Ecole Nationale Supérieure Agronomique de Montpellier ... - CIAM

Spatio-temporal Analysis of Disease Spread Provides Insights into the
Epidemiology of European Stone Fruit Yellows
Gaël Thébaud, Claude Castelain, Joël Chadœuf
and Gérard Labonne
UMR BGPI
Institut National de la
Recherche Agronomique
Montpellier, France
Station de Pathologie Végétale
INRA
Domaine Saint-Maurice
Montfavet, France
- 90 -
Unité de Biométrie
INRA, Domaine Saint-Paul
Site Agroparc
Avignon, France
Keywords: incidence curve, disease map, simulation, permutation, ESFY, apricot chlorotic
leaf roll, statistics.
ABSTRACT
European stone fruit yellows (ESFY) is caused by a phytoplasma and transmitted by
Cacopsylla pruni. As it is becoming a major threat in Europe for Prunus orchards, we need
more knowledge on many fundamental epidemiological processes of this disease. Up to now,
the spread of ESFY in an orchard has not been analysed on a statistical basis, and the
underlying mechanisms are poorly documented: How is the pathogen introduced into an
orchard? How long are the incubation, latent and infectious periods for the infected trees?
What is the current range of the dissemination? Can we estimate transmission parameters
from the observed patterns? To begin addressing these questions, we adopted a hypothesis
testing approach to the spatio-temporal correlations between the symptomatic trees. The case
study presented here is based on a long-term (17 years) survey and mapping of symptomatic
trees in a group of 4 adjacent apricot orchards. After a description of the temporal dynamics
of the disease and of its spatio-temporal pattern, we tested hypotheses on the proximity
between symptomatic trees. Our results show the unevenness of disease introduction and/or
symptom expression within this group of orchards. We also demonstrate that the infected
trees are too close from one to another to be the result of independent infections. The
epidemiological significance of the results is discussed.
INTRODUCTION
The European stone fruit yellows (ESFY) phytoplasma is the causal agent of apricot
chlorotic leaf roll and other decline diseases affecting trees of the Genus Prunus (Lorenz et
al., 1994). Symptoms of ESFY have been reported for a long time in French orchards
(Chabrolin, 1924), but the intensity of recent outbreaks highlights that this disease is a major
threat for the new cultivars of apricot (Prunus armeniaca L.) and for Japanese plum (P.
salicina Lindl.). Recently, significant epidemiological results have been produced:
identification of the vector Cacopsylla pruni Scopoli (Carraro et al., 1998), detection of wild
plant hosts for the phytoplasma (Jarausch et al., 2001b), and characterisation of vector
transmission in controlled conditions (Carraro et al., 2001). At present, one of the major
challenges is to understand the biological processes that are responsible for the spread of the
disease at the scale of an orchard, and at a regional scale.
Although an increasing number of spatial or spatio-temporal data sets are being collected,
few of them (Labonne et al., 2000; Jarausch et al., 2001a) have been used to analyse the
spread of ESFY. This work aims at (i) illustrating how a clear visual summary of the data can
suggest hypotheses and (ii) showing that a framework based on statistical tests of these
hypotheses is useful to gain insights into the epidemiology of a disease.