The

possible values of the rating system ranged from 5

(indicating best condition) to 0 (poorest condition) for

each category. These individual scores were then added to

produce an overall condition index for each tree, with 35

being the maximum value possible. Extent of crown wilt

present in the tree crown was estimated separately. The

percentage of wilt observed in the crown, as determined

from four different locations around the tree (cardinal

directions), was recorded once each summer during 1999,

2000, and 2003. In cases where landowners removed trees

that died in subsequent years, the year of tree death and

year of removal were recorded for each. Red oaks that

developed > 60 percent wilt in the last year of this study

were considered dead in the data summarization.

RESULTS

Red Oak Preventive Study

The 46 PPZL-treated red oaks ranged in size from 13 to 89

cm (5 to 35 in.) d.b.h. They were located from 4.6 to 15.2

m (15 to 50 ft) from the nearest C. fagacearum-infected
3

tree. Oak wilt was first observed in the treated trees in

1999 when one (2%) began wilting late in the growing

season and died the following year (table 1). An additional

17 trees (37%) wilted between early September 2000 and

late August 2003. Three of these were observed with active

wilt in 2003. Mortality due to C. fagacearum was higher in

trees < 9 m (30 ft) from previously wilted trees compared

to treated trees between 9 and 15 m (50 ft) from the

nearest pathogen-killed tree (P = 0.08). Of the 18 trees that

died over the 5 years, 13 (72%) were less than 9 m (30 ft)

from the nearest C. fagacearum-killed tree in 1998 when

PPZL was injected. Tree mortality due to C. fagacearum was

also higher (44%, or 8 of 18 trees) in larger diameter trees

(d.b.h. > 48 cm or 19 in.) (P = 0.10). The remaining live

trees showed no change to slight improvement in tree

condition when ratings between 2003 and 1998 were

compared.

White Oak Preventive Study

The 26 PPZL-treated white and bur oaks ranged in size

from 14 to 89 cm (6 to 34 in.) d.b.h. They were located

from 0.6 to 13.7 m (2 to 45 ft) from the nearest C.

fagacearum-infected white or bur oak. Only one tree (a

white oak) exhibited oak wilt symptoms over the 5-year

period, and none died. A few symptomatic branches

(< 10% of the crown) were observed on the one affected

Table 1.—Mortality observed in red oaks operationally treated with propiconazole in 1998 for prevention of root graft spread of

Ceratocystis fagacearum

 Distance to Tree No. treated Tree mortality by year Cumulative mortality (5 yrs)

diseased tree diameter trees 19981 1999 2000 2001-2003 Number Percent

 (m) (cm)

≤ 9 ≤ 33 11 0 0 0 4 4 36

> 33 to 48.3 5 0 0 0 3 3 60

> 48.3 10 0 02 1 5 6 60

> 9 to 15 <33.02 9 0 0 0 2 2 22

> 33 to 48.3 7 0 0 0 1 1 14

> 48.3 4 0 0 0 2 2 50

Total 46 0 0 1 17 18 39

1

Year trees were treated.

2 Wilting crown observed late in growing season, but did not result in mortality in 1999.

tree in 1999 and 2000, but no new symptoms were

observed in 2003. The condition index for all of the other

trees did not change appreciably over the 5-year period.

White Oak Therapeutic Study

The eight bur oaks injected with PPZL ranged in size from

42 to 86 cm (17 to 34 in.) d.b.h. The initial percentages of

tree crowns exhibiting wilt symptoms ranged from 5 to 30

(table 2). During the 5-year period, only one tree exhibited

new oak wilt symptoms (5% increase) and this occurred

only in 2000. The tree was injected with PPZL again in

2000 and dead branches were removed; no evidence of

new wilt was found in 2003. Dead branches were pruned

from tree crowns during winters in subsequent years after

injection. Dead branch pruning of the tree crowns in 1998

resulted in slightly decreased (< 1%) crown size (table 2).

Tree condition index over the 5 years differed by tree size.

The index slightly decreased for bur oaks < 56 cm (22 in.)

d.b.h., but increased for those > 56 cm.

