Ponderosa lemon
The ponderosa lemon originated in roughly 1887, and is believed to come from a chance seedling grown in Hagerstown, Maryland. It was later named and introduced to the nursery trade in 1900. Though often referred to as a cross between the lemon and citron, a recent genomic analysis showed it to solely contain pomelo ("Citrus maxima") and citron ("Citrus medica") DNA, perhaps being an F2 hybrid. Thus they cannot derive from true lemons, which have a mandarin orange component.
Mal secco
The main host of mal secco is the lemon; however other genera including Citrus, Severina, Fortunella, and Poncirus have all been reported to be susceptible. In addition to lemon, the disease has been observed to be highly destructive in Citron (C. "medica L".), lime (C. "latifolia Tan".), and bergamot (C. "bergamia Risso"). The most common lemons in the Mediterranean, the rough lemon (C. "jambiri Lush"), volkamer lemon (C. "volkameriana Ten. & Pasq".), and alemow (C. "macrophilla wester") are very susceptible.