Gear streaks are always in parallel to the cylinder axis.

Non-gear streaks are also parallel to the cylinder axis, but do not resemble the pitch of the gear teeth.

Scum streaks are in the round the cylinder direction.

Cause Cure
Improper packing of printing blanket, thus trying to drive the plate cylinder. Pack blankets as near to bearer height as possible.
Lack of proper bearer pressure. Properly set bearer pressure to specifications.
Gear teeth are bottoming out due to the accumulation of dirt and paper fibers. Clean the gear teeth.
Worn cylinder bearings. Replace worn bearings.
Bouncing of ink or water form rollers, one or more of the rollers are set too heavy against the plate and when it hits the plate gap it bounces from the lead edge and causes a streak across the plate. Reset the water and ink form rollers to proper specifications.
Blanket is loose or slipping. Tighten the blanket.
Dirty or worn dampening roller or cover. Clean dampening roller if dirty, replace if necessary.
Non-uniform pressure of dampening roller against plate, causing uneven wear. Reset dampening rollers, replace if necessary.