Chemical controls for agricultural pests and diseases can have detrimental effects on human health and the environment. One alternative is to introduce soil microbes, such as arbuscular mycorrhizal fungi (AMF), that can improve crop resilience to pests and pathogens. While many plants form symbioses with AMF, not all crops benefit from inoculation. We asked (1) if AMF inoculation could increase the growth of Rubus ideaus cv. Meeker (red raspberry), (2) if it would improve plant resistance to the pathogen Phytophthora rubi and the parasitic nematode Pratylenchus penetrans, and (3) if the source of AMF mattered. To answer these questions, we ran a 6-month, full factorial greenhouse experiment (4 inoculum x 4 pathogen treatments, 10 replicates, N = 160). Plants first received either AMF from a constructed community, the rhizosphere of uncultivated Rubus parviflorus, the rhizosphere of R. ideaus from a commercial farm; or no AMF. All plants received small microbes (<11 um) from mixed inocula. After 2.5 months, we introduced pest/pathogen treatments of neither, either, or both P. rubi and P. penetrans. We measured root/shoot biomass, height, shoot nutrient content, AMF colonization of roots, and nematode densities to evaluate effects of AMF on R. ideaus with and without pest/pathogens present.
Results/Conclusions
Plants receiving AMF inoculum had 5.1% higher leaf chlorophyll, yet their biomass was either unchanged or reduced compared to un-inoculated controls. Because many un-inoculated plants were colonized by wind-dispersed AMF (therefore all plants received additional AMF), observed differences in plant response suggest that early inoculation of crops can have lasting effects. High colonization for all inoculated plants (85.5% +/- 2.1%) suggests that fumigated raspberry farms can have sufficient AMF communities to colonize plants. Plants receiving P. penetrans had 315-680 nematodes/g root, with 55% lower densities in plants that also received P. rubi, suggesting an interaction between these organisms. We found no evidence that mycorrhizal inoculum altered nematode densities in roots or soil. However, a lack of biomass or nutrient differences in plants receiving P. rubi and P. penetrans indicates we did not achieve pest/pathogen densities that impact plant growth. Thus, we have no evidence for or against AMF increasing resilience of R. ideaus.
In conclusion, we observed that different sources of AMF had lasting effects on plant biomass and chlorophyll content, though they did not improve plant nutrition. Further research is needed to determine whether AMF from any source can ameliorate the effects of pests/pathogens on R. ideaus crops.