Effect of Starch Accumulation in Huanglongbing Symptomatic Leaves on Reflecting Polarized Light

Huanglongbing (HLB) or citrus greening disease is an extremely dangerous infection which has severely influenced the citrus industry in Florida. It was also recently found in California and Texas. There is no effective cure for this disease reported yet. The infected trees should be identified and removed immediately to prevent the disease from being spread to other trees. The visual leaf symptoms of this disease are green islands, yellow veins, or vein corking; however, starch accumulates on infected leaf and causes some blotchy mottle which is the finest diagnostic leaf symptom. Still it is not easy to decide the blotchy mottle is the results of starch accumulation or nutrient deficiency. Starch can rotate the polarization planar of light at a specific waveband. In this study, a vision sensor was developed based on this property of starch to detect the blotchy mottle on HLB infected citrus leaf and differentiate it from similar symptoms caused by nutrient deficiencies. A highly sensitive monochrome camera and 10 high power narrow band LEDs at 591 nm were used in this sensor. Also a polarizing film and a polarizing filter were mounted in perpendicular directions in front of the LED panel and the camera lens correspondingly. Therefore, the camera receives the minimum reflection. Since starch rotates the polarization planar of light, the sensor is able to highlight the accumulation of starch on the leaf. The narrow band polarized illumination condition was compared to non-polarized natural white light for leaf samples in healthy, HLB symptomatic, and nutrient (zinc) deficient conditions. The result showed that the developed vision sensor increasingly highlighted the HLB symptomatic areas on the leaf which contained more starch. Additionally, the separability among four different citrus leaf classes were compared before and after being ground, to investigate if the starch in ground infected leaves can be identified as good as unground leaves. The results showed that the freeze-dried ground leaves had more uniform brightness; but the starch accumulation could be identified more clearly.