A problem case study: Influence of climatic trends on late blight epidemiology in potatoes

Late blight is temporally sporadic in potato crops in the Midwest US, occurring only when microclimate conditions within canopies are favorable and inoculum is present. Increasing concern over climate change projections has prompted numerous crop-based studies on the possible agricultural implications. It is not possible to evaluate sustainability without understanding the interactions between the influence of climatic trends, host resistance, cultural interventions and fungicide efficacy in relation to late blight risk. The objectives of this case study were to report the potential impact of climate change on late blight epidemiology in potatoes. Analysis of historical data from 1948 1999 indicated that late blight risk over a standardized growing season from 1 May 30 Sep increased in the Upper Great Lakes region of the US. Predominant genotypes of P. infestans (e.g. US8) in the US appear more tolerant of temperatures close to 0C and their survival in warming conditions may explain their supremacy. As conditions become increasingly favorable for late blight development it is essential to reduce sources of initial inoculum through integrated approaches that include prediction of conditions conducive to late blight development and appropriate application of controls. INTRODUCTION Late blight of potato (Solanum tuberosum, L.) caused by Phytophthora infestans (Mont de Bary), is a major worldwide threat to the production of high quality potatoes (Fry and Goodwin 1997). Potato late blight control strategies changed following the migration of mefenoxam/metalaxyl-resistant populations of P. infestans from Mexico to North America (Fry, Goodwin et al. 1993) and necessitate cultural control methods and crop protection strategies that rely primarily on protectant foliar fungicide applications (Kirk, Felcher et al. 2001). Although fungicides have been used to manage late blight, both the efficacy and availability of commonly used fungicides have been threatened. This problem is compounded by the demand to reduce chemical input in agricultural systems (Gray and Whalon 1996; Guenthner, Michael et al. 2001) and the potential loss of commonly used protectant fungicides such as chlorothalonil and mefenoxam (Gray and Whalon 1996). In addition, the cost of protecting potato crops in the United States against late blight is estimated at $77.1million annually (Guenthner, Michael et al. 2001) and losses and control costs are estimated at $3 billion worldwide (Forbes, Goodwin et al. 1998). The influence of climate change is therefore critical to assess changes in risk of late blight development for the future. Leaf wetness duration and in-canopy relative humidity are critical variables in determining the relative risk of late blight development. As a result, changes in meteorological variables throughout the growing season that influence the amount of inProc. XXVI IHC – Sustainability of Horticultural Systems Eds. L. Bertschinger and J.D. Anderson Acta Hort. 638, ISHS 2004 Publication supported by Can. Int. Dev. Agency (CIDA) 38 canopy moisture and vapor pressure could significantly impact subsequent disease pressure. This study addresses recent climate trends (Karl and Knight 1998) and their potential impact on potato late blight disease risk in the Upper Great Lakes region of the U.S. This historical perspective for potato late blight risk characterizes temporal trends in the greater Michigan region from 1948-1999. In North America, the probability that infected potato stems or foliage will emerge from an infected tuber is difficult to estimate as several factors can influence the fate of the infected tuber (Lambert, Currier et al. 1998; Powelson and Inglis 1999), temperature being one of the most important (Kirk, Niemira et al. 2001). The survival of viable host tissue from infection through dormancy to re-emergence the following spring is vital for survival of P. infestans (Zwankhuizen, Govers et al. 1998). Many investigators have used in vitro and soil assays to study the optimal and lethal upper temperatures for growth of Phytophthora spp. (Zentmyer 1981; Bollen 1985; Juarez-Palacios, Felix-Gastelum et al. 1991; Coelho, Mitchell et al. 2000). No studies have been found that examine the ability for P. infestans mycelium to survive at temperatures below zero. P. infestans can survive within infected tubers at 3C as stored seed (Kirk, Niemira et al. 2001), however the fate of mycelium of P. infestans within potato tubers exposed to temperatures below 0C has not been monitored. The objectives of this case study were to report the potential impact of climate change on late blight epidemiology in potatoes. METHODS AND MATERIALS Climate Trends Historical hourly air and dew point temperatures were extracted from the National Climatic Data Center’s (NCDC) Surface Airways data-set (NCDC, 1948-1999) for seven first order National Weather Service (NWS) stations in the greater Michigan region. The influence of climate on disease risk is quantified with a modified Wallin disease severity index (Wallin 1962). The index is simple and completely dependent on meteorological variables, without considering irrigation, other cultural practices or pathogen biotype changes could impact late blight risk. Potato late blight disease severity values (DSV) were calculated for each day from May 1 through Sept 30 at each location every year. DSV were based on a modified Wallin method used by Michigan State University Late Blight Lab (Baker, Andresen et al. 2000). A relative humidity threshold of 80 percent was used to classify hourly values as conducive for late blight if the associated air temperature ranged from 7.2 to 27°C. Trends in the timing and accumulation of disease severity values were quantified using a non-parametric slope estimator (Sen 1968). The probability of detecting a statistically significant difference between risk indicators, derived as secondary variables from weather data, was estimated at the p=0.05 significance level using Kendall’s tau b non-parametric correlation coefficient (SAS statistical software package). Kruskall-Wallis one-way analysis of variance on ranks was also used at the p=0.05 level to test for statistically significant differences between potato late blight risk indicator estimates at various locations. Thermal Tolerance Briefly, 50 plates of each of four isolates of Phytophthora infestans [US1, 8, 11, 14 (Goodwin, Schneider et al. 1995)] were prepared 48 h prior to introduction to the temperature treatment. Plates were labeled with culture ID numbers and exposure times and transferred to a PTC-1 Peltier-effect temperature cabinet controlled by a PELT-3 Peltier-effect temperature controller (Sable Systems International). The PTC-1 chambers were positioned in temperature-controlled environment chambers, 1.8 m volume at 5C. Plates were removed after exposures of 1, 4, 8, 12 and 24 h and in a second experiment after exposures of 1, 2, 3, 4, and 5 days. Temperature treatments were 0, -3, -5, -10 and 20C (experiment 1) and 0, -3, -5 C (experiment 2). After plates were removed from the PTC-1 Peltier-effect temperature cabinet they were stored in the light at 12C. Plates (n =