Always mow when the grass is dry with sharp blades.
Check belt drive to insure there is no slippage and operate at rated blade tip speed.
Adjust the attitude of the mower deck so that the front is slightly lower than the rear.
In early spring conditions it may be necessary to reduce ground speed or mow twice. Once at a higher cut height then again at the normal cut height. Later in the season after the grass has been mowed several times, the grass is stiffer and easier to cut. In most cases, the streaking usually goes away as the season progresses.