Abstract
Citrus greening disease caused by greening organisms (GOs; Liberobacter spp.) is one of the most destructive diseases of citrus in Nepal and Thailand. The 16S rDNA fragments of the GOs were detected by PCR in leaves showing one of the seven typical symptoms: mottling (type I), chlorosis with green netlike veins (type II), severe chlorosis with green main veins (type III), pale green on young leaves (type IV), vein yellowing (type V), vein corking (type VI) and yellow blotching (type VII). Leaves were collected from GO-infected ‘Suntala’ mandarin trees in Nepal. Sequence analysis of the 16S rDNA fragments and the 16S/23S intergenic spacer regions of Nepalese and Thai isolates of GOs indicated that these isolates were closely related to Indian and Chinese isolates of L. asiaticum. Symptom types III, V and VI were considered to be optimum for PCR diagnosis because of the high amount of GO DNA amplified. The accuracy of diagnosis by the naked eye can be improved by observing as many leaves as possible with all seven typical symptoms of greening on the same mandarin tree. Symptom types III, V and VII may be common to mandarin and sweet orange, because these symptoms are very similar to those reported on sweet orange in South Africa.
