The recent warm, wet autumns and winters have provided ideal conditions for clubroot development. The condition has being particularly problematic this season, but new genetics could give growers a valuable lifeline.
Once traditionally associated with Scotland and the North of England, clubroot is becoming an increasing issue across the UK, believes Julie Smith, a principal research scientist in plant pathology at ADAS and part of a European clubroot panel.
“Clubroot is a silent disease which can be hard to spot, negatively affects yield and can wipe out entire crops. I get more calls about it now than ever before, many from growers who have never seen it.
“It has a wide range of hosts, not just oilseed rape, but also brassica-type vegetables, cover crops and cruciferous weeds such as Shepherd’s Purse, Charlock and Runch.
"The incidence of clubroot is exacerbated by close rotations, it often goes undiagnosed and is almost impossible to eradicate, making ‘clean’ land a finite resource.
"Globally, it is estimated that 10% to12% of land is affected by clubroot, with yield losses typically 10% to 15%, but in heavily infested fields it can be up to 100%. A 10% infestation results in a yield loss of 0.3t/ha, worth £120/ha with oilseed rape at £400/t."
Growing problem
The UK does not have a specific monitoring programme for clubroot, but ADAS surveys suggest it is increasingly widespread throughout England, Scotland and Wales, she says
"With so much attention focused on cabbage stem flea beetle oilseed rape growers may assume that any crop damage is due to this pest, but that’s not always the case and you cannot rely solely on its above-ground appearance.
"Clubroot can occur in patches, particularly if the land is wet, so areas where crop is discoloured, or plants are stunted must be investigated. The effects may also be evident at flowering when you will see patches where this is delayed and/or the crop is shorter.
"These signs indicate the presence of clubroot, so it is essential to uproot plants to check for the characteristic galls. A soil test is also recommended."
Developing sustainable methods for managing clubroot involves the targeted use of resistant varieties and applying soil treatments in affected areas, she advises.
"Good machinery hygiene is critical to minimise the transfer of pathogen between fields.
"The average piece of farm equipment can transfer 250kg of soil from one field to another, so if clubroot is present try to work your ‘clean’ fields first before going into infected fields and if you can’t do that, wash machinery before moving.
"Other biosecurity measures include wearing clean footwear between fields, avoiding using infected green manures/mulches and creating breaks to minimise the transfer of pathogen."
100% focus on clubroot tolerant varieties
The first time Shropshire farmer Rob Morgan noticed signs of clubroot in his oilseed rape was about eight years ago in a field which had previously grown a continuous cover crop and a wild bird food mix with kale. Now all 30ha of the crop are clubroot tolerant varieties
“The first signs of clubroot weren’t easily visible in the field but showed up clearly when we flew a drone over the crop.
“We pulled up some plants from those areas and the symptoms confirmed the presence of clubroot. The first indication of it in oilseed rape was that some areas had a slightly purple tinge, and some plants were stunted."
The decision was made to stop growing the wild bird mix, address some soil pH issues and switch to varieties of oilseed rape which are tolerant to clubroot, he says
“We would love to be able to grow a Clearfield variety with clubroot resistance, but none is available."
"The clubroot tolerant variety DSV Crocodile CR has performed well in the past and now DSV Cromputer CR looks like continuing this, he says.
"DSV Cromputer came through the cold months well and has looked good throughout. Until the crop is harvested you never really know how well it will yield, but based on what we have seen so far we will be growing it again next season."
New genetics
DSV UK's Sarah Hawthorne says DSV Cromputer CR is one of the latest varieties helping to bridge the gap between clubroot protected and other varieties, in many cases outyielding them.
"Currently on the AHDB Candidate List for 2024/25, DSV Cromputer CR is a high vigour variety which is fast to establish, producing tall plants with a compact canopy and offering a significant yield increases over existing clubroot-resistant varieties.
"This is combined with TuYV (Turnip Yellow Virus) protection and improved stem canker resistance, plus it's a strong yielding variety with a gross output of 99.9% and a seed yield for the east/west region of 99.8%. Oil content is good at 45.4%, too."
Incidence of clubroot has definitely been increasing in the UK over the last five years, and while no official advice is currently available on how to deploy clubroot-tolerant varieties, they should not be over-used, she stresses.
"Growers should do their bit to protect the genetics of these varieties by only planting them where the disease is a known issue and after addressing other causal issues such as drainage, calcium and boron levels, controlling weeds effectively and delaying sowing.
"DSV Cromputer, for example, should not be drilled before the third week of August and grown at a minimum 1:5 rotation in soil free from brassicas.
“Where clubroot-tolerant varieties are used frequently or in heavily infested soil, strains which break this resistance are more likely to develop. Such varieties are a great ally in the fight against clubroot, but they should certainly be used with care."