3.3. Compared to D. dianthicola, D. solaniexhibited a fitness advantage in potato tubers
We compared the D. dianthicola and D. solani ecological
traits in potato tubers. Aggressiveness of the pathogens 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 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 and D. 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 107 CFU per tuber
(Figure 4a ) and 49 ± 8 for D. solani and 7 ± 4 forD. dianthicola with an inoculum of 105 CFU 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 symptom
incidence 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). Symptom incidence 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.
solani and 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. solanias 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 (mean value ± SE) 2 x 109 ± 3 x
109 to 9 x 109 ± 9 x
109 cells per gram of rotted tissues. 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. solani regardless 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, caused more damage and was
more competitive than D. dianthicola . Remarkably, symptom
severity of D. solani was decreased in the presence of D.
dianthicola .