For. Snow Landsc. Res. 76, 3: 361–367 (2001)
Chestnut blight in Europe: Diversity of Cryphonectria
parasitica, hypovirulence and biocontrol
Cécile Robin1and Ursula Heiniger2
1UMR Santé Végétale, INRA Bordeaux, BP81, F-33 883 Villenave d’Ornon, France
2WSL, Swiss Federal Research Institute, CH-8903 Birmensdorf, Switzerland. email@example.com
Abstract [Review article]
A review of the chestnut blight situation in Europe is given. Since its introduction in 1938 into
Italy, the blight has spread throughout Europe with the exception of the UK, the Netherlands and
northern France. Recently the blight has begun to have much more impact on chestnut in north-
west Spain and Portugal. Hypovirulence is widespread. In most countries healing cankers have
been observed and the presence of dsRNA has been confirmed. However, hypovirulence is still
absent from newly infected areas in northern France, northern Switzerland, Portugal and north-
western Spain. The year of the first observation of healing cankers is weakly correlated with the
year of the first occurrence of the blight. Although vc type diversity may be low at a local level,
over 40 vc types have been identified in Europe. Vc type EU-2 is dominant in Western Europe
whereas EU-12 is dominant in the south. In order not to jeopardise natural biocontrol with
hypovirulence, strict quarantine regulations should prevent the introduction of new vc types.
Keywords: hypovirus, vegetative compatibility, epidemiology, Castanea sativa
Chestnut blight, caused by Cryphonectria parasitica (Murr.) Barr (Syn. Endothia parasitica
[Murr.] And.), is one of the major diseases of chestnut (Castanea spp.) and has caused serious
damage in orchards and in forests since its introduction in Europe (ANAGNOSTAKIS 1987).
The hypovirulence, which is an attenuation of fungal pathogenis, has reduced the impact of
this disease and has been used as a biological control method in Europe (NUSS 1992;
HEINIGER and RIGLING 1994). During the COST G4 Action “Multidisciplinary Chestnut
Research”, the common objective of the 14 participating countries was to update infor -
mation on the distribution of the blight and the hypovirulence, to standardize protocols and
methods used in different countries to study the biology and vegetative compatibility of the
fungus and to improve biological control in Europe. This paper is a short review of the
recent findings obtained in Europe during this project.
2 Distribution of chestnut blight
C. parasitica, the causal agent of chestnut blight was first observed in Europe in 1938 near
Genova, Italy. From Genova this disease spread rapidly within Italy and to neighboring
chestnut (Castanea sativa Mill.) growing areas (for a review see HEINIGER and RIGLING
1994). A recent survey in France has revealed a significant northward progression of the
blight (DEVILLEBONNE 1998). Similarly, areas with only scattered chestnut stands, such as
Swiss chestnut stands north of the Alps (HEINIGER and STADL ER 1990) and in the Rhine
valley in Germany (SEEMANN et al. 2001), are also infected. The disease is also spreading
and gaining high impact in the northwest regions of Spain and in Portugal. Today only the
scattered stands of chestnuts in the Netherlands and the coppice stands in southern UK are
free of the blight (Fig. 1).
362 Cécile Robin, Ursula Heiniger
3 Distribution of hypovirulence
Unlike in the USA where chestnut blight virtually eliminated the American chestnut (C.
dentata Borkh.), the European chestnut recovered from the disease due to the natural
occurrence of hypovirulence, caused by the dsRNA hypovirus CHV1 (HEINIGER and
RIGLING 1994, ALLEMANN et al. 1999). In most countries healing cankers were observed and
the presence of dsRNA was confirmed (Table 1).
There is a weak correlation between the year of the first observation of chestnut blight
and the appearance of healing cankers (Fig. 2). The hypovirus did not reach all the chestnut
blight affected areas and is still absent from newly infected areas in northern France and
northern Switzerland, Portugal, and north-western Spain (Pereira and Heiniger, unpup -
lished results). In Germany only one hypovirulent isolate was found in one of several blight
infested areas (SEEMANN et al. 2001).
ALLEMANN et al. (1999) reported considerable molecular variability within the European
CHV1 population, which led to the definition of five different CHV1 sub-types. Tracking
hypovirus is now possible using molecular markers, which allow the assessment of the
spread of hypovirulent strains used as biocontrol agents (HOEGGER et al. 1998; ROBIN et al.
Fig. 2. Correlation of year of first canker report and year of first hypovirulence (hyv) report in
European countries (data from authors cited in Table 1).
Fig. 1. Map of Europe showing the
presence of chestnut blight and the
year of its first observation.
year of first canker
year of first report of hyv
1930 1940 1950 1960 1970 1980 1990 2000
R2 = 0.47
For. Snow Landsc. Res. 76, 3 (2001)
The natural spread of CHV1 is still not fully understood. Although CHV1 infested C.
para sitica strains grow well in vitro, their growth is limited in the chestnut bark and sporu -
lation is rare. However, PROSPERO et al. (1998) found heavy sporulation of virulent as well
as hypovirulent strains on fallen wood and stacked fire wood.
