Verticillium stem stripe: Reinforced resistance results released

Variation in resistance to verticillium stem stripe in oilseed rape varieties has been relatively consistent across Recommended Lists (RL) trials, according to the latest disease data. AHDB Senior Crop Protection Scientist Catherine Harries explains.

AHDB released the first RL data for verticillium stem stripe in 2023. The new data covers three harvest 2024 trials, boosting the total number of verticillium trials to nine (across harvests 2022, 2023 and 2024).

With useful variation identified, the resistance data could help farmers manage this soilborne disease.

The next edition of the RL (2025/26, due online on 2 December 2024) will include updated disease ratings based on three categories:

• Moderately resistant (MR)
• Intermediate (I)
• Susceptible (S)

The selected trial sites are judged to have a relatively uniform distribution of the disease, with artificial inoculation used (as appropriate) to help ensure this.

All sites are in Cambridgeshire – one of the first UK counties to be severely impacted by the disease.

Each site features four replicates of each variety to minimise site variation effects.

Trial management is like the main oilseed rape trial series, with an exception: fungicide applications are limited to a single autumn treatment targeted at stem canker.

Additionally, the approach is destructive, so plots are not harvested (plants are pulled up early as part of the disease assessment process).

Crops are carefully monitored to determine the optimum assessment timing – too early or too late would potentially underestimate symptom incidence and severity.

In each plot, 30 stems are assessed for external (shredding and black microsclerotia) and internal (grey discolouration, revealed by scraping stems) symptoms, with the percentage of the stem circumference affected recorded.

The data is used to calculate a verticillium stem stripe index: 0 to 100, where 0 represents no symptoms on any stem and 100 represents all stems severely affected or dead.

Due to the shifting nature of varieties tested in RL trials, relatively few varieties feature in all nine trials (the number of data points ranges from 3 to 9).

Across the trials, variety performance is relatively consistent.

Based on a three-year mean, the most resistant variety had verticillium disease index rating of 9.7, with the most susceptible variety scoring 43.3 – this variety (Quartz) is included as a susceptible control (which helps us validate the data).

The latest data has an LSD (least significant difference) of 11.62.

Only differences between variety means that are larger than the LSD are statistically significant at the 5% level – meaning the result is only likely to occur by chance fewer than 1 in 20 times.

This means that many varieties have significantly lower levels of disease than more susceptible varieties.

Such varieties may help farmers manage long-term disease pressures, as they are associated with fewer fungal microsclerotia that form on stem tissue and plant debris. This can help reduce pathogen pressures in subsequent crops.

However, resistance does not infer tolerance: some susceptible varieties may yield relatively well, even under relatively high disease pressures.

Access further information via ahdb.org.uk/verticillium-stem-stripe