Nursery Detection
Reliable detection of a pathogen in nurseries requires good
knowledge of the pathogen itself, epidemiology and disease development,
and the different types of nursery management. Last, but not
least, a well-trained staff with enough specialists is necessary
for monitoring and survey.
Sampling
If the disease symptoms are not characteristic,
samples must be taken to the laboratory for detailed analysis.
The quality of the sampling method and the sample is as important
for reliable monitoring and surveys as the specificity and sensitivity
of the detection method.
Important questions to consider
include the following: - Where
to take samples on a big field with seedlings, on a container
stand, and so on?
- Which part of the plant should be
sampled?
- How many samples must be taken?
- When
is the optimal season to take samples?
Of course, statistical rules can be used to calculate the
optimal number of samples and to optimize the sampling procedure
in the field. They are, of course, very helpful in determining
where the pathogen is present in the field. To detect Phytophthora
on plants, it is best to look at dips/depressions/swales in
the field or at a container stand. In these dips the soil moisture
is much higher and persists longer than in the surrounding area
because the water accumulates there after rainy periods or after
irrigation. And since P. ramorum (like all other Phytophthora
species) needs water to develop and to infect, latently infected
plants or disease symptoms will be first found in very moist
places.
Which part of the plant should be sampled depends on the
plant species. On Rhododendron, P. ramorum was
detected from leaves and twigs. But experience with plant material
from nurseries has shown that the detection rate can be higher
from twigs than from leaves (Werres, unpublished data). That
may be due to application of fungicides (see below). Concerning
Viburnum, only samples from the stem base enable sufficient
detection, not the wilting twigs. To summarize the experience
with sampling in nurseries: If it is not certain whether the
disease symptom really is caused by P. ramorum, it is
always better to take the whole plant to the laboratory if possible,
rather than just twigs or leaves.
It is very difficult to say how many samples must be collected
for adequate monitoring, but it helps if samples are taken from
different parts of the plant. Samples from different plants
within a diseased area should be examined. The detection rate
can be higher with younger than with old nursery plants, so
more samples should be collected for monitoring old plants.
The best season for sampling in nurseries is late spring
to autumn. Observations in nurseries have shown that, especially
at high temperatures in combination with heavy rainfall, disease
symptoms can occur within a few days after infection. Very often
the assortment of plants in the nursery changes within a season.
Therefore, more than one sampling date is needed within a season.
Detection methods
Detection methods for monitoring
or surveys should be specific for P. ramorum, sensitive to detect
latent infection, and robust toward influencing factors from
the nurseries. In addition they should give rapid results and
be easy to handle.
Initial diagnosis of P. ramorum
infection in nurseries is usually based on disease symptoms.
A reliable diagnosis by disease symptoms is possible only when: - the
kind of symptom(s) that can develop is known;
- the symptom
itself is characteristic for the pathogen.
P. ramorum causes different symptoms depending on
the plant species (see
http://cemarin.ucdavis.edu/symptoms.html). Presently, disease symptoms caused by natural infection with
P. ramorum in European nurseries are only known from
Rhododendron, Viburnum, and in 2002
in United Kingdom two findings from Pieris and one finding from Camellia
(http://www.defra.gov.uk/planth/publicat/sudden.pdf).
Status on P. ramorum findings in Europe is available
on
http://www.eppo.org. Symptoms on other possible hosts in nurseries are mainly known
from artificial inoculation of a single part of the plant. On
Rhododendron, P. ramorum causes mainly a twig
and shoot dieback (figure 1a, b). However, brown to black discoloration
of single non-wooden twigs is characteristic not only for P.
ramorum, but also for other Phytophthora species
like P. citricola, P. cactorum, and P. syringae
(figure 2a, b). So diagnosis solely by disease symptoms is not
sufficient for P. ramorum. The wilting symptoms on Viburnum
are very unspecific. To verify the diagnosis P. ramorum
for this host, it is necessary to look for cambial necrosis
at the stem base.
