Arbuscular Mycorrhiza and Antagonistic Microbial Consortia Reduce Phytopathogenic Pressure and Improve Rhizosphere Functioning of Sugar Beet Under Short-Rotation Cropping Systems

Short-rotation sugar beet (Beta vulgaris L.) cultivation in the Western Forest-Steppe of Ukraine is often accompanied by increased phytopathogenic pressure and impaired rhizosphere functioning, creating a need for biological tools to stabilize the plant–soil system. This study evaluated the effects of arbuscular mycorrhiza and an antagonistic microbial consortium on pathogen pressure, rhizosphere activity, yield, and technological quality of sugar beet under different crop rotations. Field experiments were conducted in 2023–2025 using a three-factor design that included rotation, mycorrhizal inoculation, and microbial inoculation. The highest phytopathogenic pressure was recorded in the maize–soybean–sugar beet rotation, where the cumulative frequency of dominant pathogens reached 94.0% and the root rot severity index in the control was 28.6%. Arbuscular mycorrhiza reduced disease development by 14.6–16.4%, whereas the antagonistic consortium reduced it by 25.6–27.9% relative to the control. Their combined application was most effective, decreasing root rot severity to 9.6–17.1% and increasing root colonization, available phosphorus, and dehydrogenase activity in the rhizosphere. The highest yield (80.5 t/ha) and sugar content (18.5%) were obtained in the soybean–winter wheat–sugar beet rotation under combined inoculation. AMF can improve phosphorus acquisition and mycorrhiza-induced tolerance, whereas antagonistic fungi can directly suppress soil-borne pathogens through competition, antibiosis, and mycoparasitism, their combined use may provide complementary protection in disease-conducive rotations. Overall, integrating arbuscular mycorrhiza with antagonistic microorganisms is a promising approach for reducing pathogen pressure and improving sugar beet performance in short-rotation systems.