Sweetpotato research looks to healthy roots

One of their not-so-secret ingredients is planting their fields with vigorous cuttings of "disease-free" sweetpotato vines, harvested from on-farm nurseries, or sourced from dedicated cutting suppliers.

In both cases, those cuttings come from special sweetpotato plant beds (sometimes referred to as seedbeds), that start life as carpets of hand-placed sweetpotato roots, covered with a 2cm to 5cm layer of soil and carefully nurtured until the precious sweetpotato "sprouts" emerge.

These original roots preferably come from a supplier who has generated certified planting material, using multiplication techniques that minimise the risk of virus infection.

Currently the bulk of Australia’s commercial planting material originates from Eric Coleman’s farm west of Rockhampton, isolated from the main sweetpotato production areas.

Investigating the best techniques for generating vigorous sprouts from nursery plant beds has been a substantial research target for sweetpotato scientists in recent years.

Queensland Department of Agriculture and Fisheries' (DAF) principal horticulturist Craig Henderson, (Henderson RDE) and former a DAF scientist at Gatton, said when the research was first started, there were nearly as many different styles of nursery plant beds as there were growers.

"Although we’re only half way through the current project, we’ve already highlighted several must-do issues you need to address for successful sweetpotato sprout nurseries," Mr Henderson said.

"When forming the beds, growers need to get them at least 20cm above the ground surface, so the bedding roots are well drained, with plenty of air circulation through the soil pores.

"Similarly, only cover the roots with a few cm of soil, so the sprouts can come through quickly and easily.

"If you have poor aeration, fungal and bacterial diseases will destroy your nursery roots before you’ve got a decent number of sweetpotato sprouts to plant out."

The Australian sweetpotato industry is starting to move quickly to a range of different types and cultivars, many sourced from the breeding program at Louisiana State University in the United States.

These new sweetpotatoes come with a range of benefits, such as nematode resistance, more reliable shapes and sizes, as well as diverse colours, flavours and prospective human health attributes.

However, they also come with several challenges.

"Some of the new cultivars can be tough to work with in our Australian nursery plant beds," Mr Henderson said.

"Because they have different climates and production systems in the USA, we’re having to find our own way a bit.

"For example, one of the new nematode resistant cultivars needs a certain amount of heat before it will sprout.

"So we’ve tried pre-heating the roots before bedding, and increasing bed temperatures with plastic mulch.

"Unfortunately, the same cultivar is very prone to breaking down and rotting in warm conditions.

"It’s going to take some working out to develop an effective strategy, but we’ve already got a few clues.

"We keep providing new ideas and suggestions in our management guides that we update on the industry websites."

Although he’s moved to Victoria, Mr Henderson said he still values conducting research at the DAF Gatton Research Facility.

"Because it’s isolated from the main sweetpotato production areas, we get less hammered by the sweetpotato pests and diseases, and so our research experiments have less complications," he said.

"We can get more precise management of agronomy. We can also do destructive measurements, or implement weird treatments, without interfering with a commercial grower’s operations.

"Probably the biggest benefit is access to terrific DAF scientists like Sandra Dennien, who has a real green thumb when it comes to sweetpotatoes."

According to the science, the current best-practice nursery plant beds should be able to generate around 250 sprouts per square metre every 18 to 21 days for at least four sequential harvests.

"We’d like to see most of the industry capable of hitting that mark in the next few years, even with the challenges of new cultivars and changing pest management pressures," Mr Henderson said.  

"In reality, the best growers will probably kick past that pretty soon, as they continue to improve.

"That’s why I still love working with this industry, even though it means a bit more travel on my part."

The current sweetpotato research is supported by Horticulture Innovation Australia, through the vegetable levy and support from the Australian Government, along with the Queensland Department of Agriculture and Fisheries and Australian Sweetpotato Growers (Inc).