1.2 The Economic Importance of Wheat Yellow Rust and Historical Epidemic
China is the largest country for wheat production and consumption in the world, annual production accounts for 17% of the world. Wheat is the second most important cereal and food crop in China currently. The total cultivated area is 24.27 million hectares and production is 131.44 million tons (From National Bureau of Statistics, 2018), which plays an important role in both domestic and global food security. While, yellow rust, caused by Puccinia striiformis f. sp. tritici (Pst), is the most destructive disease on wheat, which causes significant yield and economic losses and severely threatens the security of wheat production due to its wide transmission and high frequent epidemics. It has been announced as the most important crop disease nationally by the Ministry of Agriculture and Rural Affairs of China on Sept 15th, 2020.
Historically, the earliest record of wheat yellow rust in China was in an earliest Chinese book, Ancient Chinese Agricultural Practices, written by Jia Sixie during Northern Wei Dynasty (533-544), it is about 1,480 years ago. More recently it has been recorded in the book Agricultural Proverbs in 1836. In both books, yellow rust was called Jaundice, like a human disease. The former book recorded that the yellow rust was related to spring rainy condition (Li and Zeng, 2002; Chen and Kang, 2017).
Currently, China is regarded as the largest epidemic zone in the world with more than 20 million hectares of wheat crop infected by the disease every year since 1950s (Wan et al., 2007; Chen et al., 2009), the first outbreak of yellow rust disease, which caused yield losses up to 6.0 million tons. Since then, several epidemics occurred in 1964, 1990, 2002, 2017, 2019, and 2020, and annual yield losses averaged about 1 million tons. The disease is more prevalent in the winter-wheat growing areas of the northwest, southwest, and north, and the spring-wheat growing areas in the north-west of China, covering 16 provinces (Li and Zeng, 2002).
Yunnan located in southwest of China, bordered on Myanmar to the west, neighboring the Himalayas, with currently 339,200 hectares wheat cultivated (From National Bureau of Statistics, 2018). Complex terrain and physiognomy, quite different altitudes, as well as diverse climate have resulted in year-round wheat cultivation. This year-around wheat provides green bridge for rust pathogen, which makes the pathogen over-summering and over-wintering easily and infect wheat constantly (Li et al., 2021). Berberis spp. and Mahonia spp., the alternate hosts of Pst (Jin et al., 2010; Zhao et al., 2013; Wang and Chen, 2013), are distributed widely in Yunnan Province (Ying et al., 2011; Fig. 1.1). Collectively these factors have caused frequent epidemic in the past, and Yunnan acts as an epitome of China. The outbreak of wheat yellow rust in Yunnan, not only causes local yield losses, but also provide original incursion sources for other parts of China (Li et al., 2021), which plays a crucial role for large scale disease epidemics.
Comprehensive research on the epidemiology of wheat yellow rust has been carried out among a nationwide network of colleagues working for more than six decades. These studies attempted to understand the over-summering, over-wintering, inter-regional dispersal of inoculum, resistance gene utilization and resistance mechanisms, race monitoring and population genetics, pathogenicity mechanisms, and integrated management in China. The progress of each aspect is introduced as follows.
Fig. 1.1 Wheat cultivation under diverse agro-ecological zones and Berberis spp. infected by yellow rust pathogen naturally in China