Australian papaya dieback: Evidence against the calcium deficiency hypothesis and observations on the significance of laticifer autofluorescence

The cause of the dieback disorder of Carica papaya (papaya, papaw, or pawpaw) is unknown, although it is a severe problem for the Queensland industry. In this study, the progression of morphological and anatomical symptoms during a growing season in Yarwun is documented. Most plants which displayed a brown discoloration of the vascular tissue developed external symptoms. The discoloration initiated in the stem zone which supported leaves, and developed acropetally into the stem apex, and basipetally into the trunk and in part of the root system. The discoloration was autofluorescent under blue or ultraviolet light, and was associated with laticifers in xylem and phloem tissue and in ray parenchyma. Laticifer autofluoresence was not observed in calcium-deficient plants, nor in the browning associated with nematode damage, but this symptom was present in association with tissue damage caused by Amblypelta lutescens (Distant) and in plants suffering root rot. Laticifer discoloration may represent a general stress reaction by the plant. Tyloses developed in xylem elements, associated with the wilting of the crown, following development of leaf chlorosis and the bending of the stem tip. Dieback-affected apical tissues were lower in total calcium content than healthy tissues, but were not below an experimentally determined critical concentration. It is suggested that the low calcium content represents a secondary symptom of the disorder, caused by a tylosis-induced decrease in hydraulic conductivity and consequently in calcium transport.