FRAC_Pathogen_risklist.doc

1
PATHOGEN RISK LIST
(December 2005)


Purpose

Information is provided about the risk of pathogens to develop resistance to fungicides under specific agronomic conditions.

Introduction

Because no scientific criteria are available to accurately determine the risk of a pathogen to develop resistance, our classification is based on experience and reported resistance claims over the last 40 years. Generally, the risk increases when a pathogen undergoes many and short disease cycles per season, the dispersal through spores over time and space is high, sexual recombination is mandatory in the disease cycle and the competitive ability of resistant individual is at least as high as that of the wild type (in the absence of selection pressure). Furthermore, the risk is considered as high when resistance evolved already after few years of product use.

Examples to illustrate pathogen risk

It is quite easy to detect single isolates of a pathogen with reduced sensitivity to a given fungicide but only their frequency over time and space will decide whether product performance will be affected significantly. Therefore, we consider the pathogen risk as medium to high only if resistance was reported in commercial situations for more than one fungicide class.

Wheat powdery mildew is considered as high risk pathogen because resistance evolved to six different chemical classes within 2 to 5 years, whereas wheat brown rust is a low risk pathogen because no resistance evolved to the major fungicide classes (DMIs, QoIs, morpholines) used against this pathogen, even not after 25 year (DMIs). Eyespot in wheat bears a medium risk, resistance evolved to MBCs and prochloraz (DMIs) only after 10 to 15 years.

An interesting case is Phytophthora infestans that developed resistance quite rapidly to the phenylamide fungicides but not at all to dimethomorph, iprovalicarb, fluazinam, cymoxanil, azoxystrobin and fenamidone (QoI fungicides), cyazofamid (QiI fungicide), propamocarb, and organotins. Therefore, we re-classified P. infestans as high risk pathogen for the RNA

2
polymerase target only and as a medium risk pathogen for all other modes of action (see Table 1 and 2).

Pathogen risk classes

The following plant pathogens (Table 1) from major world markets have evolved resistance to fungicides in a time span sufficiently short to be a serious threat to the commercial success of more than one fungicide class.

Table 1: Plant pathogens accepted as showing a high risk of development of resistance to fungicides (adapted from EPPO 2002, FRAC Monograph No. 3, Russell, 2003)
Pathogen
Crop
Disease
Botryotinia fuckeliana (Botrytis
cinerea)
various, especially
grapevine
grey mould
Erysiphe (=Blumeria) graminis
wheat/barley
powdery mildew
Mycosphaerella fijiensis
banana
black sigatoka
Penicillium spp.
citrus, various
post harvest rot
Phytophthora infestans (RNA
polymerase)
potato/tomato
late blight
Plasmopara viticola
grapevine
downy mildew
Pseudoperonospora cubensis
and related spp.
cucurbits, various
downy mildews
Pyricularia spp.
rice, turf
rice blast, leaf spot
Sphaerotheca fuliginea and
related spp.
cucurbits, various
powdery mildews
Venturia spp.
apple, pear
scab
The following pathogens (Table 2) are regarded as posing a much lower risk because resistance is not a major problem or has been slow to develop. In some cases this due to the pattern of product use. Cases of specific isolates being classed as resistant may be known in some instances, but in commercial practice resistance has not created major disease control problems. The EPPO Guideline does not list these and decisions on baseline production must be made on individual case reviews.

3
Table 2: Plant pathogens accepted as showing a medium risk of development of resistance to fungicides
Pathogen
Crop
Disease
Bremia lactucae
lettuce
downy mildew
Cercospora spp.
sugar beet, peanuts, various
leaf spots
Gibberella fujikuori*
rice
bakanae
Monilinia spp.
various
Monilia rot
Mycosphaerella graminicola
(Septoria tritici)
wheat
leaf spot
Mycosphaerella musicola
banana
yellow sigatoka
Peronospora spp.
various
downy mildews
Phytophthora infestans (target
outside RNA polymerase)
potato/tomato
late blight
Pyrenophora teres
barley
net blotch
Rhynchosporium secalis
barley
leaf blotch/scald
Sclerotinia spp. (especially
homoeocarpa, sclerotiorum)
various (turf, oil seed rape)
Sclerotinia diseases,
dollar spot
Tapesia spp.
wheat/barley
eyespot
Uncinula (= Erysiphe) necator*
grapevine
powdery mildew
* The EPPO Guideline lists these pathogens as high risk pathogens of which baseline
sensitivity is normally requested
In some cases the financial outlay in establishing baselines will not be justified by the small markets involved irrespective of their risk of resistance development. Typical pathogens and diseases are given in Table 3. Pathogens in this group are of local importance, but in commercial market terms are considered as minor pathogens. Decisions on baseline production must be made on a case by case basis. For certain pathogens (e.g. Phytophthora infestans), resistance occurred only to one chemical class (phenylamides) but not to others and therefore, the pathogen is considered as low risk pathogen.

