Phytophthora infestans sporangial germination and tuber rot reduced by soil residues of fungicides

Foliar sprays of Difolatan were equal to or slightly better than those of maneb for the control of late blight when applied at equivalent rates of active ingredient. Difolatan foliar sprays, in general, resulted in less late blight tuber rot than did maneb sprays. Foliar sprays of Bordeaux mixture or Difolatan produced residues of these fungicides in the soil. These residues were greater in the surface soil (0 to 1/2 inch depth) than in the 1 to 2 inch layer, but were not sufficient to inhibit indirect germination of the sporangia ofP. infestans in soil leachates. Similar results were obtained with applications of Bordeux mixture or Difolatan to the soil in the absence of any foliar spray. Residues from foliar sprays of Difolatan plus soil treatments of Difolatan or Bordeaux mixture, made before and after the last cultivation, inhibited indirect germination ofP. infestans sporangia in soil leachates from the surface soil, but not from the 1 to 2 inch layer of soil. Application of foliar sprays of Bordeaux mixture to plots given soil treatments of Difolatan before the last cultivation resulted in fungitoxic residues in the surface soil and in the 1- to 2-inch layer of soil, but not when Difolatan soil treatments were made after the last cultivation. Similar results were obtained when maneb foliar sprays were applied to plots that had received soil applications of Difolatan.