The 13 white oaks (Q. alba) treated with PPZL ranged in

size from 25 to 69 cm (12 to 27 in.) d.b.h. The initial

percentages of tree crowns exhibiting wilt symptoms

ranged from 5 to 45 (table 2). Of eight oaks < 56 cm (22

in.) d.b.h., one exhibited new oak wilt symptoms in 1999,

but none of the trees exhibited symptoms in 2000 and
4

Table 2.—Incidence and extent of crown wilting in operationally treated bur and white oaks infected with Ceratocystis fagacearum prior to and following propiconazole treatment

Oak species D.b.h. No. trees Intitial percent wilt (1998) Trees exhibiting new crown wilt Cumulative percent of crown

(cm) evaluated Average Range Year Number Crown wilt removed by 2003

 (percent) Average Range

Bur 42-86 8 16 5-30 1999 0 0 16 5-30

2000 1 5 17 5-30

2001-2003 0 0 17 5-30

White < 56 8 14 5-35 1999 1 5 14 5-35

2000 0 0 14 5-35

2001-2003 0 0 14 5-35

≥ 56 5 21 5-45 1999 1 25 26 5-45

2000 2 5, 5 28 5-50

2001-2003 0 0 28 5-50
5

2003. Of five trees > 56 cm d.b.h., one tree had new wilt in

1999 and two had new wilt in 2000. Pruning of wilting

and other dead branches from tree crowns resulted in a

slight (2%) reduction in crown size. Tree condition index

over the 5 years differed by tree size. The index slightly

decreased for white oaks < 56 cm (22 in.) d.b.h., but

slightly increased for those >56 cm.

DISCUSSION

Red Oak Preventive Treatment

The observed 100-percent survival rate of red oaks over 2

years and the 61-percent survival rate over the total 5 years

of this study suggest that preventive macro-infusion

treatments with the PPZL product at 1.4 g a.i./ 2.54 cm

d.b.h. (0.045 oz/1 in.) provides limited suppression of oak

wilt spread via root grafts. It appears that the ability of the

chemical treatment to suppress disease progression

decreases over time (years). Many factors likely influence

efficacy of PPZL in preventing root graft spread of the

pathogen. Such factors include distances over which root

grafts can be expected, frequency of root graft transmission,

rapidity with which C. fagacearum moves through

connected root systems, longevity of PPZL activity in trees,

and PPZL application rate.

Root graft transmission of the oak wilt pathogen to healthy

oaks commonly occurs within 15 m (50 ft) of a diseased

oak of the same species. Heimlick and Fox (1961) reported

pathogen spread and subsequent death in trees up to 15 m

(50 ft) from a disease source. In another study in which

trees were injected with radioisotopes, radioisotopes were

detected in trees up to ~15 m (50 ft) from the injected tree,

presumably translocated between trees through root grafts

(Kuntz and Riker 1955). The rate of spread of oak wilt

through root grafts, especially in red oaks, can be as much

as 12 m (40 ft) per year (Bruhn et al. 1991). In our study,

all preventively treated red oaks were less than 15 m (50 ft)

from diseased trees and thus had a high probability of

becoming infected with C. fagacearum within several years

after the death of the adjacent oak wilt-killed tree.

The length of time required for transmission of the fungus

from infected to adjacent healthy trees varies. By some

accounts the transmission from an oak wilt-killed tree to a

healthy tree via root grafted roots is rapid due to the loss of

upward vascular movement caused by vascular plugging in

the diseased tree (Menges and Kuntz 1985). However,

Himelick and Fox (1961) found that while the majority of

oak wilt centers remained active year after year, a few other

centers remained dormant for up to 6 years and then reactivated, i.e., newly wilted trees were observed. With such

a wide time span over which transmission may occur, it is

possible that the pathogen can spread after the PPZL is

degraded or no longer active in a treated tree. Osterbauer

and French (1992) injected trees with PPZL at a rate of

0.43 g a.i./ 2.54 cm d.b.h. (0.014 oz / in.). They were able

to detect the PPZL activity at 1, 2, 8, and 12 months after

injection, but not 20 or 23 months after injection.

Comparing these earlier results to our study suggests that

re-treatment with PPZL two growing seasons after initial

treatment may be beneficial. For example, if a tree is

treated in year 1, re-treatment would seem appropriate just

before fall leaf coloration of year 2, or soon after full leaf

expansion in year 3.

Chemical application rate is an additional factor to

consider. Red oaks injected with 0.42, 0.56, 0.7, and 0.84

g PPZL / 2.54 cm d.b.h. (0.015, 0.020, 0.024, and 0.030

oz / in.) to prevent root graft transmission of the pathogen

had a combined survival rate of 100 percent at < 14

months and 76 percent at 24 months (Johnson 2001).

Trees treated with the highest rate (0.84 g PPZL / 2.54 cm

d.b.h.) had a 95-percent survival rate after 2 years. Results

of our study can be compared to those of Johnson (2001).

Treatment with PPZL at 0.84 g a.i. / 2.54 cm d.b.h.

