Learn Turkish: Bağcı, mahsul kuruyacak diye korkuyor. - The vinegrower is afraid that the crop will dry up.

This is the standard way to express anticipatory concern in Turkish. You treat the feared event as a direct quote. Literally, the sentence means the vinegrower is afraid, thinking: 'The crop will dry up.' You use the future tense (kuruyacak) because the feared event hasn't happened yet, followed by diye to link it to the main verb (korkuyor).

Could we use a verbal noun with the ablative case instead, like mahsulün kurumasından korkuyor?

Yes, you can use the verbal noun with the ablative case (-dAn korkmak), which translates formally to 'fearing the drying of the crop.' However, using the diye construction with a conjugated future verb (kuruyacak diye korkuyor) is much more common and conversational in everyday Turkish for expressing worries.

Why isn't mahsul in the accusative case here?

Because inside the quoted phrase (mahsul kuruyacak), mahsul is the subject of that specific action, not a direct object. It is the thing doing the drying up, so it stays in its bare dictionary form.

What is the purpose of the comma after Bağcı?

The comma clarifies that Bağcı (the vinegrower) is the subject of the main verb korkuyor. Because the sentence immediately introduces another noun (mahsul) which acts as the subject of the inner clause, the comma visually separates the main subject from the embedded fear clause.