Fitness of Venturia inaequalis isolates with resistance to multiple chemical classes of fungicides in vitro

Background. The development of fungicide resistance in the apple scab pathogen suggests the development of measures to manage the development of resistance that can functionally delay the emergence of resistant populations of the pathogen. Knowledge of the pathogen's fitness is essential to applying this approach and assessing how successful it will be, since resistance may come with a fitness cost.

Purpose. To evaluate the fitness of sensitive Venturia inaequalis isolates and isolates with multiple fungicide resistances to osmotic and oxidative stress in vitro.

Methods. The size of isolates from four groups (a group with baseline sensitivity and three groups with different multiple resistances to fungicides) was evaluated on potato-glucose agar with the addition of various concentrations of NaCl (2, 3, 4, and 6%) and H₂O₂ (2, 5, and 10 mM).

Results. The inhibition of mycelium growth in variants with the addition of NaCl relative to the control occurred on average by about 10% with an increase in concentration of 1%. In all variants of the experiment, the average size of isolates in the initial group was smaller than in the pathogen groups with resistance to fungicides. Oxidative stress has the following effect on the average size of the isolates: in the control, it was 12.2 mm; at a concentration of H₂O₂, it was 2 mM–6.7 mm; and at 5 mM–1.3 mm. There was a tendency for the isolates to have a higher average size in the group with simultaneous resistance to two fungicides, compared with other groups at concentrations of H₂O₂ of 2 mM and 5 mM.

Conclusion. The absence of significant lower values of the average size of isolates in groups with multiple resistances to fungicides in comparison with the baseline group under in vitro stress conditions shows the absence of fitness cost.