Disease survey highlights

Brown rust risk map
Disease survey highlights
Defra Oilseed Rape Survey 2017 / 2018: Highlights


Introduction
Eighty crops were surveyed in the 2017/2018 season with the number of crops monitored in each region being proportional to the area of oilseed rape grown. Crops were assessed on three occasions during the season: in the autumn during leaf production, in the spring during stem extension and in the summer at pod ripening.


Headlines

  • With the exception of light leaf spot, pod disease levels were generally lower in 2018 compared to 2017 despite foliar disease levels earlier in the season in autumn and spring being generally higher than in the previous year.
  • On the stems, levels of phoma canker were the highest since 2011.
  • Light leaf spot was the most dominant disease on oilseed rape in summer, slightly more prevalent than in 2017 and slightly higher than the historical average
  • The incidence of Sclerotinia stem rot was lower than in the previous year and was only recorded in two regions in the survey (the East and the Midlands & West).
  • Disease severity was higher at each assessment timing compared with the previous year.
  • Verticillium wilt was found in all regions in the survey this summer, except for the North and South East but overall incidence was significantly lower than in 2017 and the second lowest in the eight years that this disease has been monitored..
  • Surveyed crops received an average of 3.2 fungicide applications (the same as in 2017) and 2.2 insecticide applications (a reduction of 0.3 applications per crop).

Incidence of foliar diseases


Light leaf spot

Nationally, the incidence of light leaf spot was lower than in 2017 during the autumn and spring but higher at the summer assessment timings (Figure 1).

Light leaf spot in the autumn was only recorded on plants in the North region, where there were 18% crops and 5% plants affected. This gave a national mean of 4% crops and 1% plants affected, which was lower than in the previous autumn when 10% crops and 1% plants were affected. The incidence of light leaf spot this autumn was also lower than the long-term mean (2005/6-2014/15) of 9% crops and 1% plants affected.

In the spring, light leaf spot affected 64% crops and 15% plants, which was just slightly lower than last year (68% crops and 15% plants affected). The disease affected crops in all regions, with the North showing the highest incidence (88% crops and 40% plants 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 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).

Figure 1. Percentage of plants affected by light leaf spot (autumn and spring samples refer to leaves and summer to pods) ** Please note – no data available for autumn and spring 2016 **.

Phoma

Nationally, the incidence of Phoma was higher in the autumn but lower in the spring and summer than in the previous year (Figure 2).

Ninety-one per cent of crops were affected with Phoma leaf spot in the autumn, this was higher than the previous autumn when 79% crops were affected. Within crops, 40% plants were affected with phoma leaf spot which was also higher than in the previous year when 27% plants were affected. Phoma leaf spot was recorded in all regions in the survey, with the highest incidence in the North and South East (where 100% crops were affected in both regions) whereas incidence across the rest of the country was uniformly consistent at 87% of crops affected.

Phoma leaf spot affected 90% crops and 32% plants, which was slightly lower than last year when 91% crops and 36% plants were affected. Phoma leaf spot affected crops in all regions with the Midlands & West and South West having 100% crops affected. The region with the lowest incidence was the South East, with 80% crops affected.

Phoma pod spot affected 4% crops and 0.1% plants in the summer; this was lower than in the previous summer when it affected 13% crops and 1% plants. The incidence of phoma pod spot was also lower than the long term mean (2007-2016) of 13% crops and 1% plants affected. Phoma pod spot was recorded in all regions of the survey, with the exception of the South East. The region with the highest incidence of phoma pod spot was the South West, with 13% crops and 0.5% plants affected.

Figure 2. Percentage of plants affected by Phoma leaf and pod spot ** Please note – no data available for autumn and spring 2016 **

Dark leaf and pod spot

Nationally, the incidence of dark leaf spot was higher in the autumn and spring, but lower in the summer compared with last year (Figure 3).

