## 2 Material and Methods  
### Sample Preparation  
According to the guidance of the local senior agronomist, citrus trees of disease were roughly divided by visual inspection method for convenient sampling [24]. The samples were all collected from an orchard planting base in Jiangxi province. Samples were collected from 5 divided regions of the base so as to present their differences. Three citrus trees were randomly selected from each region and the distance among the three trees was over 12 m. A total of 180 leaves were collected with 12 leaves picked from each tree on the same layer in four directions (east, south, west and north). The leaves were loaded by self-sealing bags with labels and brought back to the laboratory for pretreatment (such as washing and airing). Samples were further screened and divided by polymerase chain reaction (PCR), and were synthesized by Genscript company in Nanjing [25; 26]. The PCR technique is generally adopted by the conventional nuclear acid extraction method, which is expanded. The cycle is composed of three steps: high-temperature variation, low-temperature annealing and medium temperature elongation. The denaturation process should be kept at a high temperature of 95 \({}^{\circ}\)C left and right; the reaction temperature decreased to 60 \({}^{\circ}\)C left and right during annealing, while the reaction temperature increased to 72 \({}^{\circ}\)C left and right during extension stage, so as to ensure that the target was fully amplified [27]. According to [PERSON]'s report of the OI\({}_{1}\)/OI\({}_{2}\) primers and [PERSON]'s report of A2/J5 primers. The sequences of OI\({}_{1}\)/OI\({}_{2}\) and A\({}_{2}\)/J\({}_{5}\) were: 5\({}^{\prime}\)-GCGGTGATCCAATACGAGGGA-3\({}^{\prime}\), 5\({}^{\prime}\)-GCCCTGAGACTTGGCAACCCAT-3\({}^{\prime}\), 5\({}^{\prime}\)-TATAAAGGTTGACCTTTGAGTTI-3\({}^{\prime}\), 5\({}^{\prime}\)-ACAAAAGCAATAGCACGAACGA-3\({}^{\prime}\). The polymerase chain reaction test was positive in diseased leaves, and negative in those without disease, as shown in Figure 1. At the same time, to show homogeneity and representativeness, the number of each category sample was the same, and since the symptoms of nutrient deficiency samples were similar to those of citrus yellowing samples, we selected samples with nutrient deficiency for comparative experiments. (The OI\({}_{1}\)/OI\({}_{2}\) primers are abbreviated as O\({}_{1}\)/O\({}_{2}\) for convenient labeling in Figure 1.)  
Figure 1: The results of common polymerase chain reaction test for citrus leaves. NOTE: The numbers, 1, 2, and 3 represent the label of the citrus tree to which the leaves belong; “Lack” represents the zinc deficiency samples. “M” stands for DNA molecular weight standard; “Blank” is only for comparison; slight, moderate and serious indicate the disease grade. It could be concluded from Figure 1 that the test results of the polymerase chain reaction, with OI\({}_{1}\)/OI\({}_{2}\) as the primer, were clearer. In Figure 1, the slight, moderate and serious citrus greening successively showed bright bands and the color became brighter with an increase in seriousness of the disease while the normal leaves showed no bright band. Different from those of leaves with citrus greening, the bright bands of nutrient deficiency leaves may have a connection to the lack of nutrients in leaves [28; 29]. The infection situation of five types of leaves and the color diagram of representative samples are shown in Table 1.  
By considering the influence of disease infection on citrus yield and the results of conventional polymerase chain reaction testing, the experimental samples were divided into five levels: Level 1 was the slight grade of citrus greening, Level 2 was the moderate grade of citrus greening, Level 3 was the serious grade of citrus greening, Level 4 was nutrient deficiency, and Level 5 was normal [30]. During leaf preservation period, 24 damaged samples were eliminated in the process of spectra collection in case they affected the experiment.