Patterson, 50, is on trial charged with three murders and one attempted murder over the lunch on July 2023 at her home in Leongatha, Gippsland, in Victoria's southeast.
Her estranged husband Simon's parents Don and Gail Patterson, both 70, and Gail's sister Heather Wilkinson, 66, all died in hospital after being diagnosed with death cap mushroom poisoning from eating the meal.
Patterson has pleaded not guilty to all offences and claims she did not intentionally kill any of her lunch guests, saying it was a "terrible" accident.
The trial being held in Morwell entered its third week on Tuesday where the jury was introduced to mushroom expert Thomas May.
Dr May said he was a mycologist, a scientist specialising in the study of fungi, at Victoria's Royal Botanic Gardens.
He discussed the origins of amanita phalloides, known as death cap mushrooms, and how "citizen scientists" often posted their whereabouts to websites such as iNaturalist.
"It is found in Gippsland, but with only three reports from Outtrim, Loch and Morwell," he told the jury of 15 people on Tuesday.
"The records for Outtrim are from May 2023?" prosecutor Nanette Rogers asked.
"Correct," Dr May replied.
"And the record from Loch is from 18 April 2023?" the prosecutor continued.
"Correct," he said.
He said the third record, found in Morwell, was historical and from "somewhere in the last 20 years".
Patterson is accused of serving up her lunch, steak covered in mushroom paste and encased in pastry, on July 29, 2023.
Outtrim and Loch are in the Gippsland region, where the lunch occurred.
The accused triple murderer told doctors and toxicologists she bought the mushrooms for the meal from Leongatha Woolworths and a Chinese grocer in Melbourne's southeast.
Dr May confirmed "there is no evidence that amanita phalloides occurs in China".
Earlier, doctors who treated Patterson at Monash Hospital spoke to the jury about how she did not appear unwell and was discharged.
Patterson had taken herself to Leongatha Hospital earlier that day, on July 31, saying she was suffering diarrhoea and abdominal pain.
Her four lunch guests, including church pastor Ian Wilkinson who survived the lunch but became very unwell from eating it, were also being treated at Melbourne hospitals at that time.
Monash emergency doctor Varuna Ruggoo assessed Patterson on August 1 and found she was "clinically well" after performing several tests.
Patterson had been cleared of potentially suffering from death cap mushroom poisoning by another doctor, Laura Muldoon, she said.
"She wrote in her notes that there was no concerns about that type of poisoning because ... Ms Patterson's liver function tests were all within normal limits," Dr Ruggoo told the jury.
Dr Ruggoo then deemed Patterson was fit to be discharged, at 1pm that day.
Gail Patterson and Heather Wilkinson died at the Austin Hospital on August 4, then Don Patterson died the following day.
The trial before Justice Christopher Beale continues.
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