Individual oilseed rape varieties definitely respond differently to pressure from a widespread soil-borne fungal disease, AHDB-funded research has revealed. Andrew Blake reports.
Verticillium wilt, which has halved yields in some trials on infected land, first emerged as a problem in the crop a decade ago. The disease affects many other crops and sowing tolerant or resistant varieties is usually the only practical and economic way to counter it.
Given the extent to which its incidence and severity has risen, the AHDB deemed a research project* to back a system of resistance ratings for oilseed rape varieties was merited.
That work has recently ended, the key finding being the consistency with which varieties rank in terms of their resistance to the disease, says NIAB pathologist Jane Thomas, who has led the work.
Two approaches were used to assess resistance – either inoculating land with no history of the disease, or using sites where previous crops had been severely affected by it.
She says: “After the first year’s trials, we were able to select a sub-set of six varieties which showed a wide range of symptom development – either high or very low levels.”
Those six were then used in yield trials where blocks were either inoculated with the disease or left without inoculation. Disease development was assessed and the trials taken to yield.
“Over the three years of variety testing, we’ve seen very consistent results,” says Dr Thomas.
“The level of disease expression overall did vary, but the more susceptible or more resistant varieties ranked in the same order from year to year, and from site to site, whether inoculated or naturally infected.
“This has given us a high degree of confidence that the differences we’ve seen reflect resistance under genetic control, so resistance rankings could be produced for varieties on the Recommended List [RL].”
None of the varieties tested, which included all those on the RL and the candidates, were completely resistant to verticillium wilt, she adds.
“All showed symptoms to some extent.
“The yield results were rather variable, but the variety ranking orders in terms of disease expression were exactly as expected.”
In some trials, high background levels of verticillium in the soil meant there was limited differentiation between the inoculated and control blocks in terms of disease levels and one trial developed poorly after flea beetle damage.
“However, in a trial with very little disease in the control blocks, there was a statistically significant cut in yield in two of the three susceptible varieties and a non-significant reduction in the third.”
There was no yield penalty in the three more resistant varieties, but in the susceptible varieties it was between 0.45-0.69 tonnes/hectare.
“While the effects of site and season are likely to influence verticillium’s impact, yield losses do occur in some circumstances and magnitude of the loss looks to be related to variety susceptibility.
“The surprise in this project has been the high degree of consistency of variety ranking orders. While we’d observed this before in smaller scale tests, we didn’t know whether larger scale testing with the whole of the RL would give us consistent data. Now it has and it’s been on a par with the reproducibility seen, for instance, with stem canker tests.
“The work we’ve done certainly indicates that variety rankings are possible, but if or how they might be introduced is under discussion with the AHDB.”
Using partially-resistant varieties could help manage the disease, Dr Thomas believes.
“If a grower has an infected site, choosing a more resistant variety should not only help reduce yield loss, but also lessen the return of infective microsclerotia to the soil, so keeping levels lower.”
Resistance to verticillium wilt has never been a key breeding target, according to Limagrain’s Vasilis Gegas.
He says: “That’s mainly because breeders have concentrated on stem canker and light leaf spot, and it’s difficult to screen for wilt on a large scale.
“Although this and other research suggests there is considerable variation in verticillium wilt tolerance in the modern breeding germplasm, the genetic underpinning of this variation remains only loosely defined and definitely under studied.”
One of the main bottlenecks has been the lack of reliable, cost-effective and high throughput screening methods, says Dr Gegas.
“It’s encouraging to see that this project’s protocol has provided good reproducibility across years and sites.
“However, establishing a relationship between yield penalty and variety rankings based on verticillium wilt severity and incidence, especially at farm level, appears more challenging.
“As with most diseases, the extent of yield loss due to wilt is determined by the interaction with various other stresses and agronomic practices.
“The way varieties develop and grow can also significantly affect the way the disease manifests itself at yield level. So it’s important to consider any symptom-based disease ranking within the wider context of the ‘ideal’ variety and farm-specific needs.”
Managing the disease, the incidence of which was particularly bad in 2017, requires an integrated approach, says Dr Gegas.
“Variety choice should be coupled with correct agronomic practices well before the seed hits the ground. As with other soil-borne diseases, such as clubroot, growers would benefit significantly by carefully assessing the risk field by field.
“Soil testing for the pathogen’s presence should be the starting point. Previous history of infection at field and farm level, length of rotation, cultivation practices, soil type and source of seed – farm-saved versus certified – are all parameters that should be considered when it comes to mitigating against losses.”
July 2015-November 2018 (three full testing years)
Funding: AHDB £117,000
In-kind support from oilseed rape plant breeders: providing seed, information on test systems in breeding programmes, and discussion on assessment techniques