It is an implication of systemic infection by the aphis‐borne viruses that potato plants with large haulms should be specially vulnerable to infection, and this implication is supported by evidence in the field.Haulms are reduced in size by short days, low temperatures, under‐nutrition and varietal differences. Certain observations are adduced to show that these factors reduce vulnerability. It is suggested that the transference of the potato from the short days, infertile soils and primitive cultivation on the Andes to the long summer days and productive farming of Europe and North America enhanced its vulnerability to aphis‐borne virus diseases: the further effects of changes of temperature and manurial practices in the last century need elucid