Treatments of sporangia and zoospores of Phytophthora infestans race 1.2.3.4 with methionine or riboflavin for durations of up to 8 h under fluorescent light did not affect its colonization of rye-seed agar. In contrast, exposure of sporangia and zoospores to methionine-riboflavin mixture for 2 h or more resulted in the failure of race 1.2.3.4 to grow when transferred to rye-seed agar medium. Hyphal growth of races 1.2.3.4 and 0, when incubated in liquid synthetic medium, was inhibited by free riboflavin. Potato plants, Solanum tuberosum cv. Kennebec, when pretreated with a methionine–riboflavin mixture and then spray inoculated with race 1.2.3.4 developed fewer diseased leaves per plant than plants pretreated with water, methionine, or riboflavin. Key words: Solanum tuberosum, hydroxyl radical, superoxide anion.
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