Ecole Nationale Supérieure Agronomique de Montpellier ... - CIAM

The spread of European stone fruit yellows is regulated by the life cycle of
its vector and by the growth rate of the hosted phytoplasma, as assessed by
real-time PCR
G. Thébaud, M. Yvon, and G. Labonne
Institut National de la Recherche Agronomique, UMR BGPI, CIRAD TA 41/K, Campus
international de Baillarguet, 34398 Montpellier cedex 5, France.
ABSTRACT
The spread of ‘Candidatus Phytoplasma prunorum’ by its vector Cacopsylla pruni has
been intensively studied these last years, but some issues about the biology of the vector and
about the transmission processes remain unclear and prevent a clear understanding of the
epidemiology of the disease. In this work, we obtained new information on the overwintering
of C. pruni and we measured the evolution of the quantities of phytoplasma in the insects
after acquisition. The cycle of C. pruni was completed for the first time, demonstrating
directly that it is a univoltine species. From the data obtained at the overwintering sites, we
hypothesized that the overwintering is conditioned by long distance migrations from Prunus
to conifers (in the range of several tens of kilometers). When C. pruni are grown on an
infected plant, they accumulate the phytoplasma and then multiply it so that at the end of the
overwintering period the phytoplasma concentration is at its uppermost value. Adults of the
new generation, though highly infected, had low transmission efficiency. Reimmigrants that
arrive healthy to reproduce on their host plant did not appear to have enough time to acquire
the phytoplasma from an infected plant and then to transmit it to another plant. It seems thus
that the infected overwintering reimmigrants are the main efficient vectors of the phytoplasma
and that the dissemination of the disease require considering both local and regional scales.
Keywords: Apricot, psyllid, Q-PCR, secondary spread.
INTRODUCTION
European stone fruit yellows (ESFY) is a disease damaging mainly apricot (Prunus
armeniaca) and Japanese plum (P. salicina) orchards in Europe. It is due to a phytoplasma for
which the name ‘Candidatus Phytoplasma prunorum’ has been proposed (Seemüller &
Schneider, 2004). It is spread by the psyllid Cacopsylla pruni (Carraro et al., 1998).
C. pruni is a European and Middle-Asiatic species (Lauterer, 1999). In the entomological
records, the species is described as a univoltine species reproducing on Prunus sp., mainly P.
spinosa, and overwintering mainly on conifers (Ossiannilsson, 1992; Hodkinson & White,
1979; Lauterer, 1999). In France, its distribution, host preference and period of presence on
Prunus have been studied (Labonne & Lichou, 2004), and the insect has been found in all the
areas where it was searched, either on wild or cultivated Prunus. The period of presence on
Prunus corresponds to the reproduction of the insect. It extends, with some variations
according to the geographical area, from the beginning of February to the end of June or July.
Two successive morphs were observed during this period: a dark-winged form corresponding
to the reimmigrants coming back on Prunus after overwintering; a light-colored form
corresponding to the adults of the new generation. However, the knowledge about the
overwintering period is very poor and the biological cycle of the species has not been
completed until now, so that the assertions about it are not demonstrated.
C. pruni has been found infected by the phytoplasma in several countries (Italy: Carraro
et al., 1998; Poggi Pollini et al., 2004; France: Jarausch et al., 2001; Spain: Laviña et al.,
2004; Switzerland: Ramel et al., 2001; Czech Republic: Fialová et al., 2004; Bosnia-
Herzegovina: Carraro et al., 2005). Both reimmigrants and adults of the new generation are
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