Detection of citrus huanglongbing using an Iodo-starch reaction

A rapid, low cost method for the detection of physiological changes associated with the causal agent of huanglongbing (greening) in citrus was developed. A leaf of citrus was cut into 5 x 5 mm pieces, crushed and softened with hot water for 10 min. A 100 microl sample of sap containing the infectious agent placed in a micro plate with 40 microl 5 mM iodine solution turned purple as a result of the iodo-starch reaction. Leaf samples from trees with deficiency symptoms gave a reaction ranging from light blue to blue, and healthy tree samples became yellow or brown, a result that distinguished diseased trees from healthy ones with the naked eye. Moreover, diseased samples had 20 times the optical absorbance as healthy ones at 540 nm. A comparison of PCR results with the iodo-starch assay resulted in 75%, agreement for leaf samples, and 95% for trees.