Mushroom cook Erin Patterson deliberately sourced death cap mushrooms, blitzed them into a powder and concealed them in beef Wellingtons to poison her lunch guests, a jury has been told.

Crown prosecutor Nanette Rogers SC laid out the allegations in the Victorian Supreme Court on Monday as she began her closing address in Patterson’s triple-murder trial.

Patterson, 50, has pleaded not guilty, denying she intentionally served the poisoned lunch to her former in-laws Don and Gail Patterson, Gail’s sister Heather and her husband Ian Wilkinson.

Don, Gail and Heather all died after the July 2023 lunch at Patterson’s home in regional Victoria, while Ian survived.

Patterson has also pleaded not guilty to the attempted murder of Mr Wilkinson.

Rogers opened her final address to the jury by claiming Patterson had carried out “four calculated deceptions”.

The first was fabricating a cancer diagnosis so there would be a reason to have the four guests over for lunch, the prosecutor said.

Rogers alleged Patterson “planted the seed” of the fake cancer claim at least a month earlier when she told Gail she was undergoing medical tests for a lump on her elbow.

Patterson told Gail she needed to have an MRI after a successful biopsy but there were no records of any tests or diagnosis, the prosecutor said.

“She never thought she would have to account for this lie,” Rogers told the jury.

“She did not think her lunch guests would live to reveal it. Her lie would die with them.”

The prosecutor alleged Patterson’s second deception was the lunch itself and her decision to source the death cap mushrooms and conceal the poison in individual beef Wellingtons.

Rogers pointed to iNaturalist website posts detailing death cap mushroom sites in Victoria’s Gippsland region and phone-tower data that showed Patterson’s mobile in those locations.

The prosecutor said the jury could safely infer Patterson had found and collected death cap mushrooms from those sites in April and May 2023.

Patterson then bought a dehydrator and dehydrated those mushrooms, before blitzing them into a powder to use in the individually portioned beef Wellingtons, Rogers alleged.

The prosecutor told the jury Patterson chose to serve the beef Wellingtons as small parcels instead of the recipe-mandated log form so she could avoid eating the death caps.

Patterson also served her own meal on a different coloured plate for the same reason, Rogers said.

The lunch cook denied using a different plate but the prosecutor told the jurors they should instead believe Mr Wilkinson’s evidence as he was a reliable witness with a clear memory.

Rogers alleged Patterson’s third deception was feigning illness after the lunch, adding Patterson falsely claimed she had continuous “explosive diarrhoea”.

She noted Patterson was able to drive her son’s friend home and go for a two-hour-long round trip the day after the lunch without needing a bathroom.

“You might think a person who genuinely fears pooing her pants … would not take such a risk,” the prosecutor said.

Patterson presented to the Leongatha Hospital emergency department two days after the lunch, complaining of diarrhoea and pain.

She left within a few minutes against the doctor’s advice because she knew she had not consumed the death cap mushrooms, Rogers alleged.

“She realised what she had done was going to be uncovered,” the prosecutor told the jury.

“She fled back to her house to try and figure out how to manage the situation.”