Nursery Crop Science
Sudden Oak Death Survey and Other Disease Updates
Colleen Y. Warfield and D. Michael Benson
Department of Plant Pathology
North Carolina State University
A pilot survey of commercial nurseries in five southern states in the Appalachian
region, including North Carolina, is currently underway in cooperation with
the USDA. This survey is voluntary and was designed as a pilot program to gather
information on the incidence and distribution of the disease known as Sudden
Oak Death (SOD) or Phytophthora canker caused by the pathogen Phytophthora ramorum.
This disease occurs in forests in California and Oregon; has been reported in
one nursery in California and more recently in an Oregon nursery; and in nurseries
in the Netherlands, Germany, the United Kingdom as well as other European countries.
The recent discovery of P. ramorum infecting Pieris and Viburnum in an Oregon
nursery was an uncomfortable reminder of the potential threat of this disease
to the nursery industry in North Carolina. There is no current evidence that
suggests this pathogen has been introduced into North Carolina or the southeastern
United States. However, due to the shipment of plant material from potentially
infested areas of California, Oregon and Europe the potential exists for this
pathogen to be introduced into North Carolina sometime in the future.
While we do not expect to find P. ramorum in our nursery survey, we are also
isolating and identifying any species of Phytophthora causing aerial dieback
that we do find on the crops surveyed. This information, together with the data
obtained from the four other southeastern state surveys, should enable us to
determine the predominant species of Phytophthora affecting nursery crops in
this region of the country. Knowing the occurrence of Phytophthora species in
specific geographic regions will help to identify pathways that this fungus
may using in order to spread across both native habitats as well as state borders.
In addition, identification of specific Phytophthora species may have important
implications in the future management of Phytophthora diseases. Data is currently
being generated, but a trend has begun to emerge which suggests that high levels
of fungicide resistance to mefenoxam (Subdue MAXX) are being observed within
certain species of Phytophthora.
The symptoms caused by P. ramorum are not unlike those observed for the more
common Phytophthora species found in nurseries. One key difference is that P.
ramorum has thus far only been found to infect the aerial parts of plants including
the leaves, green and woody stems, but not the roots. Preliminary studies in
California have shown that a film of water on the leaves must be present for
a minimum of 12 hours in order for leaf (in this case, bay laurel) infection
to occur. Temperatures in the 15-20∞C range were most favorable for spore
production. Therefore, we are probably more likely to see symptoms caused by
P. ramorum during the early spring months in North Carolina, as opposed to late
spring and early summer as would be more typical with the common Phytophthora
species.
Today we will discuss the symptoms associated with this pathogen, modes of pathogen
survival, and steps you can take to help minimize losses in your nursery should
this pathogen be introduced.
