A contagious disease caused serious damage to Gracilariopsis lemaneiformis cultivation in China

The yield of Gracilariopsis lemaneiformis in China was reduced by at least 40% in 2023 compared with 2022. Based on the investigation of this study, the decrease in production was caused by a disease locally named ‘Baotou’. The first sign of ‘Baotou’ disease was observed with the appearance of many light red branches in the thalli of G. lemaneiformis. During the onset of ‘Baotou’ disease, G. lemaneiformis lost its original red color and eventually decayed. One of the most typical morphological characteristics of ‘Baotou’ is that the closer the thalli were to the attachment rope, the more they decayed. Compared with the flat and regular epidermal cells of healthy alga, diseased algae had a rough surface covered with mucilaginous substance and numerous bacteria were found within the mucilaginous substance. The number of decayed points was significantly increased when the healthy cuttings were placed near the ‘Baotou’ cuttings. Pathogenic bacteria, which can induce the decay of healthy G. lemaneiformis, can be obtained from the ‘Baotou’ diseased G. lemaneiformis. The ‘Baotou’ disease is contagious. Once the G. lemaneiformis are infected with ‘Baotou’ disease, they should be harvested timely to prevent them from infecting the healthy alga.