AHDB’s newly improved phoma leaf spot forecast suggests the oilseed rape disease will develop relatively late this autumn.

The 2021 forecast has been refined, partly in response to industry concerns that the service had become disconnected from field realities, with the new model focusing on damp, rather than wet, conditions.

Originally developed in the early 2000s, the model uses temperature and rainfall information (15 July to 26 September) to simulate the development of Leptosphaeria maculans – a key pathogen responsible for phoma leaf spot and phoma stem canker.

Accounting for subsequent crop infection, the forecast predicts the starting week when 10% of oilseed rape plants could potentially show symptoms. This level of infection relates to a phoma treatment threshold for relatively susceptible varieties (disease ratings of 7 and below).

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Robert Saville, who manages disease research at AHDB, said: “Phoma spore maturation on stubbles is favoured by warm, wet weather. Until this year, the model used total rainfall data to help simulate disease development.  However, it is how damp, not how wet, stubbles are that matter. Once damp, further rain on that day does not progress disease development.

“Heavy rainfall events, which have become more frequent, distorted the model’s predictions – with it suggesting that disease is more advanced than it is. In fact, AHDB research, based on observation and consultation, found that the model predicted threshold breaches around two weeks earlier than seen in the field. Although better than the late prediction, it was clear we needed to improve the model.”

To account for dampness, the model now only considers daily rainfall events up to 10 mm and total rainfall up to 200 mm, with additional rainfall having no influence. As a result of the changes, the refined model (compared to the previous one) tends to predict later start dates for threshold breaches.

Robert said: “It is critical that any forecast does not cry wolf if the industry is to have confidence in it. The tendency for the historic forecast to predict relatively early-risk years was problematic. Naturally, people are more nervous of elevated early risks, as the pathogen moves more easily within smaller plants.”

The 2021 forecast suggests that most relatively susceptible winter oilseed rape crops are unlikely to reach the 10% threshold until after the middle of October. However, there are some exceptions – most notably at some sites on Wales’ southern coast, which are predicted to hit the threshold in early October. Some sites, especially towards the UK’s north-eastern coastline, are not forecast to hit the threshold until November.

Another development is the number of sites considered by the model. Historically, the phoma forecast covered around 30 sites across England and Scotland. However, the revised model forecasts risk at hundreds of sites across England, Scotland and Wales, which helps to reveal variation. The less regional variation, the more reliable the forecast is.

The new forecast also presents the equivalent forecasts for 2019 and 2020, revealing that 2021 is shaping up to be a relatively late year for phoma. Compared with the long-term average, it has been relatively dry, with just a few wetter spells – especially at the start of August.

To access the forecast and phoma management information, visit: ahdb.org.uk/phoma-leaf-spot-forecast