What’s in a name? Phosphonates as fungicides

Phosphonate fungicides have very good efficacy for Pythium diseases and other diseases caused by oomycete fungi when applied preventatively; but are thought to have poor efficacy when applied post-infection (after disease symptoms and signs are apparent) see picture above --Effects of reagent-grade potassium phosphite
(H₃PO₃) and potassium phosphate (H₃PO₄) on symptom
development of Pythium blight of creeping bentgrass.
Potassium phosphite has good efficacy against this
disease when applied preventatively.

Literature cited

1. Adams, F. and J.P. Conrad. 1953. Transition of phosphite to phosphate in soils. Soil Science 75:361-371.
2. Anonymous. 2005. Greenbook turf and ornamental reference for plant protection products. Vance Communication Corp., New York, NY.