XB YANG1, P SUN1 and B-H HU2

1Iowa State University, Ames, USA; 2General Station of Pest Monitoring and Forecasting, PR China

Background and objective
Over the past two decades, increases in abundance of previously minor infectious diseases have been observed by botanical and medical epidemiologists. For instance, grey leaf spot (Cercospora zea-maydis) has become a major corn disease in the USA. Sudden death syndrome (Fusarium solani f. sp. glycines) emerged from being a minor disease in the 1970s to become a widespread problem in US soybean-producing regions. The underlying evolutionary mechanisms of infectious diseases have received considerable interest as they are fundamental to long-term prediction and management of agroecosystems. Different hypotheses, such as the consequence of global climate change, have been raised. However, investigations are scarce because of a lack of long-term pest records on a macro-scale. The General Station of Pest Monitoring and Forecasting of the Ministry of Agriculture, PR China, has monitored and systematically recorded the seasonal occurrence of various pests of major agricultural crops since 1950, through its network of stations at provincial, legislative regional and county levels. Computerization of these data makes the analysis of such a vast amount of information possible. We report preliminary results from analyses of long-term dynamics of plant diseases in China.

Materials and methods
Our study focuses on disease evolution in two major crops, wheat and rice. The study has two phases: (i) to detect trends of individual plant diseases; and (ii) to identify the driving agricultural or climatic variables by statistical analyses. Four diseases were selected: (i) wheat stripe rust (Puccinia striiformis) which is a cool-season disease; (ii) wheat powdery mildew (Erysiphe graminis); (iii) wheat scab (Fusarium spp.), a mid-season disease in central and southern production regions; and (iv) rice blast (Pyricularia oryzae), a disease in tropical and temperate regions. Long-term instrumental climatic data of PR China were from the Institute of Atmospheric Physics, Chinese Academy of Sciences and the Numeric Data Package (NDP) prepared by the Carbon Dioxide Information Analysis Center (CDIAC), USA. The database has records from 270 stations, with 65 of them containing monthly means, extremes, or totals of barometric pressure, air temperature, precipitation, RH, sunshine duration, cloud amount, wind (direction, frequency, speed) and days with snow cover.

Results and discussion
Our results show that one of the climatic trends in China is an increase in average temperature with a significant increase in minimum temperature, which confirms results of a previous study. The prevalence (acreage of occurrence) of wheat scab and rice blast increased drastically in the 1970s. Powdery mildew, which was an unnoticeable disease in Chinese wheat production before the 1970s, has become a leading yield-limiting factor. In contrast, levels of stripe rust in northern wheat-producing regions have shown a decreasing trend since the 1970s, perhaps because of unfavourable warmer seasons (late spring and early summer) as indicated by results of our analysis. Four monthly climate variables (average temperature, average minimum temperature, average maximum temperature and difference between monthly maximum and minimum temperature, DMMT) were used in regression against disease prevalence. DMMT appears to have a better correlation with disease prevalence.