Spread's parent company Synchemicals, which also owns Vitax, has found that the wholesaler is not viable, particularly after Bayer stopped working with the firm. It might close, with disposals of parts of the company possible.
Campbell said: "There's been a consolidation in the wholesaler market in recent years. It will become increasingly difficult for small manufacturers to get their products to small retailers. Larger retailers will be able to make up minimum orders direct, and with bigger manufacturers such as Gardman, Scotts, Westland and Bayer you're already able to go direct.
"Over recent years wholesalers have developed their own ranges, for instance in tools with Solus' Yeoman, and can distribute directly. The end of Spread only leaves Solus and Decco, so lots of people between now and Glee will have to think of their options," he added.
"There is a lot of upward price pressure anyway, so the potential for Solus to put prices up because of its near-monopoly does not have a lot of scope."
GCA chief executive Gillie Westwood said: "I don't know how many people were using Spread anymore. I question whether the days of the distributor are numbered, with so many garden centres buying direct and buying containers.
"Solus is diversifying into own brands and Gardman is becoming more like Solus."
North Yorkshire-based Deans Garden Centres co-owner Richard Deans said: "It will probably mean that Solus now more or less has a monopoly.
"Decco is still doing hort products to a degree but its service has been questionable in my area. Also, smaller family wholesalers in my area aren't there anymore. Range-wise there were not many products we could get exclusively from Spread. Now it means there are going to be more bottlenecks in the supply chain if there are fewer suppliers. You can place orders 'direct' from Scotts or Bayer but it often still comes from the wholesaler. If there's one less wholesaler I don't know how that's going to work unless Solus can increase its warehousing size. It may try to claw back some of the margin."
Kershaw Garden Centre representative Mark Yates added: "A lot of people who dealt with Spread did so because they didn't like dealing with Solus. So I think there's going to be an opportunity for Decco. I got chems and tools - pretty much everything - from Spread. I'm looking at Decco because, historically, I think Solus' service has not been as good."