DIAGNOSIS The 'Recycler' design mulches finely, which can lead to more clumping and sticking, especially with damp grass. The packed debris traps moisture against the steel deck.
FIX After each use, hose out the underside. The real trick is to do it while the debris is fresh. Use the washout port on the deck: connect a garden hose, run the mower for a minute to sling water around, then tip it and scrape out the rest with a putty knife or stick. For existing rust, dry it thoroughly and scrub the spots with a wire brush, then spray a light coat of silicone spray (not oil) on the clean, dry deck to prevent future sticking.
IF THAT DIDN'T WORK If the rust is deep or the discharge chute is corroded shut, you might need to replace the deck shell. Toro sells it as a part, but it's a big job.
HEADS UP Never use a pressure washer directly on the deck seals or the motor housing. The high pressure can force water into the bearings and electrical connections. Garden hose pressure only.