Pathways of movement for Phytophthora ramorum, the causal agent ...

Davidson, J. M., and Shaw, C. G. 2003. Pathways of movement for Phytophthora ramorum, the
causal agent of Sudden Oak Death. Sudden Oak Death Online Symposium.
www.apsnet.org/online/SOD (website of The American Phytopathological Society). doi:10.1094/SOD-2003-TS
Pathways of movement for Phytophthora
ramorum, the causal agent of Sudden Oak
Death
Jennifer M. Davidson and Charles G. (Terry) Shaw
Phytophthora ramorum and the suite of diseases it causes,
including Sudden Oak Death, are relatively new to science. As
such, there are far more questions than answers regarding nearly
all aspects of this pathogen's biology. However, since the
discovery of P. ramorum 3 years ago, much has been learned
about the transmission biology of this pathogen, especially in oak
woodlands.
BACKGROUND
Phytophthora species have been studied for nearly 150 years, so
extensive background knowledge exists on the transmission
biology of this genus. In general, Phytophthora species that infect
aerial parts of plants spread through a cycle of production of
sexual, or more often, asexual spores, movement of spores, and
infection of new host tissue. The new infection can then serve as
another source of spores to begin the cycle again. Appropriate
environmental conditions (temperature and relative humidity), as
well as survival of the pathogen (either as spores or mycelia), are
necessary for each step of the cycle. Transport of infected host
tissue harboring a viable pathogen can also introduce
Phytophthora to new areas where the pathogen may establish if it
can complete the cycle described above.
The first step, production of spores, occurs on or within living
plant tissue. P. ramorum has thus far only been found to infect
aerial parts of plants, including leaves, green stems, and woody
stems, but not roots. At 15 - 20 ºC, in culture and on foliar hosts
such as bay and rhododendron, P. ramorum readily forms
sporangia that are highly deciduous, a characteristic consistent
with dispersal from aboveground plant parts. Both attached and
detached sporangia of P. ramorum produce zoospores (asexual
spores) in culture. In addition, P. ramorum prolifically produces
chlamydospores (asexual spores) in culture and on some foliar
hosts, indicating a possible mechanism for dormancy and survival
in adverse conditions. Oospores (sexual spores) have not been
observed in nature, perhaps because P. ramorum appears to
require two mating types to produce sexual spores. Currently, the
European population only has one mating type, and the North
American population has the opposite mating type.
Once spores are produced by aboveground Phytophthora species,

they are commonly dispersed through several mechanisms 2 , 3 .
Often the spores are splashed or carried to the ground by rain or
sprinkler drops. Spores landing in streams or irrigation runoff then
potentially can travel long distances. Likewise, spores landing in
the soil of a forest or nursery floor can be transported in soil
sticking to shoes, equipment, or vehicles. For a few Phytophthora
species, such as P. infestans, sporangia are dispersed in air
without water drops (e.g., without rain, sprinklers). Finally, spores
and hyphae may be spread through movement of infected plant
material. As described below, spores of P. ramorum, like spores of
the majority of aboveground Phytophthora species, are known to
move via all of these mechanisms except dispersal in air without
water drops.
A final step in the reproductive cycle involves successful infection
of new host tissue. Reaching a susceptible host is essential. In
addition, preliminary studies indicate that optimal infection of bay
leaves by zoospores of P. ramorum occurs at 20° C. A film of
water on the leaves for 12 hours appears to be necessary for
infection by P. ramorum (D. Huberli, unpublished data).
PATHWAYS
Research to determine each component of the transmission cycle
for P. ramorum will allow us to better evaluate possible pathways
for pathogen spread. These pathways include transmission in
natural ecosystems and spread through human activities such as
commerce and recreation. Below, we present brief summaries
characterizing pathways for both of these main areas. The concern
is basic - movement of spores or infected plants may serve as a
pathway for pathogen spread. What do we know about the
mechanisms and potential for this movement to actually happen?
Pathways in natural ecosystems.
The life cycle of P. ramorum and mechanisms of spread have been
studied in coast live oak and tanoak woodlands in northern
California. Although all of the host species (currently, there are
more than 20) have not been examined, dominant hosts such as
coast live oak, tanoak, coast redwood, and bay are being tested
for spore production. A major source of spores in these forests
appears to be infected bay leaves 1 . To date, sporulation has not
been observed on intact bark of oak or tanoak trunks. Studies are
underway to determine whether sporulation occurs on tanoak and
redwood leaves and small branches.
Once spores are formed in these forests, rain carries them onto
other plants, into streams, or onto soil 1 . P. ramorum has been
detected in streams throughout the rainy season, and in some
cases, throughout the dry summer months (Davidson, P. Maloney,
S. Tjosvold, unpublished data). However, it is unknown whether
spores can move from stream water up to infect new plant tissue.
Given that infection patterns in forests do not seem to follow
stream courses, this may be a rare event. P. ramorum has also

