Vessel blockage by myelin forms in citrus with and without rough-lemon decline symptoms

Abstract

Vessels in xylem of citrus trees with wilt symptoms described as rough-lemon, sandhill, and young-tree decline, or blight, were often found plugged, generally at constrictions of vessel ends. Diseased trees had significantly more vessels plugged than healthy trees. The plugging substance was composed of a dense network of cylindrical strands, with a diameter of about 0.3 μm, which were identified as myelin forms. Electron microscopy revealed that these strands were composed of concentric bands of electron-dense material of varying intensity. These strands stained blue-black with Sudan black B and light green with Nile blue sulfate and were positive for unsaturated lipids with osmium. They reacted positively for phospholipid with orange G – aniline blue, and choline was detected by the acid haematin test. The myelin forms originated from accumulations of lipid on the inside walls of vessels that formed by migration of lipid through vessel pits from adjacent parenchyma cells. Large quantities of lipid bodies formed in ray parenchyma, and lipid-like bodies were observed in phloem cells.