(Johnson 2001) is as successful over a 2-year period as 1.4

g a.i./ 2.54 cm d.b.h. (0.045 oz /1 in.) (our study), while

lower rates (Johnson 2001) had lower success rate in

preventing wilt 2 years after injection. Perhaps increasing

the application rate may lengthen the time of disease

suppression.

White and Bur Oak Preventive Treatments

Preventive treatment of white and bur oaks with PPZL has

been proven to be very effective in protecting these trees

from oak wilt symptom development, perhaps by

preventing pathogen spread of C. fagacearum through root

grafts. Although preventive treatment is effective, the

current viewpoint of arborists is to not treat preventively

due to the observed success of operational therapeutic

treatments. An arborist will usually not treat a white or bur

oak unless the tree is in an area where disease pressure and

aesthetic value are very high and/or the client requests

treatment.
6

White and Bur Oak Therapeutic Treatments

PPZL applications provided very good control of oak wilt

in white and bur oaks already infected at time of treatment.

This finding supports the commonly recommended

treatment of these species. Although preventive treatment

of white and bur oaks at high risk for oak wilt is valid,

therapeutic treatment of infected white and bur oaks, when

done promptly, arrests disease progression with minimal

crown loss. In a previous study, trees with up to 50-percent

affected crown were successfully treated with PPZL

(Osterbauer 1991, Osterbauer et al. 1994).

Concluding Remarks

Results of our studies provide valuable documentation on

the efficacy of PPZL in operational preventive treatment of

red and white oaks and in therapeutic treatment of white

oaks. However, further studies with untreated controls and

randomly assigned treatments are needed to provide

additional efficacy information on preventive treatments

particularly for red oak species. Such studies are currently

underway at the University of Minnesota and at the North

Central Research Station, USDA Forest Service.

LITERATURE CITED

Appel, D.N. 2001.

The use of Alamo for oak wilt management. In: Ash, Cynthia

L., ed. Shade tree wilt diseases. St. Paul, MN: APS Press:

101-106.

Bruhn, J.N.; Pickens, J.B. 1991.

Probit analysis of oak wilt transmission through root grafts in

red oak stands. Forest Science. 37: 28-44.

Himelick, E.B.; Fox, H.W. 1961.

Experimental studies on control of oak wilt disease. Illinois

Agricultural Experiment Station Bulletin. 680: 1-48.

Johnson, J. 2001.

Effects of propiconazole fungicide on fungal mat formation by

Ceratocystis fagacearum and the long distance spread of oak

wilt from infected black and northern pin oaks. Green Bay, WI:

University of Wisconsin–Green Bay. 111 p. M.S. thesis.

Juzwik, J.; Schmidt, T. 2000.

Oak wilt and oak decline in the Upper Midwest USA. In:

Oszako, T.; Delatour, C., eds. Recent advances on oak

health in Europe. Warsaw, Poland: Forest Research

Institute: 139-145.

Juzwik, J.; Cook, S.; Haugen, L.; Elwell, J. 2004.

Oak wilt: people and trees, a community approach to

management. Gen. Tech. Rep. NC-238. St. Paul, MN: U.S.

Department of Agriculture, Forest Service, North Central

Research Station. CD-Rom version 2004 v 1.3.

Kuntz, J.E.; Riker, A.J. 1955.

The use of radioactive isotopes to ascertain the role of root

grafting in the translocation of water, nutrients, and diseaseinducing organisms among forest trees. In: Proceedings

International conference on the Peaceful uses of atomic

energy. New York, NY: United Nations. 12: 144-148.

Menges, E.S.; Kuntz, J.E. 1985.

Predictive equations for local spread of oak wilt in southern

Wisconsin. Forest Science. 31: 43-51.

Nair, V.M.G. 1995.

Chemotherapeutic control of oak wilt by the use of “Alamo”

propiconazole. Alamo Research Symposium, Bloomington,

MN. 28 p.

Osterbauer, N.K. 1991.

Evaluation of propiconazole as a treatment for oak wilt in two

species of Quercus. St. Paul, MN: University of Minnesota.

41 p. M.S. thesis.

Osterbauer, N.K.; French, D.W. 1992.

Propiconazole as a treatment for oak wilt in Quercus rubra

and Q. ellipsoidalis. Journal of Arboriculture. 18: 221-226.

Osterbauer, N.K.; Salisbury, T.; French, D.W. 1994.

Propiconazole as a treatment for oak wilt in Quercus alba and

Q. macrocarpa. Journal of Arboriculture. 20: 202.

Prosser, T.; Zwack, J.; Johnson, B. not dated.

Macro-infusion guide. St. Louis Park, MN: Rainbow Treecare

Scientific Advancements.