Scratch method for simple, rapid diagnosis of citrus huanglongbing using iodine to detect high accumulation of starch in the citrus leaves. Jpn. J. Phytopathol. 73: 3-8 We demonstrated a rapid and simple diagnostic method (scratch method) for citrus huanglongbing (HLB) by detecting high accumulation of starch in the citrus leaves with the iodine-starch reaction. The average quantity of starch was 514.2 mg/kg in HLB-infected citrus leaves and 85.6 mg/kg in healthy leaves (Welch's t-test p<0.01), a significant difference in starch levels between diseased and healthy leaves. Based on this result, we devised a scratch method using abrasive paper for HLB diagnosis. Scratch the surface of a citrus leaf at least 20 times with abrasive paper, put the abrasive paper into 1 ml water in a vinyl pack, add to about 25 µl iodine solution 50 mM for dyeing starch. Iodine-reacted solutions show mostly 'dark brown or black' and 'yellow or orange' which are HLBpositive and negative, respectively. The scratch method and PCR assay of field samples showed more than 90% agreement. In addition, the scratch method did not give false HLB-positive reactions for healthy, nutrient-deficient or other disease leaves infected with Citrus tatter leaf virus and Hop stunt viroid. Therefore, the scratch method will be useful for rapid, simple detection of HLB in the field.