ARS | Publication request: HISTOLOGY OF SWEET ORANGE STEM PITTING CAUSED BY AN AUSTRALIAN ISOLATE OF CITRUS TRISTEZA VIRUS

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: July 2, 2002
Publication Date: October 20, 2002
Citation: Brlansky, R.H., Howd, D.S., Broadbent, P., Damsteegt, V.D. 2002. Histology of sweet orange stem pitting caused by an australian isolate of citrus tristeza virus. Plant Disease 86:1169-1174

Interpretive Summary: Pitting symptoms in the trunks, limbs, and twigs of sweet orange trees infected with severe strains of the citrus tristeza virus are associated with tree decline and reduction in fruit size and yield. Investigation of the pits or grooves in the wood indicated the presence of a yellow gum material. Electron microscope examination of these pits showed irregular growth of phloem cells with proliferation of cells into the grooved areas. In addition to the yellow gum material associated with the pits, there was a higher number of viral inclusions. There was a disruption of normal cambium growth in the pitted areas resulting in abnormal xylem vessels carrying water which may ultimately cause stunted trees and smaller fruit. The effect of stem pitting on water and nutrient transport remains to be investigated.

Technical Abstract: Some strains of the citrus tristeza virus (CTV) cause stem pitting in sweet orange (Citrus sinensis (L.) Osbeck). This abnormality causes tree decline and reduction in fruit size and yield of affected citrus trees. Stem-pitting symptoms can occur on trunks, on all sizes of limbs, and on the twigs where fruit are produced. Variously sized pits or grooves in the wood often contain a yellow gum. Irregular growth of the phloem occurs in the area of these xylem pits. The histology of stem pitting caused by an Australian CTV isolate was studied in sweet orange using light and electron microscopy. Using scanning electron microscopy, details of the wood pits containing the gumming material were revealed. In thin sections of bark tissue, outgrowths of the phloem tissue were found at various intervals that corresponded to the pits in the wood. Higher numbers of viral inclusions were detected in the phloem outgrowths than were present in the other sieve elements.