Potential to include verticillium wilt disease ratings in Recommended List

NIAB, which led the study, carried out inoculated and naturally infected trials over three seasons. These found highly consistent differences between varieties in the level of symptom expression.

Although no varieties were found to be completely resistant, several varieties showed consistently low infection levels. Associated with fewer fungal spores, these varieties could play an important role in reducing the amount of disease returned to the soil, says AHDB.

Both RL and Candidate List varieties were tested, some of which are no longer listed.

Based on a mean verticillium disease index (0–100 scale), the most resistant variety was Catana, which had a score of 26.2. Harper was the most susceptible variety with a score of 60.0. This translated to a provisional disease resistance rating of 7.6 and 2.1, respectively (1–9 scale).

Three varieties with high levels of partial resistance and three relatively susceptible varieties were included in four trials to investigate yield effects.

Although these were variable, susceptible varieties showed significantly greater yield depression in one trial, and the more resistant varieties showed little or no effect throughout all trials.

Oil content was not significantly affected by the presence of infection. Surprisingly, thousand seed weight was also unaffected in the trial that showed significant yield depression.

This indicates that components of yield, such as pod number or seeds per pod were affected, rather than seed size, says AHDB.

There was an indication that some varieties may be tolerant to infection, having a small and non-significant yield penalty, despite high infection levels.

The RL team will now look at how to implement a reliable ratings system, using the research results.