I am currently doing some historical studies if the vitamin hypothesis fits also the temporal relationship of allergy prevalence. While ordering RKI files for my next trip to the Berlin document center, I found that farming and lower allergy sensitization is known much longer than I anticipated. Here is Blackley CH. Experimental Researches on the Causes and and Nature of Cattarhus Aestivus (Hay-fever or Hay-asthma). London: Balliere, Tindall and Cox, 1873:
The farming observation therefore does not go back to 1989 but to 1873, yea, yea.