Phosphonic or phosphorous acid (H3PO3), not to be confused with phosphoric acid found in fertilisers, releases phosphite ions upon dissociation and it is used to to control root rot, downy mildew, late blight & phytophthora.

Phosphorous acid is not a source of available phosphate or nutrient as it is not easily metabolized by plants the same way phosphate fertilizers do not provide disease control.

Advantages: It is a Systemic fungicide and it is pH neutral.

Sold under different registered names such us: Phosic 600 (R); Anti Rot; etc