Summer disease assessment

Defra has carried out annual stratified surveys of diseases of commercially-managed winter oilseed rape since 1987. Between 80 and 100 randomly selected crops are assessed for symptoms of disease at three growth stages during the cropping year, at mid leaf production (December), stem extension (March) and pod ripening (June) and information about the agronomic practices associated with each crop are collected. The following report summarises the results of the survey carried out at pod ripening in 2018.

Stems and pods of twenty-five plants selected at random from each surveyed crop were assessed visually at pod ripening for symptoms of disease.

This summer, 89% of crops were affected by phoma canker. This was a higher proportion than last year, when 78% crops were affected and was the highest proportion of crops affected since 2011 (96%). Levels of infection within crops were also higher than last year with 29% stems affected compared with 21% stems in 2017. Canker incidence this summer was also higher than the long-term mean (2007-2016) of 85% crops and 26% stems affected. The proportion of crops affected by total phoma (phoma canker and phoma stem lesions) was also higher this year, with 93% crops and 33% stems affected compared with 84% crops and 25% stems affected last summer and 91% crops and 31% stems affected in the long-term. The highest incidence of phoma canker occurred in the East, where 94% crops were affected and the region with the lowest was the Midlands & West with 80% crops affected.

Light leaf spot incidence in 2018 was slightly higher than last year. On the pods, the disease affected 62% crops and 17% plants affected compared with 61% crops and 14% plants affected in 2017. The percentage of crops affected was slightly higher than the long-term mean (2007-2016) of 59% crops affected, however, within crops, the incidence was slightly lower than the long-term mean of 22% plants affected. On the stems, light leaf spot affected 93% crops and 44% stems which was higher than last year (83% crops and 36% stems affected). The proportion of crops affected was higher than the long-term mean (84% crops affected) but incidence within crops was lower than the long-term (45%). Regionally, the highest incidence of light leaf spot on the stems was in the South West where 100% crops were affected and the lowest in the South East (90% crops affected) and within crops, incidence ranged from 61% stems affected in the Midlands & West to 33% stems affected in the East. On the pods, the incidence ranged from 80% crops affected in the Midlands & West to 40% crops affected in the South East. Within these crops, the highest incidence of light leaf spot on the pods was in the Midlands & West (32% of plants affected) and the lowest in the North (7% plants affected).

Levels of powdery mildew on the pods were lower than last year, with 6% crops and 2% plants affected compared with 14% crops and 2% plants in 2017, and also lower than the long-term mean (2007-2016) of 22% crops and 7% plants affected. On the stems, 36% crops and 7% stems were affected which was also lower than last year (46% crops and 10% stems affected). When compared to the long-term mean of 29% crops and 9% stems affected, the proportion of crops affected this year was higher, but within crops, the incidence of stems affected was lower. The region with the highest incidence of powdery mildew on the stems was the South East, with 60% crops and 11% stems affected and the lowest was the North, where 12% crops and 1% stems were affected. The South West was the region with the highest incidence of powdery mildew on the pods (25% crops affected) and the Midlands & West had the lowest incidence, as the disease was not recorded on any surveyed pods in this region.

Sclerotinia stem rot affected 5% crops and 0.5% stems this summer, which was slightly lower than last year (8% crops and 1% stems affected) and also lower than the long-term mean (2007-2016) of 13% crops and 2% stems affected. Regionally, the highest incidence of sclerotinia stem rot was the East where 10% crops were affected. Sclerotinia stem rot was not recorded on any surveyed crops in the North, South West and South East regions. Sclerotinia disease on the pods was not recorded on any surveyed crop this summer – lower than last year when 1% crops and 0.3% plants were affected, and also lower than the long-term mean of 0.5% crops and 0.05% plants affected. The last time that Sclerotinia was not recorded on any pods in the survey was 2014.

Alternaria pod spot was recorded on 32% crops and 4% plants, a lower incidence than last year when 49% crops and 8% plants were affected and also lower than the long-term mean (2007-2016) of 59% crops and 11% plants affected. On the stems, Alternaria affected 3% crops and 0.1% stems, which was lower than last year (12% crops and 1% stems affected) and also lower than the long-term mean of 9% crops and 1% stems affected. Alternaria pod spot affected crops in all regions in the survey except for the North. The East had the highest incidence with 48% crops affected. On the stems, Alternaria was only recorded on samples from the East (3% crops and 0.3% stems affected) and South West (13% crops and 1% stems affected) regions.

Botrytis on the pods was recorded at higher levels compared to last year, but lower than the long-term mean (2007-2016). This summer, 72% crops and 11% plants were affected compared with 67% crops and 11% plants affected in 2017 and 74% crops and 20% stems affected in the long-term. On the stems, Botrytis was not recorded on any surveyed crops for the first time since the survey began in 1987. On the pods, Botrytis was recorded in all regions of the survey, with the South East having the highest incidence (90% crops affected) and the North with the lowest (53% crops affected).

Verticillium wilt has been recorded in the survey since the summer of 2011. This summer, 5% crops and 0.4% plants were affected. This was lower than in the previous year when 20% crops and 2% plants were affected. Verticillium wilt was found in all regions in the survey, except for the North and South East. The region with the highest incidence was the Midlands & West where 13% crops and 1% stems were affected.

This summer, the only region where downy mildew was recorded on the pods in the survey was the North, where 6% crops and 1% plants were affected. This gave a national incidence of 1% crops and 0.1% plants affected, which was lower than last year when 8% crops and 0.4% plants were affected. This was also lower than the long-term mean (2007-2016) of 7% crops and 1% plants affected. Downy mildew was not recorded on any stems this summer, the last time downy mildew was recorded on the stems in the survey was in 2009.