Research Accomplishment Reports 2007

Early Detection Survey for Phytophthora

J. Hartman
Department of Plant Pathology

 

Project Description

Phytophthora ramorum, a fungus-like water mold, causes leaf and shoot blight of many woody plants in forests and nurseries and stem cankers on most oak species and tanoak (sudden oak death).  The disease occurs in U.S. West coast states and is not known to exist in the forest in Kentucky and other eastern states.  The possibility that the fungus has already been brought into Kentucky on nursery stock or in soil on visitors’ shoes and car tires is real.  P. ramorum can be spread locally via rain splash, water courses, and air currents.  Early detection of P. ramorum here would allow USDA officials to identify contaminated areas and quickly eradicate the infection so that the disease doesn’t spread throughout Kentucky forests.
           

The USDA APHIS promoted the comparison of eight national scale risk models for P. ramorum, and the Northwest Coast and the central Appalachian Mountains were consistently considered high risk areas. P. ramorum has not been detected in the central Appalachian Mountains, where its introduction could have serious consequences for the state of Kentucky. Forty seven percent of the state is covered by 11.9 million acres of forests. Many understory, mid story and upper story plants found in these forests are known hosts of P. ramorum. A recent report showed that several plant species of major importance to the state and to the Appalachian range are susceptible to infection by P. ramorum.  Thus, Kentucky is in a high risk area with many known susceptible plants.

This pathogen is a nursery and forest problem and its introduction and establishment in the state could have a considerable impact on the nursery and forestry industries and cause substantial economic losses. Early detection and eradication of diseased plants followed by good practices for control of diseases in the nurseries are important to protect Kentucky’s forest resources and forestry industry as well as the nursery and landscape industry from Phytophthora ramorum.

Terrestrial surveys for P. ramorum are expensive and not very efficient, so we tested a stream baiting protocol that would allow sampling at a single stream-side site to be representative of a 5,000 to 10,000 acre watershed.  It has been shown that a single infected plant within such a watershed 2-3 miles upstream from the sampling site will produce enough swimming spores to be attracted to baits placed at the sampling site.  The baits used were nylon net bags containing healthy rhododendron leaves and placed in selected streams for a week or two at a time.  Streams were sampled monthly during the 2007 growing season.  Using stream baiting, Phytophthora swimming spores will colonize these leaves and form lesions.  At the end of each immersion period, infected leaves were removed from the bait bags and taken to the laboratory and cultured and tested for presence of P. ramorum.

The survey for this potentially invasive pathogen was conducted by scientists from the Plant Pathology and Forestry departments at University of Kentucky.  The problem of potential P. ramorum spread is being taken seriously by the U.S. Forest service which sponsored the early detection surveys in Kentucky and the many other Eastern and West Coast states that also participated in the project.

Impact

Eight streams were baited over a five month period and bait bags were placed within private property with consent of the owners where it was possible to do so. This allowed for protection of the bags against tampering and for easy access for researchers placing and collecting the bags. Streams not within private property were sampled with permission for sampling for research purposes from the appropriate organizations.

Two of the sampled streams baited Clemons Fork and Coles Fork (with watersheds of 4,000 and 5,000 acres respectively), are located within the Robinson Forest a University of Kentucky Research Forest. A third forested stream, Indian Creek (7,700 acre watershed) is located within the Daniel Boone National Forest. The other five streams drain watersheds that receive water from urban and agricultural areas and include nurseries and garden centers. Cane Run Creek drains a watershed of approximately 10,000 acres that is subject to high tourist activity and animal transit. Town Branch is within a large watershed of 14,700 acres that receives drainage from downtown Lexington and agricultural areas. The South Fork of the Elkhorn Creek has a 6,500 acre watershed receiving runoff from urban areas of Lexington. North Elkhorn Creek has an 11,000 acre watershed that drains primarily agricultural parts of Lexington, but also one of the largest new development sites in the city. Gunpowder Creek is within a 6,700 acre watershed that drains parts of Florence in northern Kentucky, including the Cincinnati-Northern Kentucky International Airport.
The survey was carried out for five months and eight streams were sampled with two bags at each sampling period resulting in 80 baiting opportunities. Some bags were lost and sampling was not possible in two streams in late summer due to drought. At the end of the of the five month sampling period there were 72 successful baiting trials yielding composite leaf samples for analysis. Leaves collected from one bait bag where used for isolations in the Plant Pathology Diagnostic Laboratory and many Phytophthora species were isolated, but not Phytophthora ramorum. The leaves collected from the second bag were sent to a partnering lab for analysis by PCR as an independent secondary diagnostic.

          Phytophthora ramorum was not isolated by culturing and no samples were positive in PCR for this pathogen in Kentucky in 2007.   This survey provides some reassurance to both the forest industry and nursery industry that P. ramorum is not yet present in Kentucky and that continued monitoring provides the opportunity to eradicate the disease early should it ever arrive here.

Publications

de Sá, P.B., Bateman, A., Hartman, J., Barton, C., Patel, R., and Adank, K.  2008.  Early Detection Survey for Phytophthora ramorum in Kentucky in 2007.  Nursery and Landscape Program 2007 Research Report, University of Kentucky Agricultural Experiment Station PR-554.