een detected in soil during the rainy, winter months. Hikers have
been shown to carry the spores of P. ramorum on their shoes as
they leave infested tanoak-redwood forests (S. Tjosvold,
unpublished data). In addition, soil from car and mountain bike
tires has tested positive for P. ramorum after these vehicles
traveled on dirt roads through infested forests (S. Tjosvold,
unpublished data). It is likely that woodland animals such as deer
also carry spores in soil stuck to their feet. Laboratory
experiments indicate that inoculum in soil can infect green leaf
litter, which can then transmit P. ramorum to aerial plant parts via
rainsplash (Davidson, unpublished data).
While these pathways provide evidence to explain the rapid
spread of P. ramorum within a forested site, the mechanisms
underlying long-distance jumps of P. ramorum-- for example over
250 km from Redway, California to Brookings, Oregon-- are still
unidentified. Clearly, long-distance transport of infested soil via
shoes, vehicles, or equipment may be one route of possible longdistance
spread. Anecdotal evidence from northern California
suggests that landscaping with infected horticultural plants at
homes within woodlands adjacent to public wildlands may have
introduced P. ramorum to new areas. Studies are underway to
investigate the potential role of birds in moving spores over longdistance
flight patterns. To date, wind dispersal of P. ramorum
without rain has not been demonstrated.
Pathways of spread through human activity.
Nursery stock. Although there are numerous nursery hosts,
information on transmission of P. ramorum is best known for
rhododendron, typically one of the most susceptible plant genera
to Phytophthora. P. ramorum has been isolated from
rhododendrons in nurseries in the U.S. (California) and Europe.
The California Department of Agriculture has traced pathogen
transmission from one nursery to another on rhododendron stock.
In Europe, P. ramorum was introduced to Majorca, Spain via a
shipment of infected rhododendrons, and many of the infections
found in nurseries in the United Kingdom can be traced to plant
transport from other nurseries. P. ramorum sporulates prolifically
from rhododendron leaves, producing both sporangia on the leaf
and chlamydospores on or within the leaf (D. Rizzo, P. Tooley,
unpublished data) (Figure 1a, b). Detached rhododendron leaves
that were dried for up to 3 months still produced sporangia upon
wetting (D. Rizzo, unpublished data). In laboratory inoculation
experiments, infection spread from one rhododendron leaf to
another via water drops that presumably carried sporangia or
zoospores (D. Rizzo, unpublished data). P. ramorum has been
isolated from irrigation water from an infested rhododendron
nursery 4 ; (S. Tjosvold, unpublished data). Fungicides commonly
used to control other Phytophthora species on rhododendron may
mask symptoms of P. ramorum, making visual diagnosis difficult.
In addition, in infested nurseries, soil or mulch in the pots of
rhododendron plants, other host plants, and even unsusceptible
plants may contain spores of P. ramorum although the plants
appear healthy.

Figure 1a. Sporangia are readily produced on susceptible rhododendron
leaves.
Figure 1b. Chlamydospores are also produced on susceptible rhododendron
leaves.
Bark and wood products. P. ramorum has been isolated from
external cankers on stems of oaks and tanoaks, including cankers
on fallen trees. Sporulation has occurred from flooded chips of
infected tanoak and the flooded, cut edges of coast live oak
cankers, indicating that mulch or firewood may be infective
(Davidson, E. Hansen, unpublished data) (Figure 2). However,
sporulation has not been observed on the outside, intact bark of
infected oak or tanoak logs. On oak, the pathogen has been
recovered 3 cm into the wood (D. Rizzo, unpublished data).
Therefore, debarking is not a sufficient treatment to eradicate the
pathogen from oak wood. It is unclear, however, if the pathogen
is infective directly from the surface of exposed wood. Heating
and drying are recommended as a treatment to destroy P.
ramorum in these hardwood species. However, studies are needed
to determine if chlamydospores are produced in bark (phloem)

