Phosphites (syn. phosphonates) (H2PO3−; HPO32−) are alkali metal salts of phosphorous acid (H3PO3), a reduced form of phosphate (PO43−). Because phosphite has one less oxygen molecule, it has a higher degree of solubility and mobility than phosphate. This characteristic gives phosphite the ability to rapidly cross the membranes of leaves or roots. It is considered an ambimobile ion, which means it has both xylem and phloem mobility (Guest and Grant, 1991; Ouimette and Coffee, 1990).
Although it is often sold as a fertilizer, phosphite cannot be metabolized as a phosphorus source by plants (McDonald et al., 2001; Ratjen and Gerendás, 2009; Thao and Yamakawa, 2009). However, phosphite has been effective in controlling plant diseases, especially those caused by Phytophthora spp. (Guest and Grant, 1991; Smillie et al., 1989), including Phytophthora diseases of palms (DeFranqueville and Renard, 1989; Pohe et al., 2003; Thévenin et al., 1995). Phosphite compounds have also been effective in controlling diseases caused by Fusarium spp., Rhizoctonia solani, and Microdochium majus in other crops (Hofgaard et al., 2010; Lobato et al., 2008, 2010).
The mode of action of phosphite is complex and is not fully understood. Phosphite has been shown to have direct effects on Phytophthora spp. and other fungi, both in vitro and in vivo. However, it is believed that the primary mode of action of phosphite is through its stimulation of natural plant defense mechanisms (Guest and Grant, 1991; Smillie et al., 1989). The benefits of phosphite as a fungicide include low cost, high mobility within plants, persistence in plant tissue, multiple action sites as a fungicide, and low mammalian and environmental toxicity (Guest and Grant, 1991). Phosphite compounds are typically applied to herbaceous plants using foliar sprays or soil drenches. However, to improve effectiveness and reduce the amount of chemical applied as well as potential environmental impacts, trunk injection is increasingly being used for application on tree crops (Guest et al., 1994, 1995; VanWoerkom et al., 2014). Since trunk injections cause permanent wounds in palms, this method is recommended only for prevention of lethal diseases.
Knowledge of phosphite distribution and dynamics in plant tissues is essential to determine the optimum timing for fungicide treatment and to predict which diseases it may be effective against. A study on the distribution and dynamics of phosphite in avocado (Persea americana) trees treated with fosetyl-Al (the active breakdown product of this fungicide is phosphite) showed that phosphite concentrations were higher in branches than in the roots or leaves (Bezuidenhout et al., 1987). Its concentration in the branches and root samples taken from avocado trees reached its peak a month after trunk injection of fosetyl-Al. However, in mature leaves, the phosphite peak was broader. As a result, old branches stored ≈50% of the phosphite, the roots around 30%, and the remaining 20% was in young branches, leaves, and leaf stems. The conclusion from this study was that phosphite was not evenly distributed among different plant organs.
Although phosphite fungicides have been evaluated for disease control in palms (Darakis et al., 1985; DeFranqueville and Renard, 1989; Thévenin et al., 1995), nothing is known about the distribution and dynamics of phosphite in palms. Because palms are arborescent monocots, they differ greatly from dicot trees in their anatomy and physiology, making it difficult to extrapolate the results of research done on dicot trees to palms. The objectives for this study are to determine the uptake, distribution, and persistence of phosphite in trunk-injected coconut palms over time.
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