DOI: 10.1111/j.1365-3180.2011.00846.x Incidence of endophyte Neotyphodium occultans in Lolium rigidum from Australia K A KIRKBY*, J E PRATLEY*, D E HUME  , M J FAVILLE  , M AN à & H WU* *EH Graham Centre for Agricultural Innovation (Industry & Investment NSW and Charles Sturt University), Wagga Wagga, NSW, Australia,  AgResearch, Grasslands Research Centre, Palmerston North, New Zealand, and àEnvironmental and Analytical Laboratories, Faculty of Science, Charles Sturt University, Wagga Wagga, NSW, Australia Received 16 August 2010 Revised version accepted 15 December 2010 Subject Editor: Karen Bailey, AAFC, Canada Summary Lolium rigidum (annual ryegrass), a widespread weed of southern Australian cereal crops, has frequently evolved herbicide resistance. It is commonly found associated with the mutualistic, endophytic fungus, Neotyphodium occultans. The objectives of this study were as follows: (i) determine the occurrence of N. occultans in L. rigi- dum populations across southern Australia, (ii) evaluate potential associations between infection frequency and herbicide resistance, climatic and topographic variables and (iii) examine genetic diversity of N. occultans from this national collection of L. rigidum using simple sequence repeat markers. By microscopic assessment of seed, we found all seed lots were infected with N. occul- tans, with highly variable frequencies, ranging from 4% to 98%. Frequencies were higher in New South Wales, South Australia and Victoria (54–62%) than in Western Australia (43%) or Tasmania (17%). A negative asso- ciation existed between elevation and mean rainfall, and infection frequency. Significant interactions were as follows: mean rainfall (growing season) and state and mean rainfall (growing season) and elevation. Significant associations were found between infection frequency and herbicide mode of action. Infection was lower in seed lots resistant to group B (sulfonylurea and imidaz- olinone) and higher in seed lots resistant to group D (dinitroaniline) herbicides. Higher infection frequencies were observed in seed lots from earlier years, with the lowest in seed lots recently collected. No significant associations were found with long-term mean tempera- ture or rainfall. Little genetic diversity was observed for N. occultans endophyte in Australian plants. These results provide a benchmark of the relationship of N. occultans with L. rigidum and suggest further resear- ch on the role this endophyte has in determining the fitness of L. rigidum under varying conditions of herbi- cide resistance, rainfall and elevation that may aid in the management of this grass. Keywords: climate, infection frequency, spatial struc- ture, survey, herbicide resistance. KIRKBY KA, PRATLEY JE, HUME DE, FAVILLE MJ, AN M&WU H (2011). Incidence of endophyte Neotyphodium occultans in Lolium rigidum from Australia. Weed Research 51, 261–272. Introduction Lolium rigidum Gaud. (annual ryegrass) is native to the Mediterranean basin and was introduced into south- eastern Australia in 1887 as a forage crop (Kloot, 1983). Seed was later categorised and traded as the ecotype Wimmera, but with no official seed certification. Since its introduction, L. rigidum has been valued for provid- ing early winter feed for livestock during pasture phases of farming. High phenotypic plasticity and genetic variability enables its reproductive success under adverse conditions. The grass forms dense stands, capable of producing up to 45 000 seeds per m 2 (Gill, 1996). To control the grass in arable crops, there has been a heavy Correspondence: Karen Kirkby, Charles Sturt University, Wagga Wagga, NSW, Australia. Tel: (+61) 428944500; Fax: (+61) 269332420; E-mail: kkirkby@csu.edu.au Ó 2011 The Authors Weed Research Ó 2011 European Weed Research Society Weed Research 51, 261–272