Plant diseases are among the major factors affecting the productivity and quality of agricultural crops. They are caused by a wide range of pathogens, including fungi, bacteria, and other microorganisms. With increasing interest in sustainable agriculture and reducing the environmental impacts associated with the intensive use of chemical pesticides, biological control has emerged as a promising approach to managing plant diseases. Among the microorganisms that have received considerable attention in this field is Bacillus subtilis, due to its properties that can help limit the activity of certain pathogens and improve plant health. Bacillus subtilis occurs naturally in soil and in the plant root zone. Certain strains can establish themselves around plant roots and utilize the surrounding environment. Their importance lies in their ability to produce a range of bioactive compounds that can affect the growth of certain pathogens. Among these compounds are substances with antimicrobial activity that may contribute to inhibiting the growth of certain fungi and bacteria responsible for plant diseases. Mechanisms of Biological Control Bacillus subtilis acts through several mechanisms, which gives it significant importance in biological control programmes. One of these mechanisms is the production of antimicrobial compounds that can affect the growth of pathogens or their ability to establish and spread. The bacterium can also compete with pathogens for nutrients and available space in the root zone, potentially reducing the opportunities for pathogens to establish and multiply. Another important mechanism is the ability of some Bacillus subtilis strains to form spores, which are structures that help the bacterium tolerate certain unfavorable environmental conditions. This characteristic gives the bacterium practical importance in some biological formulations, as it may improve its survival during storage and agricultural application, depending on the formulation and environmental conditions. The benefits of this bacterium are not limited to its direct effects on pathogens. Studies indicate that certain strains can contribute to stimulating plant defense mechanisms. This effect is associated with induced resistance in plants, whereby the presence of beneficial bacteria around the roots may help plants respond more effectively to certain biotic stresses, thereby strengthening their natural ability to resist pathogens. Role in Promoting Plant Growth In addition to its role in biological control, certain strains of Bacillus subtilis may indirectly contribute to plant growth. The biological activities of some strains may improve the availability of certain nutrients or produce compounds associated with plant growth, which may positively influence root development and overall plant health. However, these effects vary according to the strain, crop, and environmental conditions, and therefore cannot be generalized to all isolates or formulations. Use within Integrated Disease Management Bacillus subtilis can be incorporated into Integrated Plant Disease Management (IPDM) programmes, which rely on combining a range of preventive, agricultural, biological, and, when necessary, chemical methods. This approach allows diseases to be managed in a more comprehensive manner, with the potential to reduce reliance on chemical pesticides when conditions are suitable and the effectiveness of the biological control agent has been demonstrated. The success of bacterial application in the field depends on selecting an appropriate strain for the crop and target disease, as well as determining the appropriate timing and application method. Environmental factors such as temperature, humidity, and soil properties also play an important role in the bacterium’s ability to establish and persist in the agricultural environment. Factors Affecting the Effectiveness of Bacillus subtilis The effectiveness of Bacillus subtilis is not constant under all conditions, as it is influenced by several factors, including the characteristics of the bacterial strain, crop type, nature of the pathogen, temperature and humidity, soil properties, as well as the method of formulation and application. Therefore, agricultural applications require prior evaluation to ensure that the selected strain and product are suitable for local conditions and the target disease. Future Prospects The use of Bacillus subtilis represents an important direction in developing more sustainable biological solutions for controlling plant diseases. Future research can contribute to identifying the most effective strains, understanding their interactions with plants and pathogens, and improving formulations and application methods, thereby increasing their potential for agricultural use. In light of the global trend toward reducing unnecessary reliance on chemical pesticides and promoting sustainable agricultural practices, Bacillus subtilis can serve as an important component of integrated plant disease management programmes, particularly when used according to scientific principles that take into account the characteristics of the crop, pathogen, and environmental conditions. Therefore, the use of this bacterium should not be considered an absolute replacement for all control methods, but rather as a biological tool that can be integrated with other approaches to achieve more effective and sustainable management of plant diseases. Al-Mustaqbal University is the first one university in Iraq.