Oilseed rape suffered widespread Sclerotinia stem-rot in 2008

An ADAS funded survey recently carried out by CSL into commercially grown oilseed rape indicated that Sclerotinia stem-rot was more prevalent in 2008 than 2007.

Following the high incidence of Sclerotinia last year, expectations were that growers would be better prepared in 2008 and the disease would be better managed, so why wasn't this the case? An extended flowering period appears to have had a major impact on incidence of this disease this season.

The national incidence of Sclerotinia stem-rot was recorded at 6.2% stems affected, which was the highest recorded since the survey began in 1987 – incidence was also higher than 1991 (when 5.4% stems were affected), which was considered to be an epidemic year. National incidence in 2007 was also high, but this was largely due to the disease being particularly prevalent in the South West of the country. Although incidence was again high in the South West (the area which historically has seen the highest risk of Sclerotinia), the disease was also recorded at high incidence in all other regions in 2008.

Dr Peter Gladders of ADAS explains: 'Generally, the flowering period lasted almost 8 weeks, about 2 weeks longer than usually reported. The main infection occurred during wet weather in late May and early June, and sprays applied up to mid April had little effect on the final level of disease. Treatment in late April or early to mid May gave quite good control. However, two spray programmes appear to be required to protect crops over an extended flowering period'.

Light leaf spot was also more prevalent in 2008 compared to recent years and long term means, although severity on the stems and pods was low. Highest incidence was recorded in the South West and the North.

In contrast, survey data show a lower phoma canker incidence this year compared to recent years. The highest levels were seen in the Midlands & West and the Eastern regions, which historically are at highest risk. This season, phoma was less severe in the autumn, and peaked in late January 2008. Autumn fungicides gave good control of phoma leaf spot but even in untreated situations few plants have shown moderate or severe stem cankers.