This study compares the effects of the fungicide Aliette® (fosetyl-Al) on non-mycorrhizal and mycorrhizal onion (Allium cepa L.) with effects of the degradation products, aluminium (applied as aluminium chloride) and phosphonate (applied as dimethyl phosphonate). We sought to determine the extent to which the plants absorb and accumulate phosphonate and ascertain why, as shown previously, toxic effects of fosetyl-Al on mycorrhizal plants are less severe than effects on equivalent non-mycorrhizal plants.

Fosetyl-Al markedly reduced growth, especially of roots, and also inhibited mycorrhizal colonization. Dimethyl phosphonate caused smaller effects on growth and did not decrease colonization. Aluminium chloride did not affect growth of non-mycorrhizal or mycorrhizal plants, or mycorrhizal colonization. In all cases, mycorrhizal plants grew better than equivalent non-mycorrhizal plants (with no added soil phosphate), and non-mycorrhizal plants given supplementary soil phosphate grew best.

Concentrations and contents of total P in shoots were increased by dimethyl phosphonate and more so by fosetyl-Al. Concentrations of P were also increased in roots. 31P nuclear magnetic resonance (NMR) spectroscopy was used to determine relative concentrations of phosphonate and phosphate. Application of fosetyl-Al led to accumulation of more phosphonate than did application of dimethyl phosphonate, and non-mycorrhizal plants treated with fosetyl-Al accumulated much more phosphonate than did equivalent mycorrhizal plants (both with no added soil phosphate). Internal phosphate concentrations increased, especially in mycorrhizal plants, following application of both dimethyl phosphonate and fosetyl-Al. Non-mycorrhizal plants given supplementary soil phosphate also showed restricted growth when treated with fosetyl-Al even though they accumulated phosphate in relatively high amounts and had lower ratios of phosphonate to phosphate, particularly in shoots. In general, high internal concentrations of phosphonate were correlated with large reductions in plant growth among treatments.

Possible causes of these effects include conversion of phosphonate to phosphate in the tissues (especially in mycorrhizal plants), enhanced uptake of phosphate via mycorrhizal fungi and additional accumulation of P in plants with restricted growth. Competition for uptake between phosphate and phosphonate is also discussed.