Lower concentrations of microelements in leaves of citrus infected with 'Candidatus Liberibacter asiaticus'

Citrus trees affected by greening (huanglongbing, HLB) often develop symptoms that resemble those of Zn or Fe deficiency in their leaves. However, there have been few studies of mineral concentrations in infected leaves. To examine the effects of infection by 'Candidatus Liberibacter asiaticus' (the causal organism of the Asiatic form of HLB) on mineral nutrition, Citrus jambhiri (Lush.), C. reticulata cv. Siem, and C. depressa Hayata were patch-grafted with infected bark squares and grown in pots in greenhouses in Japan and Indonesia. In addition, leaves were collected from field-grown C. tankan Hayata and C. unshiu Marc. in Japan and C. reticulata cv. Siem in Indonesia, and their disease status was determined by PCR. Leaf samples were homogenized in 2-(N-morpholino) ethanesulfonic acid buffer and the concentrations of water-soluble Cu, Fe, Mn, and Zn in the macerate were determined using inductively coupled plasma atomic emission spectrometry (Japan) or atomic absorption spectrometry (Indonesia). In general, infected leaf samples had lower Fe and Zn. On average, the concentrations of Fe and Zn in infected plants were approximately half those in healthy plants. Cu was not significantly reduced by infection and Mn was occasionally lower. In C. unshiu, the concentrations of Fe and Zn were reduced before chlorosis appeared. These results suggested that the concentrations of particular elements (Fe and Zn) rather than element concentrations in general are reduced by infection by 'Candidatus Liberibacter asiaticus'.