Effects of fungicides on arbuscular mycorrhizal fungi: differential responses in alkaline phosphatase activity of external and internal hyphae

The effect of four fungicides on alkaline phosphatase (ALP) activity in internal and external hyphae of an arbuscular mycorrhizal (AM) fungus was examined. Four-week-old plants were treated with an aqueous solution of the fungicides and then harvested 3 days later. Fungicides were applied at a low and a high level corresponding to 1 and 100 times the recommended field application dose. The fungicides were the two abundantly used sterol inhibitors, fenpropimorph and propiconazole, as well as a commercial mixture of them. Benomyl served as a positive control, as this fungicide is known for its deleterious effect on AM fungi. At harvest, roots and hyphae extracted from the soil were stained for ALP activity. Benomyl inhibited fungal ALP activity of both internal and external hyphae at the low application level (1 mu g g(-1) soil) corresponding to the recommended field dose. Fenpropimorph had an intermediate effect, inhibiting both internal and external hyphae but only at the high application level (125 mu g g(-1) soil). Propiconazole decreased the activity of the external hyphae at the low application level (0.21 mu g g(-1) soil) but did not affect the internal activity at any application level. The effect of Tilt Top was similar to that of fenpropimorph. The results showed that the external hyphae were more sensitive than internal hyphae to application of fungicides.