Table 3: Plant pathogens with low risk of development of resistance to fungicides or of minor commercial importance
Pathogen
Crop
Disease
Alternaria spp.
various
leaf spots
Colletotrichum spp.
various
anthracnose
Fusarium and related spp.
various
Fusarioses
Hemileia vastatrix
coffee
rust
Leptosphaera
(=Stagonospora) nodorum
wheat
leaf spot
Phytophthora spp. (soil borne)
various
damping off
Podosphaera leucotricha
apple
powdery mildew
Puccinia and related rust spp.
wheat/barley, various
rusts
Pythium spp.
various
damping off
Rhizoctonia spp.
various
foot and root rot
Sclerotium spp.
various
blight
Tilletia spp.
cereals
bunts
Ustilago spp.
cereals
smuts

4
When the pathogen risk is plotted against the inherent resistance risk of the fungicide class, the combined resistance risk for each pathogen/fungicide combination can be estimated (Figure 1).

Figure 1: Combined resistance risk diagram based on inherent fungicide risk and inherent pathogen risk (* only most important classes and groups mentioned) (according to FRAC Monograph No. 2, by K.J. Brent and D.W. Hollomon, 1998, ** QoI fungicides have been moved from medium to high risk)
Fungicide
Classes *
Fungicide
Risk
Combined Risk
benzimidazoles
dicarboximides
phenylamides
QoI fungicides **
high = 3
3
6
9
carboxamides
SBI fungicides
anilinopyrimidines
phenylpyrroles
phosphorothiolates
medium = 2
2
4
6
multi site fungicides
(e.g.dithiocarbamates
Copper, Sulphur)
MBI-R inhibitors
SAR inducers
low = 1
1
2
3
Pathogen risk
low = 1
medium = 2
high = 3
Pathogen groups *
seed borne
pathogens (e.g.
Pyrenophora spp.
Ustilago spp.)
soil-borne
pathogens (e.g.
Phytophthora
spp .)
rust fungi
Rhizoctonia spp.
Tapesia spp.
Rhynchosporium
secalis
Septoria tritici
Erysiphe graminis
Botrytis cinerea
Penicillium spp.
Magnaporthe
grisea
Venturia
inaequalis
Mycosphaerella
fijiensis
Phytophthora
infestans
The pathogen risk should be estimated also in regard to the local intensity of disease development that is based on weather conditions, fertilization, irrigation, cultural practices and degree of resistance of cultivars. Therefore, we propose to modify the risk diagram in the following manner (Figure 2). Detail can be found in the article written by KH Kuck, “Fungicide Resistance Management in a New Regulatory Environment”, in the Proceedings of the Reinhardsbrunn Symposium 2004 (Modern fungicides and antifungal agents, Dehne, Gisi, Kuck, Russell, eds., BCPC 2005).

5
Figure 2: Combined resistance risk diagram based on inherent fungicide risk, inherent pathogen risk, and agronomic risk (* only most important classes and groups mentioned) (according to Kuck, 2005)
Fungicide
Classes *
Fungicide
Risk
Combined Risk
Agronomic
Risk
benzimidazoles

6
12
18
high = 1
dicarboximides
high = 6
3
6
9
medium = 0.5
phenylamides

1,5
3
4,5
low = 0.25
QoI fungicides

carboxamides

4
8
12
high = 1
SBI fungicides
medium = 4
2
4
6
medium = 0.5
anilinopyrimidines

1
2
3
low = 0.25
phenylpyrroles

multi site fungicides

1
2
3
high = 1
(e.g.dithiocarbamates)
low = 1
0,5
1
1,5
medium = 0.5
MBI-R inhibitors

0,25
0,5
0,75
low = 0.25
SAR inducers

Pathogen risk
low = 1
medium = 2
high = 3

seed borne
Uncinula
necator
Gibberella
Erysiphe graminis
Botrytis cinerea

pathogens (e.g.
fujikuori
Plasmopara

Pyrenophora sp.
Tapesia sp.
viticola

Ustilago sp.)
Rhynchosporium
Magnaporthe

Patho g en grou p s *

secalis

soil-borne

grisea

pathogens (e.g.
Pyrenophora
Venturia

Phytophthora sp.)
teres
inaequalis

rust fungi
Septoria tritici
Mycosphaerella

Rhizoctonia sp.
Sclerotinia sp.
fijiensis

Fusarium sp.
Monilinia sp.
Phytophthora

Cercospora sp.
Phytophthora
infestans/other
modes of action
infestans/RNA
polymerase