Leaf curl describes a symptom, not a single disease.
Look inside the curl
Aphids often hide inside curled new growth. Citrus leafminer produces winding mines. Peach leaf curl produces thickened, distorted tissue. Psyllids, thrips and water stress can create other patterns.
Photograph before pruning
Take clear photos of the top and underside of leaves and the whole affected shoot. Removing all damaged growth can erase useful diagnostic evidence.