Assessment of Citrus Susceptibility towards Candidatus Liberibacter Asiaticus-Terengganu Isolate Based on Vector and Graft Transmission Tests

Five citrus species namely Citrus reticulata, C. aurantium, C. grandis, C. madurensis and Murraya paniculata were assessed for tolerance by grafting and vector transmission tests toward Ca. L. asiaticus-Terengganu isolate. This study was conducted in an insect-proof glasshouse for a period of six months. Results of this study showed that seedlings of M. paniculata, and C. grandis showed no HLB symptoms six months after inoculation period. They were also showed negative results by polymerase chain reaction (PCR) test and recorded the lowest percentage of reduction rate of plant height and plant biomass. Based on our findings, proved that these two citrus species were resistance to huanglongbing (HLB) or citrus greening infection. However, C. aurantium was showed high tolerance to HLB infection with the trees exhibited mild leaf mottling and green vein and the infected seedlings recovered after 3-4 months after treated with fertilizer. However the pathogen was detected by PCR test in the recovered seedlings. C. reticulata and C. madurensis were very susceptible to HLB infection. Inoculated seedlings showed severe HLB symptoms such as leaf mottling and yellowing, leaf green vein, leaf defoliation, twigs dieback within six months after inoculation. The inoculated seedlings started showing the first symptoms within three months after inoculation. Infected seedlings were also showed positive results with PCR test and recorded the highest percentage of reduction rate of plant height and plant biomass. Hence, this data provide a basis for developing citrus germplasm program for tolerance to HLB disease in Malaysia.