Well, the adjectives differ according to the grammatical gender of the noun they describe.
-y or -i is the ending for masculine singular, -a for feminine, -e for neuter (and not masculine-personal plural), -i or -y for masculine personal plural. "Jabłko" is neuter, so it's smaczne.
Your first assumption should be that:
if it ends with a consonant, it's probably masculine
if it ends with -a, it's probably feminine
if it ends with -o, -e or -um, it's neuter.
There are exceptions, not even that few of them, but first start with the rule and then learn the exceptions :)