By Glenys Christian
Calum Wilson – hopes for answers from powdery scab research 
John Keer – United Kingdom potato growers running out of weed control options
Powdery scab is the biggest economic constraint for the Australian potato industry, Dr Calum Wilson, from the Tasmania Institute of Agriculture, told the Inta-Ag Conference in Pukekohe recently.
It causes 10% to 20% yield losses and is estimated to have an economic cost of A$30 million a year.
“And there are not a lot of tools in the tool basket to control this beast,” he said.
Powdery scab, which is a vector for Potato Mop Top Virus (PMTV), is particularly significant in Tasmania with huge levels of spongospora found in some soils. They infect potato crops in three phases through root infection, galling and tuber lesions.
A lot of damage can occur before the disease is visible and it can remain in the soil for decades, taking 10 to 20 years before it is no longer a risk.
A grower’s best defense is to plant clean and certified seed from varieties which are not susceptible, into soils free of the disease. In South Australia a test is offered where growers can send several kilograms of soil to be tested for spongospora levels.
It is also a good idea to avoid planting in wet soils, which powdery scab particularly likes, and to avoid excessive irrigation. Short crop rotations are best avoided and the longer between potato crops the better.
“We need new controls and research teams are looking at where its Achilles heel is,” Calum said.
One target is host resistance to root infection because it has been found that root binding varies with different potato varieties.
“We can study this rapidly in the lab to screen new breeding lines,” he said.
“We hope to get gene markers and also understand what is being bound onto in the root.”
Pathogen response in host plants is also being studied to see what triggers the virus, and soil inoculum management might be able to be used in a “germinate to exterminate” manner. Scaling up is needed and there is also the challenge of treatment cost as well as the capacity to treat the full soil profile. However mini field plots are showing promising results and three stimulation trials will be run over the next 12 months.
Another strategy, “speed to recede”, might be able to be used after a potato crop is grown, to manage soil inoculum levels. There is good preliminary data available and Calum is working on convincing potato growers to get involved.
Another possible means of control might be “diffuse to confuse” by looking into what exudes from potato roots to attract spongospora and then replicating that.
“We haven’t started that work yet and have no idea of if it will work,” he said.
“But watch this space for, fingers crossed, something new in a short while.”
John Keer, Richard Austin Agriculture’s technical manager specialising in crop nutrition, protection and soil management, told the conference that United Kingdom potato growers are increasingly moving to use toppers or flailers to dessicate their mature potato crops. “It’s not a particularly happy situation at the moment.”
He said that 10 years ago it was an absolute non-event as 100% weed control could be achieved all of the time with the range of chemicals available. But withdrawal of a number of chemicals from the market means that growers have struggled to find alternative dessicants, and new residual herbicides are unlikely to be developed because of ecotoxicity concerns and water regulations. Two dessicants are still available to growers but they work slowly and need to be applied at precisely the recommended time.
John said it was estimated that weeds could cause losses from 14% to 80% in UK potato crops due to crop competition and the difficulties that some weeds such as fat hen and wire weed, present at harvesting time.
“There are few cultural control options because post-planting cultivation destroys one flush of weeds but creates another and also overworks the soil,” he said.
Cracks in ridges mean soil water losses which disturb root systems. There is less effect when there are large weed numbers on highly organic soils, but new implements being looked at such as hoes can disturb the bottom of the ridge.
“It isn’t progress but we’re running short of weed control options,” he said.
There is a case for applying glyphosate before sowing a crop if perennial weeds are a problem. While UK growers had previously tank mixed contact and residual sprays to apply just before emergence, the timing can be tricky to manage. So some growers are looking at a separate residual spray application on settled ridges soon after planting.