Page 13 - Griffin Gazette Issue 3 - 2021

WHAT IS PHOSPHITE?
Chemically speaking, phosphite is phosphate with one less oxygen
attached. That small change means the world. Phosphite is a
fungicide and can only become a nutrient source after conversion to
phosphate. This oxidation to phosphate is carried out by soil-dwelling
bacteria. The conversion is a relatively slow process, so the phosphite
fungicides do not contribute to the overall nutrition of the plant. In fact,
when plants are nutritionally deficient in phosphorus, it is possible
for phosphite to inhibit the growth of the plant. Due to the similarities
of the molecules, phosphite can be taken up by the plant in place of Figure 1: Phosphate (left) has four oxygen
phosphate, but it can’t be further used by the plant. Phosphite is like atoms (red), while Phosphite right
chocolate syrup versus cocoa powder when sucked into a vacuum contains just 3 bound oxygens.
cleaner, it gums up the works! This effect on plants is rare, but has
been observed in citrus groves and other crops. For greenhouse
growers, the stage of plants most likely to be deficient in phosphorus
is rooted cuttings immediately after propagation, if fertilizer was not
supplied during rooting. Otherwise, phosphorus deficiency is a rare
occurrence when using most common fertilizers.

WHAT DOES IT DO?
PHLOEM
XYLEM
CORTEX Phosphite fungicides are used preventatively
against Pythium, Phytophthora, downy mildew
and other diseases. These fungicides are
liquid products formulated with the potassium,
sodium, ammonium, aluminum or other metal
salts of phosphite. Product labels often specify
the active ingredient chemically as a mixture
of ‘salts of phosphorus acid’, which probably
Phosphite is easily absorbed doesn’t make you think ‘phosphite’ unless you
Phosphite is systemic, and is easily through the leaves and know the chemistry. For example, the active
absorbed through the roots and EPIDERMIS transported down the
transported up the Xylem to Phloem. ingredient of Fosphite brand fungicide is: ‘Mono-
the rest of the plant. and Di-Potassium Salts of Phosphorous Acid’.
Fungicides containing phosphite can be applied
as a drench or spray. Because it is a rare true
Figure 2: The activity of phosphite is truly systemic, allowing systemic, xylem and phloem active, phosphite
foliar sprays to control root pathogens. Diagram courtesy of can control root rots from a foliar application as
Douglas Plant Health. well as a drench.

GRIFFIN GAZETTE 2021 | 13