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| Introduction |
| The phenylamides are a highly active class of fungicides
specifically controlling plant pathoges of the oomycetes (the downy
mildews within Peronosporales and Sclerosporales, as well as most members
of the Pythiales and Saprolegniales; Gisi, 2002). They penetrate the
plant tissue rapidly, are translocated acropetally within the plant
and inhibit rRNA synthesis in target fungi. The mode of resistance
may involve one (or two?) major gene(s) and potentially several minor
genes, the site of mutation(s) in the genome has not yet been mapped.
Phenylamide fungicides have been in commercial use since 1978. The
following active ingredients are classified as phenylamides: metalaxyl,
metalaxyl-M (mefenoxam), furalaxyl and oxadixyl (all four Syngenta),
benalaxyl (Isagro) and ofurace (Aventis) (Gisi and Ziegler, 2002).
The use strategies for PAs have been well established (see below)
and they do not create controversial issues amongst member companies,
officials and advisors. There have been no reported infringements
of use strategies for several years. Current sensitivity monitoring
data are produced by only a few research groups and the presence
of resistant subpopulations is well known in several plant pathogens
on a range of crops worldwide (Gisi and Cohen, 1996). Sensitive subpopulations
have not disappeared, even though PA-containing products have been
used in similar amounts and intensities over the past 20 years. This
strongly suggests that the recommended anti-resistance strategies
are successful. Sampling and testing methods have been published
through FRAC in 1992 (EPPO Bulletin 22, 297-322) and are still valid.
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| Sensitivity
of field populations |
General:
Resistant isolates of Phytophthora infestans and Plasmopara
viticola
existed at low proportions in wild type populations already before
PA fungicides were used extensively (1977/78) suggesting that recurrent
mutations give rise to resistant individuals at different locations
and time periods (Gisi and Cohen 1996). Resistant isolates have been
selected through the use of PAs, increased in frequency, survived
and later migrated. They can compete with sensitive isolates even
in the absence of PAs. Therefore, resistant isolates can be detected
in current populations in fields treated or not treated with PAs.
Standard sensitivity test methods (e.g. leaf disc assay) provide
a fully resistant response to PAs (used as active ingredients in
the laboratory assay versus mixture product used in the field) when
as little as 1% of the sporangia in bulk samples of field populations
are resistant (Sozzi et al., 1992). Samples for sensitivity analyses
should be taken as early in the season as possible. Those taken towards
the end of the season will provide sensitivity responses which are
a result of selection, migration, mating and competition occurring
during epidemics. The current sensitivity data provide valuable information
on the distribution of isolates over a certain time period in a given
area but should not be used to predict product performance. In most
cases mixed populations can be controlled adequately by PA-containing
products if the proportion of resistant isolates is not too high
and if the number of applications is limited (see use recommendations).
There is no cross resistance between PAs and fungicides of other
chemical classes like cyanoacetamide oximes (cymoxanil), QoIs (e.g.
azoxystrobin, famoxadone), phosphonates (fosetyl-Al), amino acid
amide carbamates (iprovalicarb), cinnamic acids (dimethomorph), carbamates
(propamocarb), dinitroanilines (fluazinam) and multisite inhibitors.
Potato and tomato late blight (Phytophthora infestans):
The sensitivity
of populations fluctuates from year to year and within the season.
In many cases, sensitive isolates predominate
early and resistant isolates late in the season for both treated
and untreated fields. In most countries, the proportion of resistant
isolates is important and has remained more or less stable for many
years. In European populations, isolates from tomato are mainly sensitive,
while those from potato may be resistant, intermediate or sensitive.
Resistant isolates may be present in field populations in high proportions,
they are in a "dynamic equilibrium" with sensitive isolates.
The dynamics of resistance evolution are driven not only by selection
through PA fungicides, but equally important are the inheritance
and genetic background of resistance, as well as fitness and migration
of isolates. The presence of resistant isolates has been confirmed
in all parts of the world. The phenotypic and genotypic structures
of current field populations suggest that they may emerge from local
processes including sexual recombination. Isolates with an intermediate
response to PAs having emerged through Mendelian inheritance may
exist in field populations. There is no genetic linkage between resistance
to phenylamides and mating type (A1, A2). The proportion of A2 mating
type isolates collected from commercial potato fields has remained
low in many countries (e.g. UK, France, Germany, Switzerland), whereas
in others such as Mexico, the USA, the Netherlands and Scandinavia
it can reach 50% and more. On tomato in private gardens, the proportion
of A2 mating type isolates may be much higher than on potato. Although
populations from tomato and potato can be separated from each other
phenotypically and genotypically, there is a certain proportion of
overlap in field populations. The aggressiveness of isolates is highest
on the host of their origin and can be significantly lower for isolates
collected from potato when tested on tomato.
Grape downy mildew (Plasmopara viticola):
Much less information on
resistance is available for this pathogen compared to P. in-festans.
