Appendix A. Sensitivity analysis for the control parameters.

Sensitivity analysis for the control parameters

The Evolution of Fungicide Resistance Resulting from Combinations of Foliar-Acting Systemic Seed Treatments and Foliar-Applied Fungicides: A Modeling Analysis

<div><p>For the treatment of foliar diseases of cereals, fungicides may be applied as foliar sprays or systemic seed treatments which are translocated to leaves. Little research has been done to assess the resistance risks associated with foliar-acting systemic seed treatments when used alone or in combination with foliar sprays, even though both types of treatment may share the same mode of action. It is therefore unknown to what extent adding a systemic seed treatment to a foliar spray programme poses an additional resistance risk and whether in the presence of a seed treatment additional resistance management strategies (such as limiting the total number of treatments) are necessary to limit the evolution of fungicide-resistance. A mathematical model was developed to simulate an epidemic and the resistance evolution of <i>Zymoseptoria tritici</i> on winter wheat, which was used to compare different combinations of seed and foliar treatments by calculating the fungicide effective life, i.e. the number of years before effective disease control is lost to resistance. A range of parameterizations for the seed treatment fungicide and different fungicide uptake models were compared. Despite the different parameterizations, the model consistently predicted the same trends in that i) similar levels of efficacy delivered either by a foliar-acting seed treatment, or a foliar application, resulted in broadly similar resistance selection, ii) adding a foliar-acting seed treatment to a foliar spray programme increased resistance selection and usually decreased effective life, and iii) splitting a given total dose—by adding a seed treatment to foliar treatments, but decreasing dose per treatment—gave effective lives that were the same as, or shorter than those given by the spray programme alone. For our chosen plant-pathogen-fungicide system, the model results suggest that to effectively manage selection for fungicide-resistance, foliar acting systemic seed treatments should be included as one of the maximum number of permitted fungicide applications.</p></div

Flow diagram of the disease sub-model for leaf layer <i>i</i>.

<p>Healthy leaf area not lost by senescence (rate <i>σ</i>(<i>t</i>)) was infected at rate <i>ε</i> by primary inoculum (ascospores) and at rate <i>ρ</i> from secondary inoculum (pycnidiospores). The mean infectious period was 1/μ and the mean latent period was 1/ <i>δ</i>. The pathogen model included two strains; a fungicide sensitive strain, <i>S</i>, and a fungicide resistant strain, <i>R</i>. Parameters <i>δ</i>, <i>ρ</i> and <i>ε</i> for the sensitive strain were reduced according to the fungicide dose response functions.</p

Graphical representation of the 11 leaves in the crop canopy growth model in the absence of disease.

<p>Leaf lives overlap and were comprised of a monomolecular growth phase, a lag phase and a logistic senescence phase.</p

Seed treatment fungicide depletion for the constant versus transpiration-based seed treatment uptake model.

<p>Both models were parameterized such that 99.99% of the fungicide has been depleted from the seed at the time that leaf layer 5 is dead.</p

Parameter symbols, descriptions, values and units.

<p>Parameter symbols, descriptions, values and units.</p

Time course of the sensitive and resistant pathogen strain for a seed treatment.

<p>The simulation was run for a low fungicide breakdown rate and a constant seed treatment uptake model. Blue and red lines indicate fungicide-sensitive and fungicide-resistance area index, respectively. Top panel: leaf layer 11. Middle panel: leaf layer 5. Bottom panel: leaf layer 1. A seed treatment was applied at 4.5 mg/m², which provides a HAD gain that approximates that of a T1 spray, and the simulation was run for 10 growing seasons.</p

Seasonal ascospore spore concentrations.

<p>The peak of the function was during the winter (0–1200 degree days) and declined to 1% of the peak value at the time of death of leaf 5 (2094 degree days).</p

Time course of the sensitive and resistant pathogen strain for a T1 foliar treatment.

<p>The simulation was run for a low fungicide breakdown rate and a constant seed treatment uptake model. Blue and red lines indicate fungicide-sensitive and fungicide-resistant area index, respectively. Top panel: leaf layer 11. Middle panel: leaf layer 5. Bottom panel: leaf layer 1. A T1 spray was applied at 20 mg/m² and the simulation was run for 10 growing seasons.</p

Effective lives for the constant versus transpiration-based seed treatment uptake model.

<p>Effective lives for the constant versus transpiration-based seed treatment uptake model.</p