and wood (xylem), and if so, whether these spores are destroyed
by drying. Techniques should be developed to assess dormancy
(viability) versus death of chlamydospores. Acorns do not appear
to be infected in the field, although spores may land on the
outside of acorns. To date, P. ramorum has not been found to
infect the main trunk of Douglas-fir or coast redwood. Studies are
underway to determine if redwood bark mulch contains spores
that may have landed on the outside of the bark while the tree
was standing in an infested forest. Soil on felled trees or logging
equipment from infested forests may also contain spores.
Figure 2. Hyphae of P. ramorum extend from the cut edge of a coast live oak
canker into water in a laboratory container. A similar process may occur if
infested firewood or chopped logs are stored in areas where water puddles.
Hyphae and spores of P. ramorum have not been observed on the outer
surface of intact bark of coast live oak or tanoak cankers.
Garlands, wreaths, and Christmas trees. Leaves and branches
of hosts such as bay laurel, Douglas-fir, and redwood are used in
wreaths and garlands. Douglas-fir is farmed for Christmas trees.
Some of these forest products are grown or manufactured within
infested counties in California and previously have been sold
throughout the United States. As mentioned above, P. ramorum
readily sporulates from bay leaves under moist, temperate
conditions. In addition, chlamydospores are formed in and on bay
leaves. Any treatment to kill P. ramorum in bay leaves would need
to destroy chlamydospores. Again, it is crucial to be able to
determine whether chlamydospores are dormant or dead. P.
ramorum infects the small branches of Douglas-fir and small
branches and needles of redwood. Studies are underway to
examine sporulation on these two important economic hosts. Even
without sporulation, fir wreaths and Christmas trees could serve
as an infection pathway if hyphae were able to grow from infected
branch tips and needles. Cones of redwood and Douglas-fir do not
appear to be infected in the field, although spores may land on
the outside of cones.

Greenwaste/compost. Greenwaste containing host material
from infested areas may serve as a source of spores, especially
from leaves of foliar hosts. Even with green material dried for
several months, some plant tissue, such as rhododendron leaves,
will still sporulate upon wetting. Although it has not been
demonstrated, it is likely that spores could be dispersed from
foliar hosts via rainsplash during transit in open containers, or
that infected leaves could detach and blow away. Tests indicate
that P. ramorum in greenwaste mulch is killed in compost after
being held at 55° C for 2 weeks (M. Garbelotto, S. Swain, T.
Harnik, K. Hayden, unpublished data).
Recreation and tourism. Spores of P. ramorum have been
detected on the shoes of hikers and on the tires of mountain bikes
and vehicles used on dirt roads or trails in an infested tanoakredwood
park in Santa Cruz County, California. Subsequent
outings to other natural areas by these visitors may then spread
the pathogen via infested soil. In this same study, a survey of
those visitors with infected shoes showed that many people
leaving the park were going to other parts of California, the United
States, and Europe (S. Tjosvold, unpublished data). In infested
park and recreation areas, educational signs coupled with a place
to wash soil from shoes and equipment would help lower the
probability of spread via park visitors. For popular destinations,
taking these steps may be more realistic than attempting to close
infested areas during the rainy season.
CONCLUSION
Pathways of spread in commerce mirror the pathways of spread in
natural ecosystems, with the major difference being that, in trade
pathways, more infected plant material is transported over
greater distances, increasing the potential to introduce P.
ramorum to new geographic areas. Background knowledge on the
transmission biology of other Phytophthora species as well as data
on the spread of P. ramorum in natural ecosystems can give us
insight into the way P. ramorum may spread in trade pathways.
Containment is important. Given the wide host range of P.
ramorum, it is likely that this pathogen could establish itself in
new geographic areas and cause the substantial forest destruction
that we are now witnessing in California.
LITERATURE CITED
1. Davidson J. M., Rizzo D. M., Garbelotto M., Tjosvold S.,
Slaughter G. W. 2002. Phytophthora ramorum and Sudden
Oak Death in California: II. Transmission and Survival. In
Proceedings of the Fifth Symposium on Oak Woodlands:
Oak Woodlands in California's Changing Landscape. 2001
October 22-25; San Diego, CA. General Technical Report
PSW-GTR-184, ed. R. B. Standiford, D. McCreary, K. L.
Purcell, pp. 741-9. Albany, CA: Pacific Southwest Research
Station, USDA Forest Service, U.S. Department of
Agriculture.
2. Erwin D. C., Ribeiro O. K. 1996. Phytophthora Diseases

Worldwide. St. Paul, MN: APS Press. 562 pp.
3. Ristaino J. B., Gumpertz M. L. 2000. New frontiers in the
study of dispersal and spatial analysis of epidemics caused
by species in the genus Phytophthora. Annual Review of
Phytopathology 38: 541-76.
4. Werres S., Marwitz R., Man in't Veld W. A., De Cock A. W.
A. M., Bonants P. J. M., et al. 2001. Phytophthora ramorum
sp. nov., a new pathogen on Rhododendron and Viburnum.
Mycological Research 105: 1155-65.