Biological control by applying hypovirulent strains to growing cankers is practised in
France using Grente’s method (GRENTE and BERTHELAY-SAURET 1978). Selected mixtures
of hypovirulent strains were produced by a private company using a slightly different proto-
col and applied by chestnut growers in orchards. In Hungary, Greece, the Slovak Republic
and Switzerland local virulent strains converted with French or local hypovirulent strains
are used as biocontrol agents, produced using Grente’s method and applied by specialized
technicians or by scientists (JUHASOVA et al. 1998; RADÓCZ 1998; S. Diamandis, personal
Table 1. The chestnut blight epidemic in Europe. Years of first observation of chestnut blight and healing
cankers. 1% of chestnut area affected; 2Year of first observation of chestnut blight; 3Year of first obser-
vation of healing cankers; 4Year of first isolation of white C. parasitica strains; 5dsRNA confirmed.
Country ha1Chestnut Healing White dsRNA5Communicated by
Albania 1967 1983 1984 yes L. Bashkim
Austria 45 1970 1993 1993 no E. Wilhelm, U. Kudera
Bosnia-Herzegovina 100 1961 1980 1980 yes M. Uscuplic
Croatia 50 1955 1978 1978–1980 yes S. Novak Agbaba
France 90 1956 1964 1964 yes C. Robin
Germany 0.5 1992 1992 1992 yes D. Seemann
Greece 100 1963 1986 1986 yes S. Diamandis
Hungary 100 1969 1951 1964 yes L. Radócz
Italy 95 1938 1951 1964 yes P. Cortesi
Portugal 10 1989 1992 no no C. Abreu
Rep. of Macedonia – 1974 1995 1995 yes K. Sotirovski
Romania 55 1984 – – yes D. Floarea
Slovak Republic 8 1976 1992 1999 yes G. Juhásová
Slovenia – 1950 1985 1985 – A. Solar
Spain 75 1947 1992 1992 yes C. Colinas
Switzerland 100 1948 1975 1975 yes U. Heiniger
Turkey 35 1968 1999 1999 yes M. Gurer, M. Çeliker
4 Diversity of vegetative compatibility types of C. parasitica
Vegetative compatibility (vc) is a major obstacle to the diffusion of the hypovirus. For a
Europe-wide comparison, 31 European vc types (EU-1 to EU-31) were identified and tester
sets supplied (CORTESI et al. 1998). These testers were used to study the genetic control of
vegetative compatibility in C. parasitica (CORTESI and MILGROOM 1998). 33 additional vc
types were obtained after crosses between the first 31 vc types and were added to the tester
set (EU-32 to EU-64). The vic genotype was determined for these 64 vc types, with 6 vic
genes each having two alleles. In France and Switzerland, 10 vc types incompatible with all
these 64 testers were found, showing that at least one additional vic gene or one additional
allele is needed to entirely describe the genetic determinism of vc types (ROBIN et al. 2000a).
The set of vc tester strains has to be increased accordingly.
364 Cécile Robin, Ursula Heiniger
Studies on vc type diversity were carried out in several European countries with the
available EU testers and results were used to update the map of the dominant vc types in
Europe (Fig. 3), already set up by HEINIGER et al. (1998). C. parasitica populations showed a
clear geographic distribution at the European level. The vc type EU-12 is the dominant vc
type in southern and eastern Europe, apart from in Turkey (M. Gurer, personal communi -
cation), whereas in western and north-western Europe, the vc type EU-2 is dominant.
The number of vc types found per country varies considerably. Over 40 vc types were
found in France, whereas in some countries only 4 vc types were identified, and in Turkey
only one (Table 2). However, sampling sizes greatly varied between countries, and diversity
indices (number of vc types/number of studied isolates or Shannon Index) must be used
with care to compare populations. However, a low vc type diversity is found in two cases:
– in areas or countries where C. parasitica had only recently been introduced, e.g. Germany
(SEEMANN et al. 2001), Portugal, northern Switzerland (HOEGGER et al. 2000),
– in countries where no perithecia were observed, e.g. Greece and Macedonia (S.
Diamandis and K. Sotirovski, personal communications).
A significant diversity between C. parasitica populations may be found within countries. E.g.
in northern Italy the vc type diversity is much greater than in southern Italy (CORTESI et al.
1996), and in Bosnia-Herzegovina 2 to 29 vc types were found per site (TRESTIC et al. this
volume). Considerable variation is also observed in Spain, where in the Atlantic regions vc
types incompatible with the most common European vc types were found (ROBIN et al.
