Titchmarsh urged consumers not to be "snooty" about less-popular bedding such as begonias and marigolds - and said shoppers would have to change their habits this spring.
B&Q, Homebase and Thompson & Morgan have all already stopped selling impatiens because of the threat of impatiens downy mildew.
Titchmarsh said: "It is time to look again at bedding begonias and to cast aside ancient prejudices of brown leaves and shocking-pink flowers.
"There is no need to be snooty about replacing busy Lizzies with French and African marigolds. They are damn good plants. At Waddesdon (the National Trust manor in Oxfordshire) they have sensational displays of African marigolds.
"Last year, I planted busy Lizzies in my garden and they were a damp squib within a fortnight - the mildew went through them like wildfire. We're all going to have to do without them this year. Besides, who wants to grow the same plant every year?"
Titchmarsh hinted that he may use his ITV1 series Love Your Garden to promote busy Lizzie-alternatives when the show returns later this year. The six episodes are being expanded from half-an-hour to an hour each.
Titchmarsh also suggested that makeovers would make a comeback due to the economic climate. "When Ground Force and Changing Rooms first aired, people were not moving house and were looking to improve what they'd got. That's where we are now."
On his B&Q role, he added: "I am not just there for a fast buck," claiming that his work with the company had led to supply chain improvements so that stock kept pace with demand. Better-quality plant stock had led to an increase in turnover last year, he added.
Titchmarsh is due to publish My Secret Garden this autumn. Photographer Jonathan Buckley has spent the past five years charting the development of Titchmarsh's private family garden in Hampshire.