Authors
C. Nischwitz,
D. Langston, Jr., and
H. F. Sanders, Department of Plant Pathology, Coastal Plain Experiment Station, University of Georgia, Tifton 31793-0748;
R. Torrance, Tattnall Cooperative Extension Service, Reidsville, GA 30453; and
K. J. Lewis and
R. D. Gitaitis, Department of Plant Pathology, Coastal Plain Experiment Station, University of Georgia, Tifton 31793-0748
In the fall of 2007, onion seedlings with twisted and distorted leaves were observed in seedbeds in multiple fields in the Vidalia onion region of Georgia. Tests for viruses and bacteria were negative and chemical injury was deemed improbable because of disease distribution in the fields. Upon further investigation, fungal fruiting bodies were observed on the outside sheath of a few of the seedlings. Symptomatic plants were cut into 1-cm segments and surface sterilized in 70% ethanol for 3 min. After rinsing in sterile water, the segments were placed onto potato dextrose agar amended with tetracycline. The fungus isolated from symptomatic plants fit the description of Colletotrichum gloeosporioides (Penz.) Penz. & Sacc. Conidia were aseptate, cylindrical, and hyaline. Sequencing of the internal transcribed spacer region and a BLAST search in GenBank (99% sequence similarity to C. gloeosporioides accessions) confirmed the identification. Ten onion seedlings were spray inoculated with a suspension of 1 × 107 spores/ml until runoff, and four seedlings were inoculated with water as negative controls. Plants were bagged for 12 h to maintain high relative humidity. Five plants were placed in the greenhouse and five plants placed in a growth chamber at 22°C. All plants inoculated with C. gloeosporioides developed distorted and twisted leaves 3 weeks after inoculation in the growth chamber and 5 weeks after inoculation in the greenhouse. Night time temperatures in the greenhouse (15°C) were lower than those in the growth chamber (22°C). Seedlings inoculated with water showed no symptoms. The fungus was reisolated from symptomatic plants. C. gloeosporioides has been reported to cause a disease called twister on onion in tropical regions (1). The fall of 2007 was unusually warm with maximum temperatures reaching 26°C during the day. The pathogen is present on many crops in the United States, but to our knowledge, this is the first report of C. gloeosporioides causing twister disease of onion in the United States. In Nigeria and Brazil, yield losses as much as 100% were observed in fields with infected onions (1). The impact of infection on the growth of the transplants and subsequent yield in Vidalia onions is currently unknown.
References: (1) J. P. Hill. Compendium of Onion and Garlic Diseases. 2nd ed. The American Phytopathological Society, St. Paul, MN, 2008.
