Leaky shutoff valve – Main to outdoor backyard

I have a shutoff valve in my basement that leads to outside backyard. I closed that valve during winter and I just opened it to water my newly seeded lawn.

Upon opening the valve, I noticed that the shutoff valve continiously leaks water. There is a small nut that holds the knob. I tried tightening the nut while the valve is open but it still leaks water.

What is causing the leak? Do i need to take the knob off and perhaps put some sealant tape along the thread?