Post infectional alterations caused by Xylaria polymorpha in the secondary xylem of Lannea coromandelica (Houtt.) Merr

Abstract

Xylaria polymorpha is known to cause root rot disease in hardwood trees. In the present study, trees of the species Lannea coromandelica infected with X. polymorpha showed symptoms consistent with root rot disease and also presented with a soft rot decay pattern. Bright-field microscopy, Confocal Microscopy, and Scanning Electron Microscopy revealed that fungal mycelia penetrated the S 2 layer of the fiber wall while axial parenchyma was found to be relatively resistant without much visible damage. Occasionally, separation of the parenchy- ma adjacent to fiber occurred due to the dissolution of the compound middle lamella. Ray parenchyma cells showed several boreholes having irregular shapes and sizes. Enlargement of the pits in axial and ray parenchyma was present in all the samples investigated. Xylem fibers were the most susceptible cell type and developed several tunnels through the S 2 layer. Tunnels formed in the S 2 layer of the fiber wall by the mycelia showed L- and/or T-bending. The diameter of the tunnels started narrow, increasing in size as the tunnels extended into the S 3 layer. In some instances, complete removal of the S 3 layer and fusion of the tunnels with the fiber lumen appearing as U-shaped erosion troughs was observed. At the advanced stage of decay, extensive damage was observed in the vessel walls, leaving the middle lamella and wall layer facing the vessel lumen intact. In conclusion the anatomical characteristics observed in the present study suggests that X. polymopha is an aggressive saprobe with strong ligninolytic activity causing soft rot type 2 decay in the wood cell wall of Lannea coromandelica .