The main cause is foot sweat (also see focal hyperhidrosis). Sweat itself is odorless, but it creates a beneficial environment for certain bacteria to grow and produce bad-smelling substances. These bacteria are naturally present on our skin as part of the human flora (though exponentially more smell is created when closed-toe shoes are worn). The front part of the foot is where it produces the most sweat. Therefore, more smell is created with factors causing more sweating, such as wearing closed-toe shoes for many hours. Socks generally do not cause foot odor on their own, but when worn along with shoes, socks can help to trap the hair on the feet, especially on the toes, and may contribute to the odor’s intensity by adding increased surface area in which the bacteria can thrive.
A great & helpful article with practical advices about “Home Remedies to Treat Skin Fungus” can be found under: https://www.healthambition.com/get-rid-skin-fungus/