The latest independent trials have shown promising but mixed results for biological fungicide product Serenade for controlling Rhizoctonia solani in potato crops.

The product, which contains the bacteria Bacillus subtilis, has an off-label approval for use in potatoes to control a range of disease-causing pathogens, applied either in-furrow at planting or as a seed treatment.

See also: Cover crops to control potato cyst nematodes

After Bayer CropScience acquired US biopesticide company AgraQuest in 2012, it gained the rights to market Serenade and in 2013 commissioned various trials to learn more about the product and how it is used.

Scotland’s Rural College (SRUC) potato expert Stuart Wale carried out one of the experiments and told Crops that although results were variable, it is still a useful product in an integrated disease control approach.

Dr Wale explains the bacteria contained in Serenade are very active on the soil-borne pathogen Rhizoctonia solani, which causes black scurf and stem canker.

He also suspects that it has good activity against a wider range of pathogens but, so far, trials in the UK have been limited to validate his suspicion.

“The difficulty with these biological controls is that their efficacy can be seriously influenced by the conditions at application and it has to compete with all the other soil biota,” he adds.

Testing conditions

The difficulties faced by bio-controls were demonstrated in Dr Wale’s Scottish trial, which was carried out in an unirrigated crop in the very dry, late spring and early summer of 2013.

The trial used both in-furrow and roller table applied (to seed) treatments using the variety Charlotte that was 95% infected with rhizoctonia, so disease pressure was high.

The highest seed tuber treatment of 4kg/t reduced stem canker incidence from 43% to 17% in one trial, demonstrating that there is a level of activity against rhizoctonia on infected seed.

“It is not the level of control that we would expect from a conventional seed treatment, but the severity of disease was reduced from about 1% to 0.34%, which is good for a biological organism,” says Dr Wale.

Although the product showed useful reductions in stem canker, it did not persist enough to prevent black scurf on daughter tubers.

It did, however, reduce the black scurf levels to 50% from the untreated 74% infection levels. The conventional chemistry reduced symptoms to 2.3%.

Dr Wale points out treating seed tubers with Serenade directly was clearly the better delivery method in this case, but in a trial situation it is much easier to get the correct dose and coverage on the tubers.

“This might not reflect how it would be on farm where time pressures may result in an element of variation, but in trials you have time to get it right,” he adds.

It is thought that the in-furrow application could result in a lower concentration of the bacteria around the planted tubers, which is where the product needs to be to get maximum effect.

“Soils are also full of micro-organisms for the bacteria to compete with and when it is diluted, it seems that it will have a less positive effect.

“The trial also suggests that dry conditions don’t give the product the best chance to work and it might be suited to irrigated crops or a wetter season,” explains Dr Wale.

Asking questions

A single year of trials data is difficult to draw any conclusions from, but Dr Wale does question what the role of Serenade will be and he thinks it will have to work side by side with conventional chemistry.

“Serenade wouldn’t be where it is if it gave us nothing, but we need to learn how best to use it. It can’t be relied upon by itself, but it could be a useful addition to an integrated programme,” he says.

In Bayer trials, the product has been used alongside conventional seed treatment Monceren (pencycuron), with Serenade applied in-furrow at planting.

Using this programme improved control of black scurf was demonstrated over and above that of both products alone.

This supports Bayer’s recommendation that Serenade is best applied as an in-furrow application at 5-10 litres/ha following Monceren seed treatment.

Dr Wale says that in the correct conditions, Serenade will do a very good job and could allow growers to drop the rates of their conventional chemistry.

“I expect trials to continue this season and we will continue learning about this product and how best to use it in the future,” he says.