As featured in Arable Farming Magazine
Making progress towards virus yellows tolerance
by Arable Farming September 2020
After virus yellows turned many sugar beet fields yellow this summer, Mike Abram visited a trial in Norfolk to see how close varietal resistance is to delivering a solution.
Project Goliath is well-named. Covering about 1.5 hectares, the British Beet Research Organisation’s (BBRO) sugar beet screen for varietal tolerance or resistance to virus yellows is necessarily giant.
But it is perhaps the most important set of trials for what has become the critical agronomic issue for the sugar beet industry to overcome to maintain future viability.
Managed by BBRO’s Dr Alistair Wright, it consists of 14 varieties – four from the current Recommended List (RL) and 10 potential candidate varieties, with the five main breeding houses, KWS, Betaseed, SES Vanderhave, Strube and MariboHilleshog contributing two each.
These are drilled in blocks, starting with an uninoculated block, and then blocks which are inoculated with aphids carrying either beet yellows virus (BYV)
or beet mild yellowing virus (BMYV). Each variety is replicated four times within each of the three different treatments for a total this year of 168 plots.
Designing the trial necessitated a complete rethink in how to do virus yellows trials, says Prof Mark Stevens, head of science at BBRO.
“We’re trying to identify the genetics of the future, but the only way you can compare varieties is having the equivalents without virus on the same site. Without
the neonicotinoid seed treatments that has become very difficult.”
Producing the inoculum starts in January in BBRO’s glasshouses in Norwich, with the aim for it to peak in May ready to prime every single plant with virus in the inoculated part of the trial.
Inoculation with beet mild yellowing virus, a polerovirus which is related to barley yellow dwarf virus and potato leaf roll, can take up to 48 hours, whereas beet yellows virus, which is a semi-persistent transmitted closterovirus, usually only needs about eight hours maximum and can be as quick as 30 minutes once the aphids start feeding, explains Prof Stevens.
Now in its second year in this format, the trial is showing significant differences between varieties in their responses to virus yellows.
Assessments are made of differences in canopy formation, both from the ground and using drone imagery, with multiple vegetation indexes. Leaf measurements of virus loading will be made in September and again just before harvest in November.
“There is some potential coming on stream,” says Prof Stevens. “All of the material from the breeding houses is primarily targeting beet mild yellowing virus. Unfortunately, [resistance] mechanisms seem to be virus-specific, but there are differences in symptom expression between varieties.”
Yield results in last season’s trials highlighted both the potential and problem with possible resistant varieties, Dr Wright says.
“One variety only lost 4% yield when infected with beet mild yellowing virus compared with uninoculated.”
And while results with it were not quite so good with BYV, where it lost 25%, that is still better than the 50% yield loss you would normally expect, he says.
As well as the potential larger yield penalty for BYV, the other issue, just as it is with other traits such as rhizomania and beet cyst nematode resistance, and now Conviso Smart herbicide tolerance, is the yield penalty which comes with initial introductions.
The current best varieties for virus tolerance yield lower than the current RL controls in the absence of BMYV, says Prof Stevens.
Any introduction will have to go through the RL process, but several of the varieties in the trial are already doing that, so potentially are close to being available.
“The question will then be for growers: do they grow a high yielding elite variety at 108% of the controls and take the hit if it gets virus? We know that virus is patchy and won’t always be there to the levels of 2020, and the yield penalty depends very much on when the virus infects.
“Or do you invest in first generation virus resistance?”
This approach reduces the virus risk, makes management easier and means a smaller hit if virus does come in the crop, but conversely means a lower yield in its absence.
Deciding that, or if other traits are more important, is not going to be an easy decision, but one piece of evidence which may help is data on the yield impact from virus from 50 commercial fields.
“We’ve got detailed aphid counts week-by-week from those fields and will also have drone imagery assessments of levels of virus and distribution, and we can link that all to varietal choice, aphid numbers and trigger counts.
“We’re also digging and taking virus assessments from leaves from four patches in each field with and without virus symptoms, so we have detailed information on the impact on yield from commercial crops.”
Progress is being made, Prof Stevens says, but with such high levels of virus in crops this season, it is vital growers think about 2021 now.
“Unless we get a really cold winter to take out aphids, the risk for 2021 is going to be that much higher.
“So it is critical growers take action around crop hygiene to reduce risk by spraying off dumps, remove cleaner/loader soil heaps, control groundkeepers, etc.
“And the other area which is a concern is beet for anaerobic digestion, which could stay in the field until the end of May and create the perfect 12-month green bridge.
“Cover crops also need carefully managing – brassica species building populations of aphids, even if they are not a host of the virus. If you’re not careful and get it wrong, you could have a ticking time bomb for next season.”
Increasing yield potential of resistant varieties key to economic use
Increasing yield potential in virus-resistant varieties will reduce the amount of virus needed in non-resistant varieties before the resistant ones become economic to grow, Dr Wright says.
Based on the lower yield potential from the most resistant variety to beet mild yellowing virus, 45-75% virus infection is needed in three comparison Recommended List (RL) varieties before the resistant variety becomes economic to grow, he suggests.
But increasing the resistant variety’s yield potential from under 12t/ha sugar to nearer the 13.5t/ha of sugar of some of the RL varieties and quickly the level of infection needed to justify the switch reduces significantly.
“For those growers wanting to not spray foliar insecticides, then these varieties will quickly become very appealing.”