Learn Turkish: Don salkımları bozacak diye korkuyorum. - I am afraid that frost will spoil the bunches.

In this lesson's structure, diye acts as a bridge to express anticipatory concern. Literally derived from demek (to say), diye here means something like "thinking" or "worrying." The sentence is structured almost like a quote: "The frost will spoil the bunches, thinking [this], I am afraid." It is a very natural Turkish way to express "I am afraid that [something will happen]."

Normally, subordinate clauses use the genitive-possessive (like donun bozacağı). Why is don just in its bare form here?

That's the great benefit of using diye! When you use diye to link clauses, the phrase before it acts like an independent sentence or a direct quote. Because of this, it takes a standard subject and verb (Don ... bozacak) rather than the complex verbal nouns (Donun ... bozacağından) you learned earlier. It is often much easier to build on the fly!

Doesn't the verb korkmak usually require the ablative case (-dan/-den)? Why is it missing here?

You are right that korkmak (to fear) usually takes the ablative for the thing you are afraid of (for example, köpekten korkuyorum means "I am afraid of the dog"). However, when you use a full clause followed by diye, you do not add -dan to diye or the verb. The diye clause acts as an adverb explaining why you are afraid, rather than acting as a simple noun object, so no ablative ending is needed.