Water Leak from Septic Vent Pipe

I am curious about a slow dripping leak coming from the vertical pipe that is mounted to the seweage pipe going out to my septic tank. I recently noticed that there a slow dipping leak coming from the rubber gasket around the vertical pipe as you can see in the picture. I've attached two pictures, the pvc pipe in question is the one on the right. Please let me know what the cause could possibly be. I'd like to have a plumber come out, but I am wondering if it is could be something simple I could tackle or not. Thank you for any and all suggestions.