From the AMA Rulebook Page 18: (see #8)

The following is a collation of all mandatory zero (0) scores applicable to all Pattern classes:
1. Flying behind flight line during or between maneuvers (see 6.2.)
2. Maneuver performed out of sequence (see 14.1.)
3. Execution of an illegal pass (see 14.2., 14.3., and 14.4.)
4. Touching the plane before completion of the flight (see 14.5.)
5. Maneuver not completed
6. Model ends up on back when landing
7. Any gear retracts or collapses during landing
8. Maneuver in progress and any remaining maneuvers scores zero (0) if any component of the aircraft falls off during the flight
9. Failure to take off (see 10.1.)
10. Landing outside of runway or landing zone lateral boundary

If the pant fell off before you rolled 10 meters (33' or 11 yards) you'd get a zero for the landing. Everything else you did before that would be scored.

Verne