# Winter Wheat Take-All Disease Index Estimation Model Based on Hyperspectral Data
## 2 Materials and Methods
### Data Acquisition and Processing  
Disease Investigation and Spectral Collection  
Spectral data were obtained under field-conditions, on May 21 2017, which was in the wheat grain filling stage and the \"white spike\" characteristics of wheat infected by take-all were the most obvious [23]. In order to obtain more accurate data and avoid the impact of other diseases, pests and pests on this experiment, we selected the sampling points in the field to be more uniform and the experiment was conducted after the occurrence of wheat rot disease was evaluated by botanists. According to different degrees of disease, 30 sample points in the field were selected as the sample points of canopy spectrum data; 1 m \(\times\) 1 m in the right front of each spectral measurement sample was used as the disease investigation quadrate and the investigation method was carried out according to the national standard Pesticide--Guidelines for the field efficacy trials (II)--Part 109:Fungicides against take-all of wheat (GB-T-17980.109-2004), which stipulates the judgment requirements of the disease level in Table 1. In other words, wheat samples were selected from each root  
Figure 1: The location of the field test area for the take-all wheat disease and the roots of the diseased wheat. quadratic according to the traditional five-point sampling method (in the central area and the surrounding four corners) and 1 m long double-row wheat was selected from each spike quadrant to investigate the number of total spikes and the number of white spikes. Formula (1) is the formula used for calculating the White spike rate. The roots of samples were washed with clean water and the incidence of root disease was investigated under a white background. The disease progression was calculated according to the percentage of infected roots of each plant. The disease grading criteria are shown in Table 1.  
In order to comprehensively consider the incidence and severity of wheat take-all, the disease index was calculated by combining the white spike rate and disease progression, which was used as a comprehensive index to evaluate the incidence degree of wheat take-all in this paper. Disease index (DI) calculation formula is:  
\[\text{white spike rate of wheat}\;(\%)=\frac{\text{the number of witheredwhite spikes}}{\text{the number of total spikes}}\times 100 \tag{1}\]  
\[\text{Disease index}\;=\frac{\sum(\text{Number of white spikes of each grade}\times\text{ Disease grade})}{(\text{The number of total spikes}\times\text{Maximum disease grade})}\times 100 \tag{2}\]  
According to previous research methods [24; 25], the disease index was divided into four grades: healthy (0), mild (0-30%), moderate (30-50%) andsevere (>50%). In this data, there were 7 healthy samples, 16 mild samples, 5 moderate samples and 2 severe samples. The hand-held field spectral radiometer, ASD Field SpecPro HH (American Analytical Spectral Devices, Inc.). was used to collect the spectral reflectance data of wheat canopy. The appearance of the instrument and the picture of the data collected in the field are shown in Figure 2. This equipment can continuously measure spectral data in the range of 350-1050 nm, the spectrum sampling interval is 1 nm and the spectrum resolution is 3 nm. The measurement time was chosen from 10:00 to 14:00 on a sunny, windless and cloudless day (the solar altitude angle is greater than 45\({}^{\circ}\)). Before measurement, the instrument was preheated in natural light for 20 min and then calibrated on standard whiteboard. The spectral measurement was carried out under natural light. The optical fiber probe was vertically downward and the distance from the measured wheat canopy was 50 cm and the area below the probe was ensured to have no shadow. For each sample area, a continuous acquisition of 10 spectral curves was performed and the average was taken as the representative spectral curve of that sample. Before each measurement, the instrument was optimized once and a standard whiteboard calibration was carried out.