subsetting a dataframe

How does one remove a column from a data frame when the name of
the column to remove is stored in a variable?

For Example:

colname <- "LOT"

newdf <- subset(olddf,select = - colname)

The above statement will give an error, but thats what I'm trying to 
accomplish.

If I had used:

newdf <- subset(olddf,select = - LOT)

then it would have worked, but as I said the column name is stored in a 
variable
so I can't just enter it explicity.