Is There a Future for Arbuscular Mycorrhizal Fungi (AMF) as Biopesticides?
The term “biopesticide” likely means different things to people of varied agricultural backgrounds. The term itself is a contraction of “biological pesticide”, which includes several types of pest management through predatory, parasitic, or chemical relationships. Biopesticides may include substances such as biochemical pesticides (bio-derived), microbial pesticides (bacteria, fungi, viruses), or substances produced by plants that contain added genetic materials (GM crops). Generally, biopesticides are made of living things, come from living things, or they occur naturally. They are often important components of IPM programs, and have received attention as substitutes for synthetic chemical protectants that are used against biological pests. Biopesticides tend to pose fewer risks than conventional pesticides and therefore generally require a less rigorous approval process prior to their registration for use.
The remainder of this article will highlight two studies that present promising results toward the future use of biopesticides–in this case, arbuscular mycorrhizal fungi or AMF–to control soil-borne pathogens that are major pests of soybeans. Results from both studies appear in the journal Plant Disease, and both are authored by Pawlowski and Hartman at the Univ. of Illinois. The following narrative is a summary of the conduct of and findings from those studies.
The first study yielded results that are reported in an article titled “Reduction of Sudden Death Syndrome (SDS) Foliar Symptoms and Fusarium virguliforme DNA in Roots Inoculated with Rhizophagus intraradices” (Plant Disease https://doi.org/10.1094/PDIS-07-19-1500-RE).
• There is increasing interest in incorporating AMF into agricultural production systems because of the benefits they provide, including touted activity against pathogens and pests.
• This study was conducted to determine whether or not soybean plants co-inoculated with F. virguliforme (pathogen that causes SDS) and the AMF species Rhizophagus intraradices showed reduced SDS foliar symptoms and reduced pathogen presence in soybean roots.
• Six different soybean genotypes that are susceptible to F. virguliforme were inoculated with F. virguliforme alone or in combination with R. intraradices in a greenhouse experiment.
• Plants were subsequently evaluated for foliar disease symptoms at 14, 17, and 21 days after planting using a 1 to 8 foliar disease rating scale.
• At 21 days after inoculation, dry weight of roots from plants was recorded, and roots were assayed for F. virguliforme DNA presence.
• There were visible differences in foliar symptoms and root mass between the AMF and non-AMF treatments.
• Area under the disease progress curve (AUDPC) values were 45% lower in plants inoculated with AMF compared to the non-AMF control.
• Average weight of roots of plants inoculated with AMF was 58% greater than that of the non-AMF control plants.
• Quantity of F. virguliforme DNA was 28% lower in plants inoculated with AMF vs. the non-AMF control plants.
• Nutrient concentrations in roots of AMF-inoculated plants were greater than those in roots of the non-AMF control plants.
• The authors suggested two major mechanisms that may have resulted in the AMF-mediated protection against the SDS pathogen. 1) AMF associations may have resulted in an overall increase in plant growth, thus allowing for compensation for SDS damage. 2) The reduction in SDS severity in AMF-inoculated plants could be related to increased nutrient uptake by those plants.
• The authors concluded that this study showed that inoculating soybean plants with the AMF R. intraradices reduced both SDS severity and F. virguliforme DNA in soybean roots while increasing soybean growth and nutrient uptake by AMF-inoculated plants. They inferred that susceptible soybean genotypes will benefit from the positive effects of the AMF used in their study. Furthermore, they surmised that R. intraradices and possibly other AMF species could become routine inputs for the management of SDS in soybean.
• I am not aware of any currently labeled biological seed treatments or soil-applied biopesticides for SDS control. But certainly the above results indicate that an endeavor to identify such pest controls could prove fruitful. ILeVO is a chemical seed treatment that is labeled for control of both SDS and SCN.
The second study provides results that are reported in an article titled “Suppression of Soybean Cyst Nematode by Arbuscular Mycorrhizal Fungi” (Plant Disease https://doi.org/10.1094/PDIS-01-20-0102-RE).
