Adult plant resistance (APR) is a specialist term referring to quantitative resistance that is not effective in the seedling stage but is effective throughout many remaining plant growth stages.  The difference between adult plant resistance and seedling resistance is especially important in annual crops. 
Seedling resistance is resistance which begins in the seedling stage of plant development and continues throughout its lifetime. When used by specialists, the term does not refer to resistance that is only active during the seedling stage. "Seedling resistance" is meant to be synonymous with major gene resistance or all stage resistance (ASR), and is used as a contrast to "adult plant resistance". Seedling resistance is often mediated by single R genes, but not all R genes encode seedling resistance.