Narrative:

I was working sectors combined all high altitude along with two low altitude sectors. I had approximately 6 to 10 aircraft tracked or on frequency at any given time. Aircraft X had recently come from executing a practice VOR approach and was on the way back to their departure point. Aircraft X was in level flight at 5;500 feet. I had several aircraft that were trying to talk on frequency at once checking on. As I acknowledged each; I asked for the other aircraft calling center. Aircraft X said that they had a near miss with another aircraft and if I showed anything on radar. At that time; I showed a weak primary target that appeared to be southeast bound just behind aircraft X. Aircraft X said that the other aircraft appeared to be an air tractor at about 5;400 feet. While there was nothing else I could do at that time; I did keep an eye on that primary target; and about 7 minutes later; had another VFR aircraft that I needed to vector off course as they were also at 5;500 feet. After getting the 2nd aircraft on a vector; they declared that they had the air tractor in sight and that they were now at 4;500 feet.I am not sure that I have any significant recommendations from the air traffic control side of things to make this a safer situation; but if the air tractor was using a transponder or displaying altitude; it would have made the traffic significantly easier to see as we often have numerous primary targets in this area where this happened.

Related reports:

Google
 

Original NASA ASRS Text

Title: ZKC Center Controller reported that an aircraft with no transponder caused a NMAC.

Narrative: I was working sectors combined all high altitude along with two low altitude sectors. I had approximately 6 to 10 aircraft tracked or on frequency at any given time. Aircraft X had recently come from executing a practice VOR approach and was on the way back to their departure point. Aircraft X was in level flight at 5;500 feet. I had several aircraft that were trying to talk on frequency at once checking on. As I acknowledged each; I asked for the other aircraft calling center. Aircraft X said that they had a near miss with another aircraft and if I showed anything on radar. At that time; I showed a weak primary target that appeared to be southeast bound just behind Aircraft X. Aircraft X said that the other aircraft appeared to be an air tractor at about 5;400 feet. While there was nothing else I could do at that time; I did keep an eye on that primary target; and about 7 minutes later; had another VFR aircraft that I needed to vector off course as they were also at 5;500 feet. After getting the 2nd aircraft on a vector; they declared that they had the air tractor in sight and that they were now at 4;500 feet.I am not sure that I have any significant recommendations from the air traffic control side of things to make this a safer situation; but if the air tractor was using a transponder or displaying altitude; it would have made the traffic significantly easier to see as we often have numerous primary targets in this area where this happened.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.