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​​​​​Selection of Fungicide Resistance: Simulation with Resistan

​Phil A. Arneson

​Cornell University

Date Accepted: 08 Apr 2004
 Date Published: 08 Apr 2004

Resistan Exercises
Answer Sheet for Instructors

Exercise 1. Captan

If all is working correctly, the screen at the end of the first season should look like this.)

The following is the year-end summary at the bottom of the Log file, edited to remove the extraneous columns and to line up the columns to improve the presentation.

    End Year    Active Lesions    % Res to benomyl     Profit
    1               4438.62              0.0           951.22
    2              11817.20              0.0           936.82
    3              31438.58              0.0           899.73
    4              83476.66              0.0           808.97
    5             220508.70              0.0           611.86
    6             574652.63              0.0           272.46

The parameters in this data set correspond to rates of fungicide weathering and rates of infection that would occur on a very susceptible cultivar in an unusually rainy season, and the initial inoculum (5000 ascospores/acre) corresponds to a very high level of infection the previous season. Under these conditions captan does not adequately control apple scab, and the quantity of ascospores to begin each successive season creeps upward slowly.

Exercise 2. Benomyl

The percent of the Venturia population resistant to benomyl rose quickly to nearly 100% during the second season. (See the graphic output of the second season of the benomyl simulation.) In subsequent seasons, resistance to benomyl remained at 100%, making the benomyl sprays completely ineffective. The carryover inoculum from one season to the next grew higher in each succeeding season, and by the fourth season the level of disease had risen to the point where the crop was no longer profitable.

    End Year    Active Lesions    % Res to benomyl    Profit
    1                 71.36              0.44        1015.84
    2                 61.92             95.11        1015.88
    3              11065.33            100.0          994.70
    4            1927352.82            100.0         -185.16

The interesting point is that even though the level of resistance had risen to 95% by the end of the second season, the level of disease was still very low (62 lesions per acre is less than one lesion per tree--probably undetectable), owing to the very effective control the previous season and the very low carryover of inoculum. Even in the third season, with 100% resistance, the level of disease had not yet risen to alarming level. (If you figure 100 trees per acre, it's only 110 lesions per tree.) A grower would certainly notice that the scab control was not as good as it had been, but it was still on a par with the control one would get with captan. By observation of disease control, a grower may or may not suspect benomyl resistance by the end of the third season. It would, however, be detectable by resistance monitoring. It would not be until the fourth season that the lack of disease control would make benomyl resistance obvious.

Exercise 3. Inoculum Level

Note that the percent resistance in the second season with the low inoculum level follows exactly the same pattern as we got with 10 times the initial inoculum. The only difference is that there is a lower level of disease. (See the graphic output of the second season of the low inoculum simulation.)

    End Year    Active Lesions    % Res to benomyl    Profit

    1                  7.14              0.44        1015.98
    2                  6.19             95.11        1015.99
    3               1106.95            100.0         1013.84
    4             206471.55            100.0          692.47

Looking at the year-end summary, it is clear that under the same benomyl spray schedule the rate of selection of resistance is exactly the same, regardless of the level of initial inoculum. The difference is in the level of disease. As we would expect, with one-tenth the level of initial inoculum we see one-tenth the number of lesions until the level of disease rises to the point where we begin to see the effects of density-dependent feedback. Disease control with the higher level of initial inoculum failed much faster, not because it selected resistance faster but because it started with a higher level of inoculum. The rate of selection of resistance is a function of the spray program, not of the level of disease when the spray program is started.

In other words, we do not risk a higher rate of selection of resistance if we use benomyl as a "rescue" treatment than if we simply use it to maintain disease control. If we were only observing the numbers of lesions and not monitoring resistance, it would appear as if the evolution of resistance were higher when we had a higher level of disease at the start.

Exercise 4. Reduced Dose of Benomyl

    End Year    Active Lesions    % Res to benomyl    Profit
    1               1193.25              0.03        1013.57
    2                922.79              7.48        1014.13
    3              14637.39             95.83         987.90
    4            2572315.89             99.98        -303.24

Comparing the year-end summary using a reduced dose of benomyl with that of the full dose in Exercise 2, we can see that the rate of selection of resistance is significantly lower with the reduced dose compared that with the full dose. Of course, the reduced dose is not as effective in controlling the disease, so if one is only looking at the numbers of lesions the perception is that the low dose fails faster.

