This paper describes the effects of the fungicides Benlate, Aliette and Ridomil on plant growth and on mycorrhizal development in onion plants. An attempt was made to distinguish effects on plants from those on the fungus by making comparisons between mycorrhizal plants in the absence of added phosphorus and non-mycorrhizal equivalent size plants. Vital staining techniques were used to analyse the effects of the fungicides on the living fungus both within the root and in the soil.
Benlate had no effect on shoot dry weight or root length of onion plants whereas a reduction in plant growth was observed following the application of Aliette or Ridomil, in comparison to control plants. Benlate had negative effects on the numbers of living internal hyphae, arbuscules, fungal-plant interface and living external hyphae in the soil.
Aliette had no effect on the number of living intercellular hyphae and arbuscules. However, it markedly reduced the root length and the length of infected root per plant. These combined effects led to a reduction in the area of the interface between fungus and plant. The length of external hyphae per gram of soil was reduced following the application of Aliette, though not as severely as with Benlate.
Ridomil had more complex effects on vesicular-arbuscular mycorrhizal symbiosis; treated plants showed a reduction in plant growth and also a reduction in all fungal parameters, namely the number of living intercellular hyphae, arbuscules and fungal–plant interface, the length of infected root and the development of external hyphae in the soil.
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