Alternaria leaf spot affected 65% crops and 10% plants in the autumn of 2017/8, this was markedly higher than in the previous autumn when 44% crops and 4% plants were affected. Alternaria leaf spot was recorded in all regions in the survey. Regionally, the incidence of Alternaria leaf spot was highest in the South East with 90% crops and 18% plants affected and lowest in the North with 47% crops and 4% plants affected.

Nationally, Alternaria leaf spot affected 30% crops and 2% plants this spring, which was higher than last year when 20% crops and 2% plants were affected. Alternaria leaf spot was recorded on crops in all regions in the survey, with the exception of the North. The South West had the highest incidence with 63% crops and 8% plants affected.

Alternaria pod spot was recorded on 32% crops and 4% plants, a lower incidence than last summer 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. 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.

Figure 3. Percentage of plants affected by dark leaf and pod spot (autumn and spring samples refer to leaves and summer to pods) ** Please note – no data available for autumn and spring 2016 **

Downy mildew

Nationally, the incidence of downy mildew was very similar to the previous autumn and spring but lower than last year in the summer (Figure 4).

Downy mildew affected 28% crops and 5% plants this autumn which was very similar to the previous year when 29% crops and 4% plants were affected. Downy mildew was recorded in crops in all regions of the survey except the South West. Regionally, the highest incidence was in the South East where 40% crops were affected.

Downy mildew affected 16% crops and 1% plants this spring. This was very similar to last year when 17% crops and 2% plants were affected. The disease was recorded in all regions in the survey this spring with the exception of the South West. The region with the highest incidence was the South East with 30% crops and 1.6% plants 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.

Figure 4. Percentage of plants affected by downy mildew (autumn and spring samples refer to leaves and summer to pods) ** Please note – no data available for autumn and spring 2016 **.
Incidence of stem diseases


The two most common stem diseases in the summer were light leaf spot (affecting 93% crops and 44% stems) and phoma canker (affecting 89% of crops and 29% stems) (Figure 5).

The incidence of light leaf spot on the stems (93% crops and 44% stems affected) was higher than last summer when 83% crops and 36% stems were affected and also higher the long-term mean (2007-2016) of 84% crops and 45% stems affected). Light leaf spot was recorded on the stems of crops in all regions, with the highest in the South West (100% crops and 53% stems affected) and the lowest in the East (90% crops and 33% stems affected).

This summer, 89% of crops and 29% stems were affected by phoma canker. This was higher than last year when 78% crops and 21% stems were affected. Levels were also higher than the long-term mean (2007-2016) of 85% crops and 26% stems affected. The highest incidence was in the East, where 94% crops were affected and lowest in the Midlands & West, where 80% crops were affected.

Powdery mildew (the third most common stem disease this summer) affected 36% crops and 7% stems this summer, this was lower than in the previous year when 46% crops and 10% stems were affected. Powdery mildew was recorded on stems in all regions of the survey, with the South East being the region with the highest incidence with 60% crops and 11% stems affected. The lowest was the North with 12% crops and 1% stems affected.

Verticillium wilt affected 5% of crops and 0.4% of stems in the survey. This was markedly lower than last year when 20% crops and 2% stems 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.

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.
Alternaria was also recorded on the stems in the summer, affecting 3% crops and 0.1% stems. This was lower than last year when 12% crops and 1% stems were affected, and also lower than the long-term mean (2007-2016) of 9% crops and 1% stems affected. Alternaria was only recorded in the East and South West regions of the survey this summer, with 3% crops and 0.3% stems and 13% crops and 0.5% stems affected respectively.

Botrytis was not recorded on the stems of any surveyed crops this summer for the first time since the survey began in 1987.

Figure 5. Percentage of stems affected at pod ripening
Disease severity


In the autumn, a mean of 0.3% total leaf area was affected by disease (Figure 6). This was much higher than in the previous year when there was 0.06% total leaf area affected. Regionally, the Midlands & West had the highest mean disease severity with 0.5% total leaf area affected by disease. The region with the lowest mean disease severity was the North, with 0.2% total leaf area affected.

