The systemic fungicide potassium phosphonate, three alkyl-substituted phosphonate compounds (monoethyl, dimethyl, and diethyl phosphonate), and potassium hypophosphonate were compared for their activity in vitro against 34 isolates of nine species of Phytophthora. Potassium phosphonate proved to be the most inhibitory compound against most isolates. In contrast, potassium hypophosphonate was the least inhibitory, in some cases over 100 times less than potassium phosphonate
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D. G. Ouimette (1989) studied this question.