Honeycrisp is a very difficult apple to grow. One of the most vexing things about Honeycrisp is its vulnerability to Calcium deficiencies in the fruit.
Calcium transport within the apple tree is a matter of wing-of-bat and eye-of-newt. There are root/soil interactions. There are rootstock/cultivar interactions. There are leaf/twig interactions. And then there are interactions between other nutrients and Calcium; notably Potassium competing with Calcium.
There is a lot of research going into breeding rootstocks that are specifically tuned to deliver much Calcium to the fruit. One of the things scientists discovered is that there is a large amount of variation within existing, commercially available rootstocks.
|Ironically, most Honeycrisp is probably planted on M9 rootstock...the worst choice for Calcium levels in Honeycrisp|
Commercial growers will keep spraying with Calcium supplements for additional insurance. But choosing the right rootstocks is going to be critical to home growers.