2000a, COLINAS and USCUPLIC 1998, TRESTIC et al. this volume). Although the number of
European vc types may locally still be low, on a European level it is quite high, indicating the
potential of locally increasing vc type diversity.
Fig. 3. Map of Europe showing for each country the dominant vc types of C. parasitica populations.
IC = vc types incompatible with main EU testers.
For. Snow Landsc. Res. 76, 3 (2001)
Table 2. Dominant vc types of European C. parasitica populations. 1year of first observation of chestnut
blight; 2vc types comprising more than 50% of the isolates; 3+: found, – : not found, ?: no information.
Country Chestnut Number of Major vc types Perithecia Personal communication
blight1vc types present (>50%)2found in or reference
Austria 1970 15 EU-2 – E. Wilhelm, U. Kudera
Bosnia-Herzegovina 1961 27 EU-12, EU-1 + M. Uscuplic
France 1956 40 + EU-2, EU-5, EU-1,
unknown EU-33 + C. Robin
Germany 1992 4 EU-2 – D. Seemann
Greece 1963 4 EU-12 – S. Diamandis,
Hungary 1938 18 EU-1, EU-6,
EU-12, EU-13 + L. Radócz
Italy 1938 20 EU-2, EU-1, EU-5 + CORTESI et al., 1998
Macedonia 1974 5? EU-12 + K. Sotirovski
Portugal 1989 4 ? – N. Santos
Slovak Republic 1976 8 EU-12 + G. Juhásová
Spain 1947 13 EU-2 ? C. Colinas
Switzerland 1948 28 EU-2, EU-1, EU-5 + U. Heiniger, D. Rigling
Turkey 1968 EU-1 + M. Gurer, M. Çeliker
5 Chestnut blight epidemiology
Except for some regional occurrence of perithecia in the field (Table 2, CORTESI et al. 1996,
BISSEGGER et al. 1997), we do not know much about how and when C. parasitica completes
its biological cycle in the different countries with their various climatic conditions. However,
knowledge of C. parasitica biology is essential to understand and predict disease develop-
ment, i.e. C. parasitica and CHV1 spread. Since C. parasitica is heterothallic but with a low
outcrossing rate, it has a clonal and a sexual cycle. This may have consequences for canker
spread, vc type diversity, CHV1 spread and thus disease severity (MILGROOM 1995).
In the USA, where vc type diversity is much higher than in Europe, most of the ascopores
are discharged in autumn and are the main source of primary inoculum responsible for
establishing new cankers (HEALD and GARDNER 1913; ANAGNOSTAKIS et al. 1998).
Ascospore production and dispersal have been investigated in one orchard in France
(GUÉRIN et al. 1998). Results showed that ascospores were discharged from spring until
autumn, with a peak of spore trapping in spring, after rainfall. The coincidence of the
discharge of ascospores in air with the highest chestnut receptivity to blight (in spring)
suggested a high impact of these propagules on blight incidence and distribution (L. Guérin
and C. Robin, unpublished results).
Very few data exist in Europe on the genetic structure of C. parasitica populations
assessed with molecular and neutral markers other than vc types. The latter are not poly-
morphic enough to differentiate genotypes. In southern Switzerland, vc type and DNA
fingerprint diversities were high and did not correlate. In contrast, fingerprint and vc type
diversity were very low and found to correlate in two populations with recently introduced
C. parasitica and one mating type clearly dominating. This suggests that these populations
were clonal and that sexual reproduction was not effective (HOEGGER et al. 2000).
366 Cécile Robin, Ursula Heiniger
With the exception of the UK and northern France, almost all the European chestnut grow-
ing areas have been infected by the blight caused by C. parasitica. However, chestnut blight
distribution is not an indicator of chestnut blight severity unless it is associated with the
presence of the hypovirus CHV1, the causal agent of hypovirulence. Disease management
should be applied according to the presence or absence of CHV1. In Europe there are chest-
nut sites where the disease was introduced more than 30 years ago and where the CHV1 is
well established and thus the blight severity low. But there are also sites where the introduc-
tion of the blight is recent and has not yet been followed by CHV1. Here the blight epidemic
may still turn out to be devastating.
As the success of biocontrol with the hypovirus CHV1 is negatively correlated to the
number of vc types present, an increase in vc type diversity may jeopardize biocontrol
efforts. Movement of chestnut plants and wood should be restricted and mycelial mixtures
of CHV1 infested C. parasitica for biocontrol should only contain the local vc and mating
More epidemiological studies of the natural spread of the hypovirus, as well as the epi-
demiology of C. parasitica are required in order to predict disease development better and
to improve biological control measures.
We would like to thank the members of the COST G4 action that submitted information about
the chestnut blight situation in their country: C. Abreu, C. Colinas, P. Cortesi, S. Diamandis, D.
Floarea, G. Juhásová, U. Kudera, B. Mal Lushaj, S. Novak Agbaba, C. Perlerou, L. Radócz, A.
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