Hi Jack,
I am not certain that is a deficiency. It is a lower leaf, and typically the cotyledon would also be sacrificed for its nutrients. The cotyledon seems to have only a hint of purplish edge color. The cotyledons will be completely sacrificed if a translocatable element becomes deficient.
While you were gone, did the plant suffer a water deficiency and wilt? Or, anticipating that you would be gone, did you flood its root zone?
If it is an infection, it will progress, so keep an eye on it to see what it is doing. For the moment, it is a mystery.
ZM
