Most of the collections of Gymnosporangium juniperi-virginianae from seven states and different localities in Iowa produced aecidia on Bechtel's Crab, Wealthy, Jonathan and Rome Beauty; flecks with or without spermogonia on York Imperial, Tolman, Ben Davis, Maiden Blush, Oldenburg, Turley and Grimes Golden and flecks on Delicious and Northwestern Greening.

The collections could be classified into eight groups on the basis of the differential reactions of certain of these varieties. Four of these groups were considered as possible parasitic races with the following differential reactions : The first one produced aecidia on Tolman and York Imperial; the second caused a diffuse spreading fleck on York Imperial and defoliation of Jonathan after large aecidial sori had been produced ; the third failed to infect the varieties Delicious and Northwestern Greening; and the fourth one defoliated Turley after producing spermogonia.