Nationally, phoma leaf spot was the most severe disease in the autumn, affecting 0.14% total leaf area. The second most severe disease in the autumn was powdery mildew, which affected 0.1% total leaf area.

In the spring, a mean of 0.6% total leaf area was affected by disease and this was higher than last year, when 0.05% total leaf area was affected by disease. The region with the highest mean disease severity was the North having 1.9% total leaf area affected by disease, and the lowest was in the South East, with only 0.04% total leaf area affected.

Nationally, light leaf spot was the most severe disease in the spring, affecting 0.5% of the total leaf area, followed by Phoma leaf spot which affected 0.1% total leaf area.

In the summer, a mean of 0.2% total pod area was affected by disease, which was also higher than last year when 0.1% total pod area was affected by disease. The region with the highest mean disease severity on the pods was the South East, where 0.7% total pod area was affected and the region with the lowest was the East, with 0.1% total pod area was affected.

Nationally, the most severe pod diseases was light leaf spot which affected 0.1% total leaf area.

Figure 6. Percentage area affected by the sum of all diseases (autumn and spring samples refer to leaves and summer to pods) ** Please note – no data available for autumn and spring 2016 **.
Fungicide applications

Surveyed crops received an average of 3.2 fungicide applications; this is the same proportion as last year and very similar to both 2016 (3.1) and 2015 (3.2). The South East was the region where crops received the highest number of treatments, crops here received an average of 3.4 fungicide applications and the region with the lowest was the North, where crops received an average of 2.9 fungicide applications.

In the autumn/winter, 88% crops received a fungicide treatment; this was higher than last year when 85% crops were treated at this time (Figure 7).

In the spring (GS 1-3), 89% crops received a fungicide treatment. This was also higher than last year (88% crops treated) and this was the highest proportion of spring treated crops since 2012 when 91% crops were treated with fungicide in the spring.

At flowering, 82% crops were treated with fungicide. This was lower than last year when 89% crops were treated with a fungicide at this time and the lowest proportion of crops treated at flowering since 2013 (79% crops). Treatments applied at this time are generally targeted for control of Sclerotinia, higher levels of use indicate growers perceive an increased risk of this disease.

The proportion of crops that received a post flowering fungicide treatment was 8%, which higher than last year, when 7% crops were treated at this time and was also the highest proportion of crops treated post-flowering since 2012, when 10% crops were treated.

Figure 7. Proportion of crops treated with fungicide in the four main periods
Insecticide applications

Crops in the survey received an average of 2.2 insecticide treatments. This was slightly lower than last years average of 2.5 treatments. The East was the region where crops received the highest number of treatments, with crops receiving an average of 2.7 insecticide applications and the region with the lowest was the South East, where crops received an average of 1.6 insecticide treatments. There were 8 crops in the survey that did not receive any insecticide treatment, this was a higher proportion than last year when there were 3 crops not treated, and the highest proportion of crops since 2014 when 19 crops were untreated.

In the autumn/winter, 84% crops received an insecticide treatment. This was lower than last year when 91% crops were treated at this timing and it was also the lowest proportion of crops receiving an insecticide treatment at this growth stage since 2014 when 66% crops were treated with an insecticide. (Figure 8).

The proportion of crops receiving an insecticide treatment in the spring was 12.0%, although this was a higher proportion of crops treated in the spring than last year (4% crops), it is still lower than those seen in recent years previous to 2017 and was the second lowest proportion of crops treated with insecticide during the spring since 1992, when 2.4% crops were treated.

During flowering, 20% crops were treated with an insecticide; this was lower than in the previous year when there were 28% crops which received an insecticide application during flowering and was the lowest proportion of crops treated with an insecticide at flowering since 2003, when 16.5% crops were treated at flowering.

The proportion of crops treated with insecticide post-flowering was 1%, this was lower than last year when 4% crops were treated at this time but similar to 2016 when there was also 1% crops treated post flowering.

Figure 8. Proportion of crops treated with insecticide in the four main periods