2.3 Plant inoculations by D. solani and D. dianthicola strains and experimental populations
Each bacterial strain was cultivated individually before being used for plant and tuber inoculations, either separately or assembled in populations. To constitute experimental populations of D. solani,D. dianthicola, or mixtures of the two species, cell suspensions of different strains were assembled just before plant inoculation. Potato plants (S. tuberosum var. Bintje) and hyacinths (Hyacinthus orientalis var. Delft blue) were cultivated in horticultural compost in individual pots (2 L) in a greenhouse at around 23°C (minimum 20°C maximum 28°C) with a 12-hour photoperiod. Potato plants (three weeks post-tuber plantation) and hyacinths (four weeks post-bulb plantation) were inoculated by watering the substrate with a pathogen cell suspension at 109 colony-forming units (CFU) per pot. In a wounded condition, the potato roots were wounded with a sterile knife before infection. The symptom monitoring was adapted from Raoul des Essarts et al. (2015). The number of symptomatic potato plants (blackleg disease) and hyacinths (soft-rot disease) was recorded biweekly (see examples of symptoms in Figure 1a-b ). For each strain and experimental population and unwounded and wounded conditions, the number of symptomatic and asymptomatic plants (hence two symptom classes) was counted at 60 days post infection (dpi) using 15 potato plants (up to 25 potato plants when indicated) and 8 hyacinths. A set of non-inoculated plants was used as an asymptomatic control. In order to facilitate visualization of the aggressiveness data in the figures, they are presented as a percentage of symptomatic plants.
Potato tubers (S. tuberosum var Bintje) were inoculated using a tip to inject 10 µL of pathogen strains and experimental populations at 107 (or 105 when indicated) CFU per tuber. After 5 days of incubation at 24°C, the tubers were cut in half and scored in five aggressiveness classes according to the symptom gravity in each tuber (from no symptom to the most severe symptoms;Figure 1c ). For each strain and experimental population, at least 10 tubers were inoculated. A set of non-inoculated tubers was used as an asymptomatic control. In order to facilitate the presentation of the aggressiveness data in the figures, results are presented as normalized disease severity index (DSI) of which values, ranging from 0 to 100 (arbitrary unit), increase with symptom severity. DSI was calculated using the five aggressiveness classes assigned to 10 tubers, see formula in Supplementary Methods 1 (SM1).
Symptomatic tissues from potato stems and tubers, as well as from hyacinths, were also used to quantify D. solani and D. dianthicola pathogens by Taq Man qPCR as described inSupplementary Methods 2 (SM2) . Assessing D. dianthicolaand D. solani in inoculum and plant tissues (potato stems and tubers, and hyacinths) permitted the calculation of competitive index (CI) values following the formula presented in SM1 . A CI value equal to one indicated an equal fitness between D. solani andD. dianthicola , CI value greater than one indicated a fitness advantage to D. solani , and CI value below one indicated a fitness advantage to D. dianthicola .