MYCORRHIZAE, BIOCIDES, AND BIOCONTROL .3. EFFECTS OF 3 DIFFERENT FUNGICIDES ON DEVELOPMENTAL STAGES OF 3 AM FUNGI

The effects of biocide use on nontarget organisms, such as arbuscular mycorrhizal (AM) fungi, are of interest to agriculture, since inhibiti on of beneficial organisms may counteract benefits derived from pest a nd disease control. Benomyl, pentachloronitrobenzene (PCNB) and captan were tested for their effects on the germination and early hyphal gro wth of the AM fungi Glomus entunicatum (Becker & Gerd.), Glomus mossea e (Nicol. & Gerd.). Gerd. and Trappe and Gigaspora rosea (Nicol & Sche nck) in a silty-clay loam soil placed in petri plates. Application of fungicides at 20 mg active ingredient (a.i.) kg(-1) soil inhibited spo re germination by all three AM-fungal isolates incubated on unsteriliz ed soil for 2 weeks. However, fungicides applied at 10 mg a.i. kg(-1) soil had variable effects on AM-fungal isolates. Fungicide effects on germination and hyphal growth of G. etunicatum were modified by soil p asteurization and CO2 concentration in petri plates and also by placin g spores below the soil surface followed by fungicide drenches. Effect s of fungicides on mycorrhiza formation and sporulation of AM fungi, a nd the resulting host-plant response, were evaluated in the same soil in associated pea (Pisum sativum L.) plants. Fungicides applied at 20 mg a.i. kg(-1) soil did not affect the root length colonized by G. etu nicatum, but both benomyl and PCNB reduced sporulation by this fungus. Benomyl and PCNB reduced the root length colonized by G. rosea at 48 and 82 days after transplanting. PCNB also reduced G. mosseae-colonize d root length at 48 and 82 days, but benomyl only affected root length colonized by G. mosseae at the earlier time point. Only PCNB reduced sporulation by G. mosseae, consistent with its effect on root length c olonized by this fungus. captan reduced the root length colonized by G . rosea at 48 days, but not at 82 days, and reduced colonization by G. mosseae at 82 days, but not at 48 days. Captan did not affect sporula tion by any of the fungi. G. rosea spore production was highly variabl e, but benomyl appeared to reduce sporulation by this fun Overall, G. etunicatum was the most tolerant to fungicides in association with pea plants in this soil, and G. rosea the most sensitive. Benomyl and PCN B were overall more toxic to these fungi than captan. Interactions of AM fungi and fungicides were highly variable and biological responses depended on fungus-fungicide combinations and on environmental conditi ons.