Part 107 UAS pilot reported a near midair collision while at cruise on an automated mapping mission with a low flying fixed wing aircraft. The UAS pilot took manual control to avoid the aircraft and safely returned to base.

Narrative

Operating a mapping mission On the side of a road over a 20 acre plot; fortunately near to larger pylons. DJI Matrice M30T RC Pro controller warned me in a Flashing Red Screen attention getter to descend immediately due to aircraft at similar altitude. I immediately stopped automated mapping mission took manual control and descended immediately to lower than pylons and started a return to take off point. 10 seconds later; only as I was half way back to the take of point did I hear and then immediately see the crop dusting aircraft which flew about 100 ft. west of my position at a lower altitude than I had be flying the drone at; he was sub 50 ft. AGL. I landed the drone and cancelled my mission for the day.If the Crop Sprayer had not had a transponder (Some do not) then I would have not been alerted by my system. Without that alert there was distinct possibility of him flying under or into my UAS. Crop Sprayers should all have ADS-B so we can avoid them by early notification. When they are transiting low (sub 500 ft.) you cannot hear them or see them until they are very close. It is normal to hear; see; then avoid; I heard nothing today despite being in a quiet environment with little wind.