Varieties of winter oilseed rape with resistance to the common strains of clubroot have featured on the Recommended Lists (RL) since Mendel was first recommended in 2002.

However, clubroot-resistant varieties are associated with relatively large yield penalties, so they only tend to be used when clubroot issues are expected, says Dr Paul Gosling, AHDB Recommended Lists leader.

The RL 2022/23 features four oilseed rape varieties with specific recommendation for use on clubroot-infected land including a new variety, Crossfit, that for the first time resists both clubroot and turnip yellows virus (TuYV).

Dr Gosling says: Resistance to clubroot is not tested in RL trials and we avoid drilling trials on land with a history of clubroot, so the disease is not normally seen.

Specific recommendation is based on the acceptance of breeders claims for traits, where the evidence supports this.

Despite the aim to avoid clubroot, inspection of an ailing RL trial in northern England in autumn 2020 revealed the disease was the issue, despite there being no record of clubroot in the field previously.

Usually, we would abandon such trials.

However, the disease was relatively uniform across the trial, so we persevered to assess clubroots effects on the full variety set, says Dr Gosling.

Symptoms By flowering, there were clear signs of clubroot symptoms and differences between varieties.

The most susceptible variety lacked vigour, with evidence of plant deaths seen as gaps in the plots.

As this was a northern trial, only recommended and candidate varieties for the North were included.

Crome was the only variety with clubroot resistance in the infected trial and it was associated with the lowest number of plant deaths.

At harvest, the benefit of clubroot resistance on yield was clear.

Crome yielded 158% of controls (4.50 tonnes per hectare) in the infected trial.

This compares with its North yield of 101% of controls on the current RL (trial mean of 2.98t/ha).

We recorded a wide range of varietal yields across the infected trials.

Blazen yielded particularly well at 133% of controls , whereas some varieties yielded below 2.5t/ha.

It may be that some varieties are more tolerant to clubroot, although not technically resistant; something for breeders to investigate, says Dr Gosling.

While the yield data from the RL trial was not used for recommendation purposes, the observations provide useful insights into clubroot management, adds AHDB.

Even where not previously recorded as an issue, clubroot symptoms can still appear.

Where clubroot is present, resistant varieties can effectively protect yield.

The clubroot resistance in Crome was effective at this site.

Based on a single dominant gene, the same resistance mechanism (termed ‘Mendel resistance) is present in all resistant crop varieties.

Growing these varieties gives some protection against common clubroot strains, but some can overcome the resistance.

In some locations the resistance is no longer effective.

Frequent use of resistant varieties, or use in heavily infested soil, makes it more likely that resistance-breaking strains will establish and become dominant.

Therefore, it is important to deploy integrated pest management techniques to tackle clubroot and sustain varietal resistance.

The generally mild conditions over the 2021/22 winter and periods of wet weather may have increased clubroot risk in some crops.

It is important to assess crops for disease symptoms and record infected areas to assist rotational planning, says Dr Gosling.

Known history This season ADAS is reporting the presence of clubroot in fields with a known history and which are grown in short rotation, with increasing reports of the disease compared to last season.

Relatively wet and mild ground conditions throughout winter have been favourable for the development of the disease.

ADAS adds: Below ground symptoms are often not detected in winter and are only identified when other symptoms such as stunting become obvious when the crop starts to extend and can occur in patches or across the whole field.

After the mild winter, there are reports this year of clubroot, says ProCam head of crop production Mike Thornton.

Theres little that can be done against it at this point, although applying a calcium carbonate dressing to raise soil pH is an option and its important to look after the crop nutritionally because nutrient uptake via the deformed roots of infected plants will be restricted, he says.

For the future, he advises growers to note down fields that are affected.

Infected fields Doing this helps with longer term management including rotation planning, liming of infected fields, choosing more tolerant varieties and delaying WOSR drilling, he adds.