  Fungicide mixtures produced by the agrochemical industry often contain
low-risk fungicides, to which fungal pathogens are fully sensitive, together
with high-risk fungicides known to be prone to fungicide resistance. Can these
mixtures provide adequate disease control while minimizing the risk for the
development of resistance? We present a population dynamics model to address
this question. We found that the fitness cost of resistance is a crucial
parameter to determine the outcome of competition between the sensitive and
resistant pathogen strains and to assess the usefulness of a mixture. If
fitness costs are absent, then the use of the high-risk fungicide in a mixture
selects for resistance and the fungicide eventually becomes nonfunctional. If
there is a cost of resistance, then an optimal ratio of fungicides in the
mixture can be found, at which selection for resistance is expected to vanish
and the level of disease control can be optimized.
