The black rot fungus overwinters in the field and in diseased refuse. The fungus is introduced into a field most commonly in infested seed. The fungus attacks seedlings and runners from which rain-splashed spores contaminate the surface of developing fruits. The fungus can penetrate Butternut squash directly in the field before harvest. Hubbard squash is resistant during the growing season but often shows symptoms during storage.
Storage rot phase of the disease is due to fungus entrance through harvest wounds.
Black rot of winter squash, pumpkins, and gourds is caused by the fungus Mycosphaerella melonis . Circular, black or tan spots appear on the cotyledons and stems of young plants. Seedlings can be killed and plants that escape early damage can develop lesions on stems near the center of the hill. Cankered vines eventually wilt after midseason. On the fruit, the disease appears first as a brown rot of the rind, flesh, and seed cavity often accompanied by a white and black growth of mold.