2.1.1. Field Survey Area

Field surveys of wheat yellow rust were conducted in Ningqiang county (118◦3519.5" E, 37◦3551.75" N), Shaanxi province, China from 11 May 2018 to 14 May 2018 (i.e., the filling stage, Figure 2). In Ningqiang County, wheat is a major crop, and yellow rust is the dominant wheat disease; a severe infestation occurred in 2018. The average annual temperature and precipitation of this area are 13 ◦C, and 1812.2 mm, respectively; low temperature and high humidity environment conditions are conductive to the occurrence, spread, and epidemic of wheat yellow rust [23]. Shaanxi Province is considered to be an important spring epidemic area and winter breeding area of wheat yellow rust in China [26]. Therefore, there is an urgen<sup>t</sup> demand to monitor wheat yellow rust disease in this region.

Three data types, including field survey data of yellow rust disease, multispectral satellite images, and meteorological information, were collected to develop a wheat yellow rust monitoring model on a regional scale.

**Figure 2.** Map of Ningqiang county, Shaanxi province in China. Green crosses denote field survey locations of healthy wheat, red denote field survey locations of yellow rust-infested wheat.
