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 .