"I think it is a pretty rough deal if a supermarket, or supermarket chain, is trying to effectively blackmail growers," he said.
"We live in the food-producing hub of Australasia and many of our artisan growers need a chance to get started."
Hastings district councillor Malcolm Dixon said he was "shocked" when told by a supplier it was under pressure to not support the market.
Such measures from the supermarket were "overkill".
"What we are trying to achieve with the Growers' Market is supply very cheap vegetables and produce to those who were on the bottom of the economic scale."
Supermarket managers declined to comment on why the supplier was dropped, referring Hawke's Bay Today to co-operative food distributor Foodstuffs.
Public-relations director Antoinette Shallue said the decision to no longer stock Te Mata Mushrooms was unrelated to the Growers' Market but Foodstuffs was "continuing to support Te Mata Mushrooms throughout its other stores across the North Island".
"Recently a commercial decision was made by the individual stores in the Hawke's Bay region to change their mushroom supplier to one located elsewhere in the country," she said.
"We understand the previous supplier is upset about this decision but from time to time stores have to make decisions to ensure long-term surety of supply and price for their customers."
Mr Whittaker said supermarkets were entitled to make commercial decisions.
"Naturally we are disappointed to lose any customer, especially a local customer," he said.
"However, we do think locals want to buy mushrooms grown fresh and in the Bay, not mushrooms that have been imported all the way from Christchurch."
Mr Whittaker would not comment on whether the supermarket decision was related to his support of the Growers' Market.
The market was planned to open on two blocks of Heretaunga St East but instead opened on one, due to a lack of vendors.
Hastings City Business Association manager Susan McDade said potential stallholders were worried participation would hinder chances of supplying supermarkets in the future. She said despite that, the market had 20 stalls, brought 1200 extra shoppers into the CBD and was expected to grow.