1.3 Epidemiology

Wheat yellow rust can be dispersed for hundreds or thousands of kilometers by upper air-flow infecting a large area of wheat. Studies indicated that Pst over-summers in western wheat growing regions, and over-winters in eastern wheat growing regions in China. The pathogen, thus complete its life-cycle through winter and summer by dispersal between western and eastern China.

As the over-summering is the weakest part in the life-cycle of Pst, the over-summering regions are the core areas in epidemic zone, so it has been paid much attention and studied in details. Several decades research discovered four over-summering areas in China, namely northwestern (Shaanxi, Gansu, Sichuan, Ningxia and Qinghai), southwestern (Tibet, Yunnan and Guizhou), Xinjiang, and northern (Shanxi and Hebei) over-summering regions (Li and Zeng, 2002; Ma et al., 2005). The northwestern and southwestern regions are the largest and the most important over-summering areas in China, and may overlap. These areas are usually mountainous, wheat grown from lowland (alt. 800m) to highland (alt. 2,500m) (Wan et al., 2007). The harvest period extends for 5 months from early May to early October. Sowing dates vary from late September to late October. Autumn-sown lowland wheat emerges before the harvest of highland wheat. Especially for Yunnan province, wheat grown all over the year, which means that host plants are available for the pathogen all year round in these over-summering regions.

Early studies showed that ideal environmental conditions in these northwestern and southwestern areas made them major centers of yellow rust epidemics and the source for new races. Almost all Chinese races were first detected in these regions (Wan et al., 2007), which serve as sources of inoculum and contribute to the development of new races for the other regions of China. The first “breakdown” of every resistance gene has occurred in these regions.

In contrast, the eastern plain regions, including Shaanxi, Shanxi, Henan, Hebei, Hubei, and Shandong provinces, due to high temperature and harvest time, there is no chance for the pathogen to over-summer, and therefore considered as non-over-summering regions.

Wind-borne urediniospores arriving in non-over-summering areas in eastern China can infect autumn-sown wheat seedlings when weather conditions are favourable and susceptible cultivars are grown. Under favourable conditions, the infections continue and produce urediniospores before the pathogen undergoes over-wintering.

During winter, the pathogen survives mainly as mycelium in live plant tissue, but may also survive as urediniospores under certain climatic conditions, e.g., under snow cover or in milder winters (Li and Zeng, 2002). The over-wintered urediniospores and mycelium contribute to spring and summer epidemics and regional spread.

For some mountainous western areas, the pathogen can complete its life cycle locally such as Yunnan, Tibet, West Sichuan, South Gansu, Middle and Southwest Xinjiang, where the Pst can oversummer and overwinter (Ma et al., 2005). This increases the chances for mutation and thus the break-down of resistance in newly introduced cultivars, followed by serious epidemics. The achievements of epidemiology have provided scientific basis for effective control strategies for wheat yellow rust.