Do fungicides used to control Rhizoctonia solani impact the non-target arbuscular mycorrhizal fungus Rhizophagus irregularis?

Abstract

There is growing evidence that the application of biocontrol organisms (e.g., Pseudomonas and Bacillus spp., arbuscular mycorrhizal fungi-AMF) is a feasible option to reduce incidence of plant pathogens in an integrated control strategy. However, the utilization of these microorganisms, in particular AMF, may be threatened by the application of fungicides, a widely-used measure to control Rhizoctonia solani in various crops among which potato. Prior to their application, it is thus important to determine the impact of fungicides on AMF. The present study investigated, under in vitro controlled conditions, the impact of azoxystrobin (a systemic broad-spectrum fungicide), flutolanil (a systemic Basidiomycota-specific fungicide), and pencycuron (a contact Rhizoctonia-specific fungicide) and their respective formulations (Amistar, Monarch, and Monceren) on the growth and development of the AMF Rhizophagus irregularis MUCL 41833 (spore germination, root colonization, extraradical mycelium development, and spore production) at doses used to control R. solani. Results demonstrated that azoxystrobin and its formulation Amistar, at threshold values for R. solani control (estimated by the half maximal inhibitory concentration, IC50, on a dry weight basis), did not affect spore germination and potato root colonization by R. irregularis, while the development of extra-radical mycelium and spore production was reduced at 10 times the threshold value. Flutolanil and its formulation Monarch at threshold value did not affect spore germination or extra-radical development but decreased root colonization and arbuscule formation. At threshold value, pencycuron and its formulation Monceren, did not affect spore germination and intra- or extraradical development of R. irregularis. These results suggest that azoxystrobin and pencycuron do not affect the AMF at threshold concentrations to control R. solani in vitro, while flutolanil (as formulation) impacts the intraradical phase of the fungus. These fungicides and R. irregularis thus have the potential to be used in parallel against Rhizoctonia disease in potato.

Why farmers should manage the arbuscular mycorrhizal symbiosis

“…(2) Ryan & Graham () do not include agrochemical effects in this discussion. However, fungicides (foliar applications and seed treatments) can reduce AM fungal spore germination, mycorrhiza formation, AM fungal community composition, extraradical hyphae and/or spore production (Dodd & Jeffries, ; Merryweather & Fitter, ; Wilson & Williamson, ; Hernandez‐Dorrego & Mestre‐Pares, ; Ipsilantis et al ., ; Jin et al ., ; Buysens et al ., ; Lekberg et al ., ). (3) Plant breeding in the future should occur in environments that favor interactions of plants and AM fungi (reviewed in Bennett et al ., ).…”

Arbuscular mycorrhizal fungi and their response to pesticides

“…When flutolanil (an inhibitor of fungal respiration) was tested as its formulated product (Monarch®), the threshold level for the reduction of root colonization was reduced to 0.1 mg L −1 (formulation) compared with flutolanil alone (1 mg L −1 a.i.) . This result is not surprising because formulations can include compounds that enhance the biological activity of the active ingredient or have a biological activity of their own …”

“…However, Ri T‐DNA‐transformed roots lack photosynthetic tissue, a normal hormone balance and physiological source–sink relationships compared with an autotrophic plant . Thus, in vitro systems using autotrophic plants could provide more realistic information on the effects of active substances on AMF colonization and AM symbiosis . Another limitation is the limited availability of different AMF taxa in vitro due to the laborious establishment and time‐consuming maintenance of such cultures.…”

“…Based on in vitro studies, there is evidence that fungicides such as flutolanil, azoxystrobin, fenpropimorph and fenhexamid inhibit spore germination of Rhizophagus irregularis in a fungistatic and nonfungicidal manner (i.e., after the transfer to fungicide‐free medium, spores can germinate) (Table S2). Under field conditions, AMF spores can be exposed to inhibiting active substances for a certain period of time. To overcome this threat, AMF spores remain in an ungerminated state.…”

Arbuscular Mycorrhizal Fungi and Associated Microbiota as Plant Biostimulants: Research Strategies for the Selection of the Best Performing Inocula

“…As to pesticides, their effects on spore germination are different depending on the target organisms. Several fungicides, like copper hydroxide and mancozeb, were able to inhibit spore germination of F. mosseae in vivo, while flutolanil, azoxystrobin, fenpropimorph, and fenhexamid inhibited germination of R. irregulare spores in vitro [26,27]. On the other hand, other fungicides, such as fosetil Al, metalaxyl and different herbicides, seem to exert no activity on spore germination even if the results obtained on the same substance in different investigations were often contradictory [28].…”