@oristarA,
"Resistance" is a noun, while "Resistant" is an adjective.
So "Is resistance to" is wrong, I ... think. But it can be corrected with be "Is/has a resistance to" or "has resistance to".
You can clearly see the difference between these two. Both works, I think.
"An important feature of this variety has resistance to some of the viruses that most commonly attack potatoes..."
or
"An important feature of this variety has a resistance to some of the viruses that most commonly attack potatoes..."
and
"An important feature of this variety is resistant to some of the viruses that most commonly attack potatoes..."