To directly observe the “cucumber–antagonist–pathogen” system, an experiment combining the root colonization and antagonism against R. solani Q1 of N43 was designed on water agar in Petri dishes. Cucumber seedling roots protected by GFP-tagged N43 were far from the location of attack by R. solani Q1, and the hyphae of R. solani Q1 were inhibited by the presence of GFP-tagged N43. This was an ideal simulation of the “cucumber–antagonist–pathogen” system