[Mystery] Solve the case in one minute! ! ! ! ! ! ! ! ! "Cracked"

There is a large orchard in the north of Scotland, and Mr. Assen is the owner. Whenever there was a good harvest in autumn, Asen would pick the fruits and sell them in the city, keeping some for himself.
This year is the harvest season again. Mr. Asen wanted to eat fruit, but forgot to take the fruit knife, so he asked Mrs. Asen to get the fruit knife. Mrs. Assen used a fruit knife to cut the apple into two halves, one for each person. Mr. Asen was poisoned and died after eating it. When the police arrived, Mrs. Asen was arrested and brought to justice.
Question: It is true that the murderer was Mrs. Assen, but how did she poison her?