Comparison of methods for selecting fungi pathogenic to leafy spurge

Studies were conducted to compare six inoculation methods for preliminary screening of potential fungal biocontrol agents for leafy spurge (Euphorbia esula). The six inoculation methods included 1) applying spore suspensions to surface-disinfested, detached leaves of leafy spurge in petri dishes; 2) applying spore suspensions to untreated, detached leaves of leafy spurge in petri dishes; 3) applying spore suspensions to untreated leaves on intact plants of leafy spurge; 4) placing a block of agar with mycelium (ABM) on surface-disinfested, detached leaves in petri dishes; 5) placing an ABM on untreated, detached leaves in petri dishes; and 6) placing an ABM on leaves of intact leafy spurge plants. Of 200 fungal isolates in 16 genera, 27, 16, 4, 82, 70, and 10% induced symptoms on leafy spurge using methods 1-6, respectively. Not all pathogenic isolates were identified by methods 1-3 because the spore suspensions did not stay on the leaves readily. Methods 4 and 5 often identified the weak pathogens because the detached leaves were kept at conditions optimum for infection. Method 6 identified the highly pathogenic isolates and was dependable, easy, effective, and rapid. The upper 20 leaves on leafy spurge reacted consistently to pathogenic isolates. Method 6 is now used routinely in our laboratory for preliminary screening of pathogens of leafy spurge and has been used to select several potential biocontrol isolates for