For the reduced dose of benomyl to be effective in controlling apple scab, it will have to be combined with another fungicide or with other methods of disease suppression.

Exercise 5. Fungicide Combinations

    End Year    Active Lesions    % Res to benomyl    Profit
    1                 17.56              0.02         959.96
    2                  0.19              4.87         960.00
    3                  0.04             92.06         960.00
    4                  0.73             99.96         960.00
    5                  1.75            100.0          960.00
    6                  4.16            100.0          959.99
    7                  9.93            100.0          959.98
    8                 23.68            100.0          959.95
    9                 56.47            100.0          959.89
   10                134.68            100.0          959.73

If one were to look only at the numbers of lesions, it would indeed appear as if the fungicide combination prevented the evolution of resistance. However, by monitoring resistance we can see that the combination of captan with benomyl only reduces the rate of selection of resistance. (Compare the resistance at the end of each year with those of benomyl alone in Exercise 2.)

Clearly applying a full dose of captan on top of a full dose of benomyl amounts to overkill and is too expensive to be practical. To reduce the cost of the spray program, one can play around with reducing either the dose of each fungicide in each spray or reducing the number of sprays of each fungicide. One can gain the effect of further reducing the rate of selection of resistance to benomyl by reducing the dose or number of sprays of benomyl. This has to be weighed against the cost of reducing the number of captan applications to reduce the application cost.

Exercise 6. Reduced Spray Schedule

    End Year    Active Lesions    % Res to benomyl    Profit
    1                650.46              0.01         986.71
    2                254.51              0.27         987.49
    3                112.81             12.19         987.78
    4                316.51             87.79         987.37
    5               5628.42             99.73         976.88
    6             112953.88             99.99         789.94

Reducing the numbers of benomyl sprays in the spray program and maintaining apple scab control with captan clearly reduces the rate of selection of (but does not prevent) resistance. It is also clear that this program cannot be maintained indefinitely.

If you look at the results with benomyl alone (Exercise 2), it probably would not be possible to detect resistance by monitoring until the third season, by which time the Venturia population would already be 100% resistant to benomyl. In the above program, benomyl resistance could probably be detected in the third season also, but the level of resistance would only be 12%. This would allow the grower time to shift to another fungicide to prevent incurring significant losses to resistance.

Exercise 7. Host Susceptibility

    End Year    Active Lesions    % Res to benomyl    Profit
    1                 22.75              0.19        1015.94
    2                  1.38             77.65        1016.00
    3                 27.27             99.98        1015.94
    4                692.28            100.0         1014.47
    5              17563.21            100.0          977.95
    6             438592.95            100.0          386.95

Using a variety of apple that is slightly less susceptible to apple scab substantially reduces rate of epidemic development and slightly reduces the rate of selection of resistance. In fact, any factor that reduces the rate of epidemic development will also reduce the rate of selection of fungicide resistance.

Exercise 8. Reduced Fitness of the Resistant Biotype

Looking first at the selection of resistance where there is no fitness cost, we can see that In the absence of benomyl, the resistant population survives as well as the sensitive one, and its proportion of the total population remains the same.

In the case where we gave the resistant fungus a small fitness cost, we see that first of all, the rate of selection of resistance is not quite as high as it was where the was no fitness cost. The most important effect, however, we observe when the modified benomyl residues disappear. The resistant population is not as ecologically fit as the sensitive wildtype, and in the absence of continued selection by the hypothetically modified benomyl, the resistant population gradually declines and the total population slowly reverts to sensitive. (In the real world, there appears to be no fitness cost to benomyl, and there is no such reversion to the sensitive wildtype.)

If you look at the characteristics of myclobutanil, there is a small fitness cost to resistance, so if we stopped applying myclobutanil after selecting a resistant population, we would expect to see a gradual reversion to the sensitive wildtype.

Exercise 9.

This is a chance for the students to be creative and put together a spray program that both effectively controls apple scab and avoids selecting high levels of reistance to any fungicide. You might offer a prize to the student who has the highest net profit for the 10-year period. Remind them that they can change the spray program between seasons using Select and Schedules options in the Fungicides menu.

Contact: Phil A. Arneson
Last updated: April 8, 2004
Copyright 2002 Cornell University