P-619 - Investigation of the causes of severe anthracnose-twister disease of green onion in Taiwan

Since 2022, leaf-twister anthracnose caused by Colletotrichum siamense has affected green onions in Taiwan during summer and autumn.  Symptoms range from leaf spots, yellowing and wilting of old leaves, curling of new leaves to stem rot and plant death. Inoculation experiments showed disease development occurs between 24–36°C, with 32°C being optimal, while symptoms were mild or hidden at 20°C or lower, although the pathogen could still be isolated. Younger seedlings (tillers) (0–4 weeks) were more susceptible, showing severe symptoms, whereas plants over 8 weeks showed only mild leaf symptoms, with no stem rot. However, the pathogen remained detectable in asymptomatic tissues. When these asymptomatic or mildly symptomatic inoculated green onion tillers were planted in pots (at 32°C for 3 weeks), over 90% of the plants died. This indicates that leaf-twister anthracnose in green onions has a latent infection phenomenon at low temperatures or in mature plants. Further field studies of pathogen dissemination were conducted by planting the inoculated green onion tillers nearby the field with healthy green onion seedlings which had grown for 30 days. In 30 days, the isolation rate of the pathogen is inversely proportional to the distance from the diseased scallion. These results suggest that latent infection in green onion tillers by C. siamense is a critical primary source of infection in the field. Wind and rain likely contribute to the spread of the disease to nearby green onions.