Scientists & Staff

Jennifer Juzwik

Research Plant Pathologist
1561 Lindig Ave.
St. Paul, MN, 55108
Phone: 651-649-5114

Contact Jennifer Juzwik

Current Research

As a Research Plant Pathologist, I conduct studies on invasive tree pathogens and associated insects. These studies are often conducted in collaboration with scientists, forest health specialists, and foresters in federal, state and county agencies as well as universities. Practical guidelines ranging from preventing introduction of invasive pathogens to managing or mitigating diseases caused by established pathogens are the outcomes of these studies. I also collaborate with US Forest Service Northeastern Area State and Private Forestry and state agencies in detection and monitoring activities for emerging diseases. My current research efforts concern:

  • Hickory decline and dieback research
    • Determine frequencies of decline/dieback and mortality of smooth bark hickories in appropriate forest cover types where deviations from expected levels of mortality have been observed (Indiana, Iowa, Minnesota, Ohio, New York, and Wisconsin).
    • Quantify relationships between decline/dieback incidence and a) pathogen and/or insect pest presence, and b) prior land use, fire history, soils and drought.
    • Determine the role of two newly described Ceratocystis species in decline/dieback and mortality of bitternut hickory.
  • Oak wilt research
    • Investigate temporal and spatial aspects of belowground transmission of C. fagacearum in relation to current disease management strategies. These studies include evaluation of the efficacy of propiconazole and of mechanical root graft disruption in operational, oak wilt control programs.
  • Thousand Cankers Disease (TCD)
    • Collaborate with state agencies on visual surveys for the disease in Iowa, Minnesota and Missouri
    • Evaluate trap tree method for early detection of walnut twig beetle in eastern black walnut in Tennessee and incorporate the method in Indiana and Missouri surveys for TCD
    • Investigate other bark and ambrosia beetles known to attack eastern black walnut for their potential association with the TCD pathogen, Geosmithia morbida

Research Interests

I would like to develop two areas of interest into research projects in the near future:

  • Molecular probes for detection of selected pathogens on associated insects.
  • Describe the major biotic and abiotic factors negatively impacting the health of Colorado blue spruce and of white spruce in Midwestern, urban landscapes.

Why This Research is Important

Oak-hickory forests comprise 29% of the 31 million hectares of forest land in the Upper Midwest (IL, IN, IA, MI, MN, MO and WI). The oak species group is the most important aggregation of hardwoods in the United States. Oak decline and oak wilt are responsible for much of the observed oak deterioration and mortality in the midwestern forests. In addition, new exotic pathogens, such as Phytophthora ramorum (cause of Sudden Oak Death) and Raffaelea quercivora (cause of Japanese oak wilt), are potential threats to the health of oaks in these forests. Our oak disease research addresses both the need for early detection of any unintentionally introduced exotic pathogens and for new or refined management tools for well-established pathogens, such as Ceratocystis fagacearum (cause of oak wilt in the USA).

Hickories are an important component of many forest associations in the eastern United States, particularly various oak-hickory cover types. Sites impacted by hickory decline or dieback and mortality have recently been reported to lose a high proportion of smooth bark hickories, particularly bitternut, over a very short period of time (3 to 5 years). These losses cause a significant, adverse impact to wildlife, timber value and biodiversity in affected stands. Our goal is to identify significant biotic and abiotic contributors to the decline or dieback and develop site or silvicultural guidelines for mitigating the tree losses and stand impacts associated with this problem.

Eastern black walnut (Juglans nigra) is the most economically important hardwood species in the eastern USA. Lumber from this species has a high market price and exports from the USA exceed $ 40 million annually. The species is also important for nut production, wildlife habitat, riparian buffers, windbreaks and other ecosystem services. Thousand cankers disease, caused by an interaction of the walnut twig beetle (Pityophthorus juglandis) and a fungus (Geosmithia morbida), is a threat to the health of eastern black walnut.


