A plant that has been raised in a sterile environment shows symptoms of a viral infection. how would you explain this?

The answer would be: The viral infection was acquired by vertical transmission.

Vertical transmission happens when the plant parents are infected before the plants detach from it. It could happens during asexual reproduction or in seeds of sexual reproduction.

That means, the infection spread before the plant raised in the sterile environment.

This can be explained by the concept of vertical transmission of a virus, i. e., the virus is inherited from a parent. During asexuak reproduction, the offspring is genetically identicle to the parent. Here, during the development of the offspring through the stem, root or bulb of the parent, the parent's virus is passed to the offspring. In sexual reproduction, viral transmission occurs as a result of seed infection.