Ionome Visualization of Sugarcane White Leaf Disease (SCWL) Infected Sugarcane Using Micro X-ray Fluorescence Spectroscopy

Background and Objectives : Sugarcane White leaf (SCWL) is an important disease that severely affects sugarcane production in Thailand. The infected sugarcane plants could either become symptomatic or remain symptomless (asymptomatic). The infected cutting can be used for a new plantation causing disease to spread. There are very few studies on the impact of SCWL phytoplasma to infected sugarcane particularly at the elemental level making farm management and nutrient supplementation difficult. This study aims to examine the impact of SCWL phytoplasma on nutrient distribution in sugarcane leaves and to evaluate the utilization of micro x-ray fluorescence spectroscopy for the identification of nutritional elements.

Methodology : Ten-month old sugarcane plants cv. Khon Khan 3 from the plantation in Lao Khwan district, Kanchanaburi province, Thailand, were used in this study. Leaf samples were collected from 15 sugarcane plants (one leaf per plant) with SCWL disease symptoms. These plants were divided based on the severity of the symptoms into 3 levels (5 plants per symptomatic level). Leaf samples were also collected from 10 sugarcane plants (one leaf per plant) without any symptoms of SCWL disease. The presence of SCWL phytoplasma in the samples was determined by Polymerase chain reaction (PCR). Micro x-ray fluorescence spectroscopy (μ-XRF) was used to determine element contents in healthy (free of SCWL phytoplasma), infected sugarcane without any SCWL disease symptoms and symptomatic leaf samples. Inductively Coupled Plasma Optical Emission Spectrometer (ICP-OES) analysis was used as a comparative method. The data were analyzed using one-way analysis of variance (ANOVA) followed by Tukey's multiple comparison tests and Pearson’s correlation method.

Main Results : SCWL phytoplasma was detected in all fifteen symptomatic samples and 5 asymptomatic samples. Six elements ranked based on the amount included K (the highest amount), Si, Ca, S, P, and Fe (the lowest) respectively. Correlation analysis showed a significant correlation between SCWL symptom severity and two elements; silicon (Si) and sulphur (S). Si content had a significantly strong negative correlation (r = -0.824) while sulphur (S) content had a significantly strong positive correlation (r = 0.803) with disease severity. In addition, the predictability of the calibrated model from micro x-ray and ICP-OES showed Mean Absolute Percentage Error (MAPE) of Si, K, and Ca was under 20% (15.52%, 10.44% and 9.69%, respectively). This result indicated that micro x-ray method has measurement capabilities similar to the ICP-OES method.

Conclusions : SCWL phytoplasma impacted element contents particularly Si and S in infected sugarcane. In addition, micro x-ray was applicable for the determination of element changes in sugarcane leaves. It was a reliable, rapid approach for visualizing quantitative changes at the cellular level and highlighted the impact of SCWL phytoplasma on sugarcane during the transition from asymptomatic to symptomatic stages. The outcome could be beneficial in farm management and nutrient supplementation of sugarcane plantations to delay disease progression.