Many manufacturers of systemic, single-site fungicides are now marketing them formulated
as combinations with multi-site fungicides, with the intent of "preventing"
the evolution of resistance to the systemic fungicides. Similarly, alternate
applications of fungicides with different modes of action or "cocktails"
of two or more fungicides applied in the same spray or have been used as a means of
combating fungicide resistance. The often-stated rationale for these
fungicide combinations is that they prevent the buildup of resistance
because the multi-site fungicide in the spray program kills
any mutants resistant to the single-site fungicide.
This explanation of the effectiveness of fungicide combinations is not
entirely correct, since a multi-site fungicide such as captan is no more
effective against a benomyl-resistant Venturia population than it is against
the wild-type population. However, even partial suppression of the resistant
population will reduce the rate of selection of fungicide resistance and give
the grower time to react before there is a catastrophic crop failure.
Fungicide combinations, therefore, are a good tactic, not because they
prevent resistance, but because they slow down the selection of resistance
and thus can prevent crop loss.
To see the effect of captan on the selection of benomyl resistance, let us
run the simulation using the same benomyl spray schedule as we did in
Exercise 2. This allows us to make direct comparisons using the same dose
of benomyl, with and without captan. To simplify the spray scheduling,
let us merely select the default spray schedule of captan as in Exercise 1
and superimpose it on the benomyl spray schedule.
Reinitialize the model as before, using Load Data File to load the
Venturia data file. In the Fungicides, Select... dialogue box,
select benomyl as one fungicide and captan as the other. Then click on
Fungicides, Schedules... to confirm the default spray schedules (sixteen
weekly sprays of 5 lb captan/acre, beginning on day 1, and 8 biweekly sprays
of 0.5 lb benomyl/acre, also beginning on day 1). This is equivalent
to applying a tank mix of benomyl and captan every other week and captan
alone in the alternate weeks.
Run the simulation for five consecutive seasons, an examine the year-end
summary in the Log.
What is the effect of the combination with captan on the selection of
benomyl resistance?
Applying a full spray schedule of captan on top of a full spray schedule
of benomyl is too costly to be practical and amounts to overkill as far as
apple scab control is concerned. How might you modify the spray program
to achieve adequate control at a reasonable cost for longer than just two
to three seasons?
