When inoculated back into American chestnut, white cultures give smaller cankers than yellow cultures.
They are hypovirulent. In addition, the virus causing whiteness and hypovirulence can move from a white to a yellow culture.
Here, the center canker, caused by a yellow culture, started out larger than those caused by the white cultures, but then stopped expanding.
White hypovirulent cultures were isolated from it, and they were the original yellow culture infected with the virus from the original white culture.
White culture
Yellow culture
White culture