DO NOT ADD PVC CEMENT TO THE OUTSIDE OF A GLUE JOINT.

PVC "Cement" is actually a PVC Solvent. It breaks down the plastic and "welds" the two pieces together. Adding cement to the outside of a joint will weaken the fitting and could result in a break later.

If your system is PVC glued into a Slip joint valve, and that is where the leak is, you will probably have to replace the valve.

The correct way to fix any leak, is to dig up the offending area, and replace it correctly.

A patch you may want to try, if you are looking for a short term fix, is a 2 part plumbers putty product that you would mold around the leak area. It looks like hell, and is no way to leave a system.

When you dig up the pipe, give yourself A LOT of room to work with. Remove the sod with a flat edge shovel, place to the side. Pile the dirt on a tarp or in a wheel barrow so you can put it back in the hole after it has been fixed.

When making a glued joint on PVC use PVC Cleaner (twice on valve areas!) primer, then cement on both pieces. Twist the joint 360 degrees if you can to eliminate any gaps. Do your valve connection first, then the union to the lateral pipe. Then wwist the union, not the pipe. Probably a Saturday morning type of job....

It sounds like you have a rotating valve assembly. This can or cannot be a difficult repair. Let's assume you can fix it first. It sounds like it is leaking a the base of the valve if this is the case then get a hacksaw and cut the pvc pipe flush with the base of the valve "Where the valve changes from green or black to pvc white. Now your probably thinking how do I put a new pipe in. Well were assuming that it's leaking because it wasn't glued properly in the first place. Now that the cut leaky pipe is flush with were it came out of the bottom flip your hacksaw blade vertical and cut a slight groove in the side of the pvc pipe remainig inside the valve, just a light scoring will be fine dont cut into the valve. Now take a small screwdriver and lightly wedge it inbetween the pipe and valve near the score and give it a little twist if your lucky it will seperate if so grasp the curled up pvc you just created with a pair of needlenose pliers and twist out the PVC pipe. If all goes well you can then buy a coupling and a piece of pipe and repair. If this dont work convert to individual electric valves and a electronic timer you'll be happier in the long run. Hope this helps.