Why can't plants catch a cold?

Because they do not have an immune system, don't have noses, don't have mouths, and just plain do not have the same functions as animals, insects, or humans. They don't have a heart or a brain, which means they can't catch a cold. They can get diseases however.

When humans catch a cold we have runny noses, coughs and sneezing, but a plant has not got a mouth or nose and plants live in the conditions in which we catch a cold so are immune to it.