1a
1b
Figure 1: Twig dieback (a) and wilting (b) caused by Phytophthora ramorum
Figure 2a: Twig dieback on Rhododendron caused by Phytophthora cactorum
Figure 2b: Twig dieback on Rhododendron caused by Phytophthora citricola
Further detection methods for P. ramorum are discussed
in detail on the Web site:
http://cemarin.ucdavis.edu/symptoms.html. None of the methods show all criteria to be optimal. Based
on the present knowledge from experiments in scientific laboratories
and in laboratories of the plant protection services, combinations
of different methods seem to be most effective.
Factors that can influence the detection rate
In
many nurseries, specific fungicides are used for control of
P. ramorum. Fungicides can suppress the development of disease
symptoms, possibly leading to false-negative results and more
difficult sampling. Furthermore, fungicides can influence the
detection rate in plant or soil samples depending on the method
used. For example, those methods that are based on the detection
of living propagules, like baiting and microbiological techniques,
will give false- negative results if the fungicide has damaged
or killed P. ramorum. On the other hand, these methods can prove
whether chemical control was successful. Residues of chemicals
and/or ingredients can interact with the detection method itself
so that false positive or false negative results are possible.
Therefore, it is always very important to know the cultivation
methods used in the commercial nurseries being monitored.
Monitoring
Monitoring of P. ramorum is done according
to the definition of the IPPC Standard No. 6 (1997) for a "detection
survey" (to conduct a survey in an area to determine if pests
are present). A survey in this case is "an official procedure
conducted over a defined period of time to determine the characteristics
of a pest population or to determine which pest species occur
in an area." In Europe, for example, the obligation to monitor
the occurrence of P. ramorum is laid down in the EU-Directive
2002/757/EG (2002), so each of the EU member states has to provide
data to the EU. Those data are also the background and the justification
of regulatory measures for both import and export of host plants
of the corresponding harmful organism.
The problems or preconditions for monitoring are described
below. These apply not only to P. ramorum but also to all "new"
harmful organisms.
At the beginning of monitoring in some cases, information
on the biology of the organism is lacking so the following criteria
have to be analyzed thoroughly to concentrate the usually limited
amount of workers, laboratory capacity, and financial resources:
confirmed host plants, potential host plants, risk areas, detection
methods, and scientific data to develop a "survey plan." Of
course, it is necessary to adapt monitoring according to new
available information. For example, in Europe the monitoring
of P. ramorum was first focused on Viburnum and Rhododendron
species in nurseries, but now it has to be enlarged to public
green areas. More data on potential host plants of P. ramorum
are available as the result of artificial inoculation trials,
so it has to be decided which plants should be included in the
monitoring.
Monitoring usually is carried out concerning a new organism.
Therefore, the results depend on the qualifications of the staff
carrying out the monitoring. In summary, a well-trained staff,
following good surveillance and record keeping practices is
essential for effective monitoring.
References
Brasier, C.D., Rose, J., Kirk, S.A., and Webber, J.F. 2002..
Pathogenicity of Phytophthora ramorum isolates from North America
and Europe to bark of European Fagaceae, American Quercus rubra
and other forest trees. Proceedings of the Sudden Oak Death
Symposium, Dec. 17.-18., 2002, Monterey, California. 30-31.
Proceedings of the Sudden Oak Death Symposium, Dec. 17.-18.,
2002, Monterey, California. Session "Survey/Monitoring," 23-27,
88-93.
Erwin, D.C. and Ribeiro, O.K. 1996. Phytophthora - Diseases
Worldwide. Chapter 2: Isolation and Detection of Phytophthora.
The American Phytopathological Society, St. Paul, Minnesota,
pp. 562.
Tsao, P.H. 1990. Why many Phytophthora root rots and crown
rots of tree and horticultural crops remain undetected. EPPO
Bulletin 20, 11-17.
Werres, S. 1999. Welche Proben und Informationen sind für
die Diagnose von Baumkrankheiten wichtig? In: Dujesiefken D.
and P. Kockerbeck (Eds.): Jahrbuch der Baumpflege 1999, Thalacker
Medien, 99-107.
IPPC Standard No. 6 1997: International Standards for Phytosanitary
Measures: Guidelines for Surveillance. IPPC Secretariat FAO,
Publ. No. 6.
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