US Department of Agriculture, Agricultural Research Service, Sidney, MT 59270, USA

Background and objectives
The herbaceous perennial species from the steppes of Eurasia in the genus Euphorbia and Centaurea are progenitors of the invasive rangeland weeds Euphorbia esula/virgata, (common name, leafy spurge) and Centaurea maculosa and C.. ;diffusa, (spotted and diffuse knapweeds, respectively), which infest millions of acres in the northern prairies of North America. Soil-borne fungi are typically isolated from diseased plants of these species where natural epidemics or stand declines occur and from extensive wounds caused by root-attacking insect agents for biological control such as Aphthona spp. Identification and characterization of these species for pathogenicity, host range and soil population levels are needed to understand how to optimize their use in biocontrol. Investigation of the impact of insect-pathogen syngergisms that apparently occur in the field are also needed to understand the likely importance of this phenomenon to weed biocontrol in prairies.

Materials and methods
Fungi were isolated from root tissue of E. ;esula/virgata plants attacked by root-feeding biocontrol agents such as Aphthona, Oberea or Chamaesphecia spp., and which exhibited stunting, chlorosis, wilting or necrotic stems. Fungi were similarly isolated from C. ;maculosa and C. ;diffusa at foreign sites where root-attacking insects are endemic in occupying crowns, and at sites in the USA and Canada where the insects are established as biocontrol agents and strongly impact on weed density. Artificial greenhouse soil mixes were infested with R. ;solani, F. ;oxysporum, and pythiaceous fungi. To investigate the contribution to mortality of these fungi in combination with insects, 9-month-old plants of E. ;esula/virgata were planted in the infested soil, plants were caged, and 15 adult A. ;flava per pot were placed in the cages. Plants were observed and rated for disease on a 0-6 scale.

Results and conclusions
Fungal species associated with diseased host tissue distal from wounds caused by root-attacking insect weed biocontrol agents in Eurasia and at US sites where weeds were rapidly declining in density were F. ;oxysporum, F. ;solani, F. ;proliferatum, R. ;solani and pythiaceous fungi. Furthermore, some of these species were isolated from migrating adults as well as larvae of Aphthona spp., and were pathogenic to leafy spurge in greenhouse tests. Insect-pathogen combinations of A. ;flava and one or more species of soil-borne fungi caused dramatically accelerated mortality compared with any agent alone. Based on the several lines of evidence presented, it is concluded that plant pathogen-insect synergisms are essential and that weed biocontrol programs will likely fail without consideration of them.