The primary aim of this research was to study the effectiveness of various strains of antagonist microorganisms and biological preparations against Meloidogyne hapla, in addition to their impact on the quality of tomato fruits and crop structure. Four microorganism strains and three registered environmentally safe nematicides were used in the experiment presented herein. The results showed that the strains Paecilomyces lilacinus F-22BK/6 and Arthrobotrys conoides F-22BK/4 had the greatest biological efficacy, reducing the number of galls on tomato plants by 91.8% and 88.4%, values comparable with the results of the chemical control Vydate 5G. The Metarhizium anisopliae F-22BK/2 and Arthrobotrys conoides F-22BK/4 treatments showed the best results, increasing the fruit weight by 8.6% and 9.9%, in addition to increasing the tomato yield by 5.0% and 13.3%. These strains contributed to an increase in sugar content, whereas the concentration of vitamin C was reduced in the Trichoderma viride F-294 and Fitoverm treatments, indicating a high level of oxidative stress in the latter treatments. The results of this study confirm the prospects of using biological nematicides against phytoparasitic nematodes, which will not only enable effective control of their population but also improve the quality of agricultural products, minimizing harm to the environment and human health.

Root-knot nematodes, Meloidogyne spp., are widespread phytoparasites that cause a significant reduction in the yield of tomato Solanum lycopersicum. In the Russian Federation, where the use of chemical nematicides is limited due to environmental and toxicological risks, the cultivation of resistant varieties and hybrids remains the most effective and environmentally safe method to control Meloidogyne. In the course of this study, the resistance screening of 20 tomato varieties/hybrids and 21 mutant lines from the collection of the FSBSI FRCBPP to M. hapla was carried out using a comprehensive approach that included morphological and biochemical analysis methods. Resistance was assessed by calculating the gall formation index, the degree of root system damage, and biochemical parameters of fruits—vitamin C content and titratable acidity. In addition, molecular screening was carried out using the SCAR marker Mi23 to identify the Mi-1.2 gene, known as a key factor in resistance to a number of Meloidogyne spp. Although Mi-1.2 is not typically associated with resistance to M. hapla, all genotypes carrying this gene showed phenotypic resistance. This unexpected correlation suggests the possible involvement of Mi-associated or parallel mechanisms and highlights the need for further investigation into noncanonical resistance pathways. It was found that when susceptible genotypes were infected with M. hapla, there was a tendency for the vitamin C content to decrease, while resistant lines retained values close to the control. The presence of the Mi-1.2 gene was confirmed in 9.5% of samples. However, the phenotypic resistance of some lines, such as Volgogradets, which do not contain a marker for the Mi-1.2 gene, indicates a polygenic nature of resistance, alternative genetic mechanisms, or the possible influence of epigenetic mechanisms. The obtained data highlight the potential of using the identified resistant genotypes in breeding programs and the need for further studies of the molecular mechanisms of resistance, including the search for new markers specific to M. hapla, to develop effective strategies for tomato protection in sustainable agriculture.