Modifying critical thresholds in Simcast to achieve better late blight management for moderately susceptible potato and tomato crops

A decision support system (DSS) for potato and tom ato late blight m anagem ent has been developed to provide disease m anagem ent recom m endations. The system is com prised of two disease-forecasting tools as well as a simulator. Results from simulation and field experim ents w ith the system have shown that the DSS schedules on moderately susceptible crops did not achieve sufficient disease suppression under certain circumstances. We used w eather data from 2000 to 2010 from over 140 w eather stations to generate spray schedules. We then inputted schedules into the sim ulator to get disease severity and fungicide use efficiency. We then modified the default critical value in the disease-forecasting tool, compared those results and got the critical thresholds by improving disease suppression while m aintaining similar fungicide use efficiency. The prim ary objective of this research was to improve disease suppression for m oderately susceptible cultivars while maximizing fungicide use efficiency. This change has subsequently been program m ed into the on-line version of the DSS. Introduction: Phytophthora infestans (Mont.) de Bary, the oomycete causal agent of potato late blight, is causing significant loss to potato production globally. A total annual loss of $6.7 billion was estim ated for related disease control efforts and yield loss (Haverkort et al., 2008). Late blight has proven to be equally devastating to tom ato crops as well as potatoes. In 2009, a late blight epidemic struck major US tom ato farms via contam inated tomato transplants of a major distributor (Fry et al., 2013). A typical late blight m anagem ent plan includes reducing disease introduction, survival, and infection rate of late blight. Late blight is extremely devastating and can destroy crops within a week (Danies et al, 2013). Fungicide application rem ains the m ost effective disease control method upon detection. Farmers generally adopt weekly spray schedules. This traditional spray scheme is inefficient given tha t weekly applications may not achieve the m ost efficient disease control (Danies et al, 2013). The developm ent of the USABlight Decision Support System (DSS) makes a more efficient fungicide spray plan possible. Late blight disease developm ent is typically sensitive to tem perature and humidity. The optimal temperature for disease development is 15° C and the temperature that favors sporangia germination is 4° C. Additionally, high humidity generally favors late blight development (Danies et al, 2013). Crops with different resistant levels also react differently to the disease and require different fungicide application schedules. Lastly, weather condition also affects the amount of fungicide remaining on leaves -which also influences the disease management. The DSS for potato late blight is a system that integrates available information (weather data, crop resistance, fungicide residue, etc.) to predict future disease severity and suggests fungicide application at critical timings based on the forecast. The system functions by quantifying the effect of weather in "severity values". Once the cumulative severity value goes over the critical threshold in the system a fungicide application is recommended. Field evaluations of the BlightPro DSS have been conducted in year 2010, 2011, 2012, and 2013 (Shtienberg & Fry, 1990). Researchers compared the disease suppression level and number of fungicide applications of DSS schedules to weekly spray schedules. Experiments were conducted among all three resistant levels of crops (Susceptible, Moderately susceptible, and Moderately resistant). The results elucidate that the both DSS schedules and weekly schedules significantly suppressed the disease (P<0.05) for all three resistant levels. Further, there was no significant difference between the DSS group and weekly schedules (P<0.05) among those three groups (Ian Small, 2015). As for fungicide application, the number of sprays recommended by the DSS differed among the three categories. For Susceptible cultivars, the DSS recommended 24% more fungicide applications in average than the weekly schedule with a range form -91% to +91%. For moderately susceptible cultivars, the DSS recommended averagely 15% fewer applications relative to a weekly schedule. For moderately resistant cultivars, the DSS recommended 36% less applications in average than the weekly schedule with a range from -91% to 0% (Ian Small, 2015). Therefore, field evaluations of DSS proved the effectiveness of the system. To expand the sample size of the comparison, Small et al have run computer simulations with 6912 simulations in 59 locations for 13 years (2010 to 2013). The result is present in the figure below. 7000 6000 5000 4000 300