In countries where sensitivity analyses
have been conducted, the proportion of sensitive P. viticola isolates
has remained important and more or less stable for many years ( e.g.
France, Italy, Switzerland). Since#990000 undergoes sexual recombination
every winter, the genetic diversity of the primary inoculum is very
high and resistance is inherited according to Mendelian rules, i.e.
all F1 progeny isolates are intermediate (i) in sensitivity. The
proportion of sensitive, intermediate and resistant isolates in F2
progeny is 1 : 2 : 1. Therefore, the proportion of intermediates
may be high in field populations, sensitive isolates will re-appear
and resistant isolates may not disappear. The sensitivity of populations
fluctuates from year to year and within the season (Gisi, 2002).
Sensitive, intermediate and resistant isolates can be detected in
fields that have or have not been treated with PAs and are in a "dynamic
equilibrium" with each other. The dynamics of resistance evolution
are driven not only by selection through PA fungicides, equally important
are the inheritance and genetic background of resistance, as well
as fitness and migration of isolates.
Other pathogens:
The presence of resistant isolates in field populations
has been confirmed in several other pathogens including Pseudoperonospora
cubensis (e.g. Israel, USA, Australia), Peronospora
tabacina (e.g.
USA), Peronospora pisi (e.g. New Zealand), Bremia
lactucae (e.g.
USA, UK, Italy), Pythium spp. (turfgrass in USA) and other pathogens
on a range of crops in several countries (Gisi 2002). Resistance
levels are often not uniformly distributed in an area and do not
necessarily correlate with product performance problems.
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| Use
recommendations |
The use recommendations for phenylamide-based products
have remained unchanged since 1997. The key guidelines for use are
as follows and they may be adapted to local needs:
1.
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The phenylamides should be used on a preventive
and not curative or eradicative basis. |
2.
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For foliar applications, the phenylamides should be used in
pre-packed mixtures with an unrelated effective partner and used
in a sound management programme. Where residual partners are
used, it is recommended to use between three quarters and full
recommended rates. The phenylamide dosage in the mixture depends
on its intrinsic activity and is defined by the respective company. |
3.
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The phenylamides should not be used as soil treatments against
airborne diseases. When solo formulations are made available
for soil use, strategies must be imple-mented which prevent any
possibilities for foliar applications. For seed treatment, mixtures
rather than straight phenylamides should be used whenever possible. |
4.
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The number of phenylamide applications should be limited (two
to four conse-cutive applications per crop and year). The application
intervals should not exceed 14 days and may be shorter in cases
of high disease pressure. If rates and appli-cation intervals
are reduced, the total amount of the phenylamide fungicide used
per season should not exceed that of the full rate, and the total
exposure time should remain the same. The rate of the mixing
partners should remain the same for both intervals. |
5.
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Phenylamide sprays are recommended early season or during the
period of active vegetative growth of the crop. The farmer should
switch to non-phenylamide products not later than the normal
standard application interval of the non- phenylamide product. |
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| Recent
relevant literature |
GISI, U, 2002. Chemical control of downy mildews.
pp.119-159 in P.T.N.Spencer, U. Gisi, A. Lebeda, eds., Advances in
Downy Mildew Research, Kluwer, Dordrecht.
GISI, U and ZIEGLER, H, 2002. Phenylamides (acylalanines and related):
Metalaxyl, metalaxyl-M, furalaxyl, benalaxyl, ofurace, oxadixyl.
In J.R. Plimmer, ed., Encyclopedia of Agrochemicals, John Wiley,
New York, in press.
GISI, U, HERMANN, D, OHL, L and STEDEN, C, 1997. Sensitivity profiles
of Mycosphaerella graminicola and Phytophthora infestans populations
to different classes of fungicides. Pesticide Science 51, 290-298.
GISI, U. and COHEN, Y, 1996. Resistance to phenylamide fungicides:
A case study with Phytophthora infestans involving mating type and
race structure. Annual Review of Phytopathology 34, 549-572.
GISI, U and OHL, L, 1994. Dynamics of pathogen resistance and selection
through phenylamide fungicides. pp. 139-146 in S. Heaney, D. Slawson,
D.W. Hollomon, M. Smith, P.E. Russell, D.W.Parry, eds., Fungicide
Resistance, BCPC Monograph No 60.
GISI, U, 1992. FRAC methods for monitoring the sensitivity of fungal
pathogens to phenylamide fungicides. PA-FRAC of GIFAP. U. Gisi, ed.,
EPPO Bulletin 22, 297-322:
Williams, R and Gisi, U. Monitoring pathogen sensitivity to phenylamide
fungicides: principles and interpretation, pp. 299-306.
Sozzi, D, Schwinn, FS and Gisi, U. Determination of the sensitivity
of Phytophthora infestans to phenylamides: a leaf disc method, pp.
306-309.
Staehle-Csech, U and Gisi, U. Determination of the sensitivity of Plasmopara
viticola to phenylamides, pp. 314-316.
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