3.4 Dickeya solani exhibited a fitness advantage overD. dianthicola in potato tubers.
We compared the D. dianthicola and D. solani ecological traits in potato tubers. Aggressiveness of the pathogens (i.e., disease severity) was compared using five symptom classes and a disease severity index (DSI) was calculated summarizing the observations of the symptom classes on 10 tubers.
In a first tuber rotting assay, each strain was inoculated separately at 107 or at 105 CFU on each set of 10 tubers. The two different inoculum loads allowed comparing the capacity of the pathogens to initiate maceration and exploit tubers in two contrasting ecological conditions. The assay was performed five times, so we collected symptom class information from 50 tubers for each bacterial strain and for each inoculation load. Kruskal-Wallis tests revealed differences between the two treatments, D. solani andD. dianthicola, both a high (k=3.2; DF=1; p=0.07) or a low (k=43.6; DF=1; p= 4 x 10-11) pathogen load. The DSI values were calculated for each set of 10 tubers and mean values ± SE across the five replicates reached 63 ± 13 for D. solani and 47 ± 4 for D. dianthicola with an inoculum of 107CFU per tuber (Figure 4a ) and 49 ± 8 for D. solani and 7 ± 4 for D. dianthicola with an inoculum of 105CFU per tuber (Figure 4d ).
In a second tuber assay, we used strain mixtures for each of the two species and an assembly of the two species to inoculate 10 tubers for each of 107 and 105 CFU per tuber. This assay was performed twice. Kruskal-Wallis tests revealed differences between the three treatments, i.e., D. solani ,D. dianthicola and the species mixture both at a high (k=51.3; DF=2; p=4.2x10-11) and a low (k=22.4; DF=2; p=1.3x10-5) pathogen load. At both bacterial loads, pairwise comparisons (Post-hoc Tukey tests) showed that aggressiveness was different between D. solani and D. dianthicola (at a high load: F=2.7; p=0.04; at a low load: F=8.6; p< 10-11). Aggressiveness also differed between D. solani and the species mixture (at a high load: F=2.9; p=0.01; at a low load: F=2.2; p=0.07). A DSI value was calculated for each set of 10 tubers. Mean value ± SE of the DSI values reached 83 ± 0 (D. solani ), 63 ± 3 (D. dianthicola ) and 63 ± 8 (D. solaniand D. dianthicola ) with an inoculum of 107 CFU per tuber (Figure 4b ) and 69 ± 14 (D. solani ), 16 ± 6 (D. dianthicola ) and 48 ± 10 (D. solani and D. dianthicola ) with an inoculum of 105 CFU per tuber (Figure 4e ). Altogether, these tuber assays revealed a higher aggressiveness of D. solani compared to D. dianthicola and to the species mixture.
The qPCR analyses of 10 symptomatic tubers revealed that the abundance of the D. solani and D. dianthicola strain mixtures ranged from 2 x 109 ± 3 x 109 to 9 x 109 ± 9 x 109 cells per gram of rotted tissues (mean values ± SE). These data indicated no important differences in the capacity to exploit tubers in terms of population yield. In co-infection assays, qPCR quantification of the pathogens in 10 symptomatic tubers revealed an advantage in favor of D. solaniregardless of the initial load (CI median value = 5.7; Figure 4c and 4f) . The CI values were different from one (Kruskal-Wallis tests: k=16.3; DF=1; p-value of 5 x 10-5 with a high load; k=7.8; DF=1; p= 0.05 with a low load).
Overall, tubers constituted potato plant tissues in which D. solani initiated symptoms more efficiently (even with a low pathogen load), caused more damage (higher disease severity index) and was more competitive than D. dianthicola (outcompeting D. dianthicola ). Remarkably, the disease severity caused by D. solani was decreased in the presence of D. dianthicola .