It depends on what you are willing to tolerate and how much work you are prepared to do.

Some apple varieties are very prone to scab, others are not prone at all and a few fall somewhere in between. Generally, scab is more a foliar issue - it can affect fruit but typically it is more of a surface blemish.....peel off the skin and the fruit is fine.

On young trees, scab can create some problems with the degree of defoliation that occurs. That can weaken the tree and seriously impact both growth and fruit production. With mature established trees, it is less of an issue and the disease is more cosmetic than debilitating.

Scab is pretty weather dependant and the PNW tends to produce late winter/spring weather that is very conducive to this disease - extremely so this year. If you do wish to treat, you will need to embark on a fungicidal (liquid copper, lime sulfur, etc.) spray program beginning while the trees are dormant and continuing periodically into the season as long as weather conditions are suitable to the pathogen (with the exception of the bloom period).

It's a lot of work. What you can do to help is gather and destroy all foliage this fall - make sure the area surrounding the trees is clear and clean. Just a single dormant season spray could help also. Otherwise, you can consider replacing these apples with resistant varieties.

FWIW, I grew Gravenstein in my old garden - my favorite apple variety hands down and very prone to scab. I never sprayed it ever and while it would drop a bunch of leaves each season, it still thrived sufficiently to produce many years of good crops. The perfect apples we'd eat out of hand - any funky looking apples got converted into applesauce. YMMV

I have a 3 yr old flowering crab that does not bear fruit. In the last two years, the leaves are getting dark spots on them and eventually yellow and fall off. Is this a fungus and how can I treat this tree? I hate to lose it, as my husband gave it to me for Mother's Day..makes me sad. Thank you for any help you can offer.