Step 1: Mycotoxins are toxic secondary metabolites produced by fungi, and each is usually linked historically to the fungal genus first found to produce it.
Step 2: Citrinin takes its name directly from Penicillium citrinum, the species in which it was first isolated, and it remains best known as a Penicillium product even though a few other genera can also make it.
Step 3: Pythium, Fusarium and Helminthosporium are all significant plant pathogens but are linked to other toxins or diseases entirely, not citrinin.
So the correct genus is Penicillium.