P-303 - Considering pathogen physiology to improve potato early blight forecasting

Abstract Text: Early blight, caused by Alternaria solani, is a common and potentially destructive foliar disease of most potato cultivars, especially early-maturing varieties. In production regions of high humidity and routine precipitation, early blight is the primary reason for weekly fungicide applications on potatoes (Guenthner et al. 1999). Currently, many regions use the physiological maturity of the host to predict disease risk and initiate fungicide sprays with use of Potato Physiological Days (P-Days) which are similar to growing degree-days. We sought to assess the efficacy of P-Days and improve the modeling of initial early blight infection to further refine the timing of fungicide applications. We used historical disease scouting data from a regional crop scout consultant and corresponding archived environmental conditions. Pre-season variables such as historic land use, surrounding landscape, and overwintering conditions were included to represent the primary pathogen population risk for each site-year in the disease rating dataset. Then, we included in-season variables related to daily temperature, moisture, and wind from the two weeks prior to early blight lesion observations. Specifically, in-season variables included air temperature, precipitation, irrigation, relative humidity, dew, leaf-wetness hours, and windspeed. The P-Day model method (based only on physiological maturity of the host) has been accurate in predicting early blight onset to within +/- 1 week on average.