Effects of systemic fungicides on vesicular-arbuscular mycorrhizal infection and plant phosphate uptake

Contrasting Patterns of Functional Diversity in Coffee Root Fungal Communities Associated with Organic and Conventionally Managed Fields

The structure and function of fungal communities in the coffee rhizosphere are influenced by crop environment. Because coffee can be grown along a management continuum from conventional application of pesticides and fertilizers in full sun to organic management in a shaded understory, we used coffee fields to hold host constant while comparing rhizosphere fungal communities under markedly different environmental conditions with regard to shade and inputs. We characterized the shade and soil environment in 25 fields under conventional, organic, or transitional management in two regions of Costa Rica. We amplified the internal transcribed spacer 2 (ITS2) region of fungal DNA from coffee roots in these fields and characterized the rhizosphere fungal community via high-throughput sequencing. Sequences were assigned to guilds to determine differences in functional diversity and trophic structure among coffee field environments. Organic fields had more shade, a greater richness of shade tree species, and more leaf litter and were less acidic, with lower soil nitrate availability and higher soil copper, calcium, and magnesium availability than conventionally managed fields, although differences between organic and conventionally managed fields in shade and calcium and magnesium availability depended on region. Differences in richness and community composition of rhizosphere fungi between organic and conventionally managed fields were also correlated with shade, soil acidity, and nitrate and copper availability. Trophic structure differed with coffee field management. Saprotrophs, plant pathogens, and mycoparasites were more diverse, and plant pathogens were more abundant, in organic than in conventionally managed fields, while saprotroph-plant pathogens were more abundant in conventionally managed fields. These differences reflected environmental differences and depended on region.IMPORTANCE Rhizosphere fungi play key roles in ecosystems as nutrient cyclers, pathogens, and mutualists, yet little is currently known about which environmental factors and how agricultural management may influence rhizosphere fungal communities and their functional diversity. This field study of the coffee agroecosystem suggests that organic management not only fosters a greater overall diversity of fungi, but it also maintains a greater richness of saprotrophic, plant-pathogenic, and mycoparasitic fungi that has implications for the efficiency of nutrient cycling and regulation of plant pathogen populations in agricultural systems. As well as influencing community composition and richness of rhizosphere fungi, shade management and use of fungicides and synthetic fertilizers altered the trophic structure of the coffee agroecosystem.

Benomyl inhibits phosphorus transport but not fungal alkaline phosphatase activity in a Glomus-cucumber symbiosis

Short-term effects of benomyl on the arbuscular mycorrhizal fungus Glomus caledonium (Nicol. & Gerd.) Trappe and Gerdeman associated with Cucumis sativus L. were studied by measuring effects on fungal P transport and on fungal alkaline phosphatase activity. Mycorrhizal plants were grown in three compartment systems where nylon mesh was used to separate n root-free hyphal compartment (HC) and a root + hyphal compartment(RHC) from The main root compartment (RC). Non-mycorrhizal control plants were grown in similar growth units. After 6 wk benomyl was applied to the plants in three ways: as soil drenches to RHC or HC, or as u spray to the leaves. Benomyl was added in three concentrations. Equal amounts of ³² P and ³³ P were added to the HC and to the RHC respectively, immediately after the application of benomyl. Plants were harvested 4-6 d later. Hyphal transport of ³² P from the HC was inhibited when benomyl was applied to the HC at 10 μg g^-1 soil, whereas the uptake of ³² P from RHC I roots + hyphae) was reduced only at the highest dose of application to the RHC (100 μ g g^-1 soil). In contrast to the marked reduction of benomyl on fungal P transport, the activity of fungal alkaline phosphatase inside the roots was unaffected by benomyl.

The effect of fungicides on vesicular-arbuscular mycorrhizal symbiosis: I. The effects on vesicular-arbuscular mycorrhizal fungi and plant growth

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.

DEPRESSED METABOLIC ACTIVITY OF VESICULAR-ARBUSCULAR MYCORRHIZAL FUNGI AFTER FUNGICIDE APPLICATIONS

Application to soil of either benomyl or captan significantly decreased the growth of vesicular-arbuscular mycorrhizal (VAM) onion (Allium cepa L. cv. Hyper) plants 4 weeks after treatment but non-VAM plants were not affected. Fungal colonization of the onion roots, as indicated by non-vital staining with chlorozole black E, was depressed 2 weeks after fungicide application. However, decreases in metabolically active VAM fungal tissue, revealed by a succinate dehydrogenase assay, could be detected as soon as 3 d after fungicide treatment. There was little difference between the fungicides in their effect on the VAM fungi used. The usefulness of the succinate dehydrogenase assay in predicting effects of fungicide is discussed.