3.1 Little co-occurrence of D. dianthicola and D.
solani in potato fields
A monitoring of the Pectobacterium and Dickeya populations
was performed over one decade (2004-2015) in France. Annual surveys were
conducted in 541 different potato fields, all exhibiting symptomatic
plants. The samplings resulted in the collection of over 1,600 isolates
that were characterized taxonomically. Each year, we recorded the number
of fields from which we isolated each taxon. These data informed on the
dynamics of the different pathogen populations. Over the period,Dickeya was detected in 10% to 35% of the sampled fields
(Figure 2a ). A moderate increase of the percentage ofDickeya -positive fields was observed over time (F= 5.05; DFn= 1;
DFd= 9; p= 0.05; R² = 0.36). Although decreasing in incidence, the
resident Pectobacterium species remained the most widespread. As
expected, the slopes of the dynamics of the percentage ofDickeya- and Pectobacterium- positive fields diverged (F=
10.11; DFn= 1; DFd= 18; p= 5 x 10-3). Among theDickeya isolates, only two species were identified, D.
dianthicola and D. solani (Figure 2b ). The slopes ofD. dianthicola -positive (F= 1.35; DFn= 1; DFd= 9; p= 0.28) andD. solani -positive (F= 3.10; DFn=1; DFd= 9; p= 0.11) field
percentages were not different from base line and were not different
from each other (F= 0.26; DFn = 1; DFd = 18; p= 0.62). This suggests
that the D. solani invasion did not occur at the expense ofD. dianthicola in French potato agrosystems.
Aside from this national survey, we zoomed at the field level using a
more extensive sampling strategy over a four-year period (2013-2016).
Along a transect in each of 19 sampled fields, we collected ca. 30
plants with blackleg symptoms and a single isolate was characterized
from aerial symptoms of each plant, resulting in the sampling of 548
isolates (Figure 2c ). The Pectobacterium populations
were present in all the 19 sampled fields. In each of the 19 fields, the
null hypothesis that Pectobacterium and Dickeya were
randomly distributed was rejected (Chi-squared test: DF= 1; p<
0.05), meaning that Pectobacterium and Dickeya co-occurred
less often than expected under random distribution. The hypothesis thatD. solani and D. dianthicola were randomly distributed along the
transect was also rejected in each of the 16 Dickeya- positive
fields (Chi-squared tests: DF= 1; p< 0.05) but one (field
#13; DF= 1; p= 0.24). These field data showed a clustering of taxa at
the field levels, both for genera (Dickeya andPectobacterium ) and species (D. solani and D.
dianthicola ). This bias in the symptomatic plants can result from a
non-random distribution of the taxa among populations in tuber seeds
or/and soils and surface waters before plant infection, or from
competitive exclusion during the infection process of potato plants and
tubers. In the following, we focused on the two species D. solaniand D. dianthicola to compare experimentally their fitness in the
course of plant and tuber infection, and in particular to test the
hypothesis of competitive exclusion.