  • University of Minnesota, Ph.D. Plant Pathology, 1983
  • Colorado State University, M.Sc. Plant Pathology, 1978
  • Fairmont State College, B.Sc. Biology, 1976

Professional Organizations

  • Minnesota Society of Arboriculture
  • Minnesota Shade Tree Advisory Committee
  • American Phytopathological Society
  • International Society of Arboriculture
  • Northeastern Nursery Association

Featured Publications & Products

Publications & Products

National Research Highlights

Eastern black walnut branch sample collected for dissection, observation of damage types present, and isolation for fungi associated with each damage type.

Matching Causes with Symptoms: Research Improves Diagnosis of Declining Eastern Black Walnut

Year: 2020

Visual diagnosis of thousand cankers disease (TCD) in a declining black walnut is difficult in the Eastern United States because the general dieback symptoms are typical of several different disease and insect problems. Work by a Northern Research Station scientist and her partners will aid plant health specialists and laboratory diagnosticians in determining causal agents of branch dieback in this valuable hardwood species

Researcher inserting temperature probes in oak wilt
fungus-colonized oak logs prior to vacuum steam treatment.

Vacuum Steam Heat: A New Method to Kill Oak Wilt Fungus in Logs

Year: 2018

International export of oak logs from the United States requires fumigation of the logs with methyl bromide. However, methyl bromide causes severe damage to the Earth’s ozone layer. Scientists recently discovered that using vacuum steam heat to treat oak logs kills the oak wilt fungus and is environmentally friendly.

Image 1: Obtaining branch sample from white oak for evaluation of new oak wilt diagnostic tool. USDA Forest Service
Image 2: Drilling to obtain sapwood shavings for fungal DNA extraction.

New tool detects oak wilt fungus faster and more accurately

Year: 2017

Oak wilt is one of several significant diseases threatening the health of oak trees in the U.S. and is a potential threat worldwide. Accurate and timely laboratory diagnosis is critical to controlling the disease. A Forest Service scientist and collaborator have developed and validated two molecular-based diagnostic tools that greatly exceed standard isolation methods in time required and accuracy.

Thousand Cankers Disease affected eastern black walnut trees used to determine insects emerging and carry the TCD fungus. Jennifer Juzwik, U.S. Department of Agriculture Forest Service.

Ambrosia Beetles and Bark-Colonizing Weevils Carry Thousand Cankers Disease Fungus

Year: 2016

Thousand cankers disease (TCD) is a threat to the health of eastern black walnut, a highly valued species for timber and nut production in the eastern United States. Two ambrosia beetle species and a bark colonizing weevil species were found to carry the TCD fungus, in addition to the previously known walnut twig beetle that carries and transmits the fungus to healthy trees. These findings are important to the state and federal monitoring and management efforts for this disease.

City forester collecting branch sample from actively wilting white oak tree. USDA Forest Service

DNA-based Method Enhances Detection of the Oak Wilt Fungus

Year: 2015

Oak wilt is a major cause of tree death in the eastern United States. Symptoms are slower to develop in white oaks species than red oaks and oak wilt in all species may be confused with damage caused by other organisms. A Forest Service scientist and her research partners tested a commercial DNA-based detection method for use by plant diagnostic laboratories. They found that it greatly enhanced the ability to diagnose the fungus in oak wilt-affected trees.

Dorsal and lateral views of the bark-colonizing weevil found to carry the Thousand Cankers Disease fungus in Indiana. Janet C. Ciegler

Thousand-Cankers Disease Fungus Found in Indiana

Year: 2014

Thousand-cankers disease (TCD) is caused by the canker-causing fungus Geosmithia morbida when carried by the walnut twig beetle. In an Indiana-wide trap tree survey of ambrosia beetles, bark beetles, and weevils colonizing stressed eastern black walnut, scientists detected the fungus on three bark-colonizing weevils from two trees on one site. This is the first report of the pathogen in Indiana and the first report of the fungus from an insect species other than the walnut twig beetle.

“Tooth-pick” like structures of insect frass of the black stem borer attacking a walnut tree. J. McKenna, USDA Forest Service

Eastern Black Walnut Trees Plagued by More Than Thousand Cankers Disease

Year: 2013

Thousand cankers disease, caused by the interaction of the walnut twig beetle and the fungus Geosmithia morbida, has been detected in four eastern states in the native range of eastern black walnut. However, other wood-infesting beetles can attack and kill stressed walnut of all ages.

Last modified: Thursday, February 22, 2018