First report of Fusarium arthrosporioides on Medusahead (Taeniatherum caput-medusae) and preliminary tests for host-specificity

Medusahead (Taeniatherum caput-medusae ssp. asperum (L.) Nevski) is an economically important weed in the USA. Foreign exploration to search for natural enemies of this weed in its native range resulted in finding the fungus, Fusarium arthrosporioides Sherb. (syn. F. roseum var. arthrosporioides (Sherb.), isolated from the collar of medusahead in August, 2001 in Greece. Identification was made using both morphological and molecular criteria. The sequence of the Internal Transcribed Spacer (ITS) region that was amplified by PCR using ITS1-F and ITS4-A primers, was similar to F. arthrosporioides sequences published in Genbank. Colonies were grown at 20°C on half strength Potato Dextrose Agar (PDA) (DIFCO Laboratories, Detroit, MI, USA). Colonies on agar were pink on the margin and red in the middle of the upperside with an abundant floccose aerial mycelium which was initially white and turned brown with time. Cultured on Synthetischer Nährstoffärmet Aghar (SNA), macroconidia were formed in pale orange sporodochia with 3 to 5 septa and a distinct foot-shaped base. Microconidia were oval, 0 to 1 septate. Fusarium arthrosporioides, a relatively common soil-borne fungus, has never been reported on the genus Taeniatherum (Nevski). In the past, it was studied as a potential biological control agent of Orobanche (Amsellem et al., 1999). Two-week old medusahead seedlings were transplanted into autoclaved soil artificially infested with 1g of inoculum (1 month-old sterile wheat seeds inoculated with a 5-mm square side of a PDA grown colony) for 10g of soil. After 11 days, inhibition of normal root development with concomitant leaf discoloration was observed. After 3 weeks, re-isolation of the pathogen from the roots and collar was successful. Utilising the same inoculation methods, preliminary tests of our medusahead isolates for host range specificity on several cultivated wheat, barley, oat, and other grasses (Elymus repens L., Hordeum murinum L., Pennisetum clandestinum L., Phacelia tanacetifolia L.), clearly demonstrated its non-specificity.

Eleven days after inoculation 60% of the barley plants showed symptoms of infection while after 3 days, wheat exhibited severe dwarfing. Dry weight comparisons of both inoculated barley and wheat to controls were significant. To our knowledge, this is the first report of F. arthrosporioides attacking T. caput-medusae. Based on our results, F. arthrosporioides will not be considered for further studies on biocontrol of medusahead.