ESTABLISHING THE STATUS OF CLAVIBACTER MICHIGANENSIS subsp INSIDIOSUS IN LUCERNE IN SOUTH AFRICA
S. Coertze, T. Jensen, T.N. Kotzé, A. McLeod
Clavibacter michiganensis subsp. insidiosus (Cmi) is a seed-borne pathogen that causes bacterial wilt of lucerne (alfalfa, Medicago sativa). The pathogen has a limited distribution worldwide and is included in the A2 pest list of the European and Mediterranean Plant Protection Organisation (OEPP/EPPO). In South Africa, cmi was first reported in 1967 in a few regions (Western-Cape province and in the Eastern Cape), with the last doubtful listing in 1983 also reporting it from the Transvaal region (currently known as Gauteng, Limpopo, North West and Mpumalanga provinces). Subsequently, lucerne plants exhibiting bacterial wilt symptoms have not been observed in any region of South Africa. Nevertheless, for South Africa, Cmi is listed as “present, restricted occurrence” by the OEPP/EPPO. Therefore, the status of Cmi in South Africa was re-investigated through field inspections and seed testing. Field inspections of lucerne seed crops conducted in all production regions from 2006 to 2014 did not reveal any symptomatic plants. Seed testing of 67 representative commercial seed lots showed that Cmi was absent from all of the seed lots. Prior to testing the commercial seed lots, artificially inoculated seed lots were used to show that the serial dilution plate method was sensitive (0.1% detection limit). It was also shown that a published Cmi semi-selective glucose-yeast-carbonate agar medium containing kanamycin and cycloheximide was able to more readily reveal the presence of presumptive Cmi colonies than the media of the EPPO diagnostic protocol, and the International Seed Health Initiative ring test.