Abstract:Red rot and Oplidiopsis disease are the main diseases that occur during the farming of Pyropia (Bangiales, Rhodophyta) cultivated in the sea, which cause large areas of Pyropia to rot. Oplidiopsis disease is caused by Oplidiopsis sp., which is a biotrophic parasitic oomycete and there is currently no in vitro culture method for this species. The pathogen can only be identified by high-resolution microscopic observation and molecular methods. Experimental infection is typically undertaken by culturing diseased Pyropia with normal blades. In January 2019, 2000 acres of Pyropia disease occurred in a P. yezoensis cultivation area in Dafeng District, Yancheng City, Jiangsu Province, causing a large decrease of the P. yezoensis yield. We performed an investigation and pathogen identification of this disease. By macroscopic observation and studying the histopathology characteristics of the diseased thalli using a microscope, the Pyropia symptoms were found to be the same as that of the Oplidiopsis disease. The main symptoms were as follows: Pink spots appeared at the edge, base, and middle of the thallus at the beginning of the infection, which gradually faded and expanded as the disease progressed. During the late infection stage, the entire thallus became lighter and the tissue broke down and fell off. One to four spheroid parasitic oomycetes were found in one Pyropia cell, which contained multiple oil drop-like substances. Diseased thalli or their homogenates were used for experimental infection, with all causing Oplidiopsis disease symptoms in normal thalli at 10℃ and 20℃. To identify the pathogen, cox1 gene sequencing and phylogenetic analysis of the naturally and experimentally infected thalli were performed. The results showed that the pathogen sequences were clustered with O. porphyrae, O. pyropiae and O. porphyrae var. koreana with 100% similarity. In summary, the disease of P. yezoensis in Dafeng District was found to be caused by Oplidiopsis sp.
