3.1 A scarce 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 next part of the work, we focused on the two speciesD. solani and 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.