Stimulation of sporulation of Phytophthora fragariae [Red stele, fungal diseases, strawberries]

Races of Phytophthora fragariae differed in production of zoospores and sporangia when flooded with 1% soil extracts of three different soils. Steam and filter sterilization of soil extracts and ignition of soils resulted in reduced zoospore production, but dry storage of soils did not Extracts of soil pstudomonads failed to induce formation of sporangia. Distilled water amended with Ca or Mg chloride induced 70-160% more sporangia than soil extracts alone, whereas Ca ++ + Mg ++ increased production of sporangia to 300% of those produced in soil extracts. Individually, Ca ++ in flooding solutions was most effective at concentrations of 10 -3 to 10 -2 M and Mg ++ was most effective at 10 -2 M. In combination, however, Ca ++ was most effective at 3.3 × 10 -4 M with Mg ++ at 3.3 × 10 -6 to 10 -4 M. Potassium and Fe ++ were not stimulatory, and Al +++ inhibited sporangium formation at 10 -6 M in soil extract.