• Soybean cyst nematode (SCN) is the leading cause of soybean yield losses in the US. Current effective control measures depend mainly on planting seed of resistant varieties and rotating soybeans with nonhost crops such as corn. However, approximately 95% of SCN-resistant varieties contain resistance genes from PI 88788, and the SCN has become or is becoming more virulent to varieties with this source of resistance. Thus, there is a need to find complementary strategies to control SCN virulence on soybeans and protect yield.
• The goal of the study reported in the above article was to evaluate the impact of AMF on SCN cyst production, SCN juveniles in soybean roots, and SCN egg hatching. This was done under the objectives of 1) compare SCN cyst production on soybeans inoculated with multiple species of AMF to those not inoculated (Expt. 1), 2) compare SCN juvenile populations at 7 and 28 days after AMF inoculation on soybean roots with or without the AMF F. mosseae (Expt. 2), and 3) determine if F. mosseae spores or spore exudates impact SCN egg hatching (Expt. 3).
• All studies were conducted using an SCN-susceptible variety grown in a greenhouse.
• Expt. 1 results. All species of AMF had lower cyst numbers than the non-inoculated control, and the fewest cysts were produced on plants inoculated with F. mosseae.
• Expt. 2 results. At 7 days after planting, plants inoculated with F. mosseae had a lower number of juveniles than the non-AMF control plants. The AMF-inoculated plants also had a much lower number of cysts vs. the non-AMF plants.
• Expt. 3 results. By day 6 after incubation, the number of SCN juveniles on inoculated plants was lower by 27% and 62% for spores and exudates, respectively, compared to the control without F. mosseae.
• These results indicate that AMF can disrupt SCN reproduction, SCN suppression occurred early in the interaction between the fungi and SCN, and AMF spores and spore exudates had a direct effect on SCN by reducing egg hatching. The spore exudates suppressed egg hatching more than the presence of spores alone.
• The authors concluded that increasing the predictability, efficiency, and deliverability of AMF to soybean fields with high populations of SCN could provide a complementary tool for a more sustainable and effective management of SCN.
• Click here for a Univ. of Illinois summary of this research that includes quotes from the authors.
Currently, Pancho/Votivo (Bacillus firmus) and Clariva Complete Beans (Pasteuria nishizawae) seed treatment products for soybeans contain biological nematicides (shown in parentheses). However, Bissonnette and Tylka (ISU Extension and Outreach) in their publication titled “Seed Treatments for Soybean Cyst Nematode” state that “Nematode-protectant seed treatments are intended to supplement current SCN management strategies, and therefore should be used in coordination with growing varieties with SCN resistance genes and rotation to nonhost crops”. In fact, a nematicide seed treatment product should never be used instead of using a resistant variety; rather, use it on a resistant variety. This supports the accepted dogma that current nematicide seed treatments will not replace proven management practices that should be used to manage SCN in soil at infested sites. Thus, there is no supposition that any AMF or other potentially forthcoming biopesticide products will replace the accepted practices for nematode control and/or management. However, the identification and development of additional complementary control measures that could result from the AMF results shown above could certainly change current recommendations. This is especially so since this would be a biopesticide vs. a chemical pesticide.
See the article on this website for additional information about using seed treatments for soybean planting seed.
The below questions arise from the above cited research.
• Will the AMF that acted as biopesticides in these studies be potential candidates for a biopesticide product or products that can provide protection against the targeted pathogens in a field setting?
• If such AMF products prove effective in a production field, can their production be commercially scaled so that they will be available in quantities for use on a considerable acreage of pathogen-infested fields?
• Can these products be soil-applied, or must they be applied as a seed treatment?
• If they can be soil-applied, will they need rainfall to effect infiltration into the soybean root zone to achieve maximum effectiveness?
• How long will their effectiveness last once in the root zone?
• Will they, like present seed treatments, be only a supplement for control of the targeted pathogens?
These questions certainly are not meant to cast aspersion on the significance of the findings in the above cited studies; in fact, the findings from these studies are most significant. Rather, they are meant to encourage further work with these identified AMF that acted as biopesticides in the controlled environment studies cited above. This is needed to determine if they will in fact be effective in a production setting against these two very significant soybean pests/pathogens.