Methionine-riboflavin mixtures with surfactants and metal ions reduce powdery mildew infection in strawberry plants

Foliar application of a mixture of methionine (1 mM) and riboflavin (26.6 mu M) reduced the severity of powdery mildew [Sphaerotheca macularia (Wallr. ex Fr.) Jacz. f. sp, fragariae] infection in 'Earliglow' strawberry (Fragaria xananassa Duch.) plants. Efficacy of this mixture on controlling powdery mildew infection was enhanced by supplements of copper, iron, and surfactants [sodium dodecyl sulfate (SDS), Triton X-100, Tween-20, or oxyalkylenemethylsiloxane (Silwet L-77)]. Free-radical scavengers (n-propyl gallate, thiourea) and antioxidants (alpha-tocopherol, beta-carotene) reduced the efficacy of this mixture. Plants treated with a mixture of riboflavin (26.6 mu M), D,L-methionine (1 mM), copper sulfate pentahydrate (1 mM), and surfactants (SDS or Silwet L-77 at concentrations of 0.05% to 0.1%) showed a decrease in powdery mildew infection. Results of this study suggest that treatment with a mixture of methionine and riboflavin is beneficial to strawberry plants and may serve as an alternative to fungicides for controlling powdery mildew.