Patterson, 50, is accused of murdering her parents-in-law, Don and Gail, and Gail's sister Heather Wilkinson.
She faced the fifth day of a Supreme Court jury trial on Monday in regional Victoria.
During his third day on the stand, Simon Patterson was asked about his interactions with his estranged wife and their children while at Monash Medical Centre on Monday, July 31, 2023.
Two days earlier, Patterson had served up a poisonous mushroom meal at her Leongatha home before taking herself and her children to hospital after recommendations from health professionals.
While at the hospital, the couple spoke about a dehydrator Patterson had previously used to hide mushrooms inside muffins due to their daughter's distaste for them.
Defence barrister Colin Mandy SC put it to Simon that he had then asked Erin: "Is that what you used to poison them?"
"I did not say that to Erin," Simon replied from the witness box.
The jury of 15 were shown messages from a family group Signal chat between Simon, his former wife and his parents - in December 2022 - after he last week claimed she had sent "extremely aggressive" messages.
These included messages where she said it was "unconscionable" that Simon was not paying half of their kids' school fees because of his child support payments.Â
"Both parents have a duty to financially support the children they made," Patterson said in the group chat.
She told Don and Gail she wanted to make them aware of what was going on and "encourage Simon to do the right thing by his children".
"However if you don't want to get involved even to that extent I respect that and I will find a way to manage this situation on my own," she said in the chat.
The alleged "aggressive" messages have not been shown to the jury.
Simon said one message came after he contacted his wife directly about their son not sleeping enough.
"She messaged the group chat about that, and that is the message that I had in mind, that was extremely inflammatory," he told the jury.
"It was very emotive, having a crack at me and accusing me of some things in response to what I'd messaged her about."
Earlier, Mr Mandy showed the jury messages where Don and Gail had tried to help them in their child support dispute.
"Where we might be able to contribute is in encouraging you both to reopen communication and to make some small steps of reconciliation," Don Patterson had told his daughter-in-law on December 6, 2022.
"I encourage you both to look for solutions (not problems/accusations etc)."
But in the months after this, Simon said his parents began to pull back as his mother had struggled with anxiety since being diagnosed with encephalitis.
"These are messages that caused mum and dad to agree that mum wouldn't read any of the messages that popped up from Erin," he told the jury.
Patterson has pleaded not guilty to the three murder charges and attempted murder of Mr Patterson's uncle Ian Wilkinson, the only surviving lunch guest.
Her defence said what happened was "a tragedy and a terrible accident".
Simon was excused from the witness box as the trial at Latrobe Valley courts in Morwell, 150km from Melbourne, continues before Justice Christopher Beale.
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