Light leaf spot

Pathogen

Pyrenopeziza brassicae

Hosts

Light leaf spot (LLS) is an important disease of winter oilseed rape in Germany, France, Poland and the UK. In Scotland and parts of Northern England, LLS (Pyrenopeziza brassicae, anamorph, Cylindrosporium concentricum) also affects vegetable brassica crops. This web page focuses on the disease in oilseed rape.

Symptoms

Symptoms are uncommon until late autumn or early winter. On green leaf tissue, very small white spots (spore masses) are visible. These develop into discrete lesions with pink-tinged centres. In the most severe cases, whole leaves can be killed. During the winter, severely affected plants may also be killed. Surviving plants can become stunted or distorted. When disease spreads to stems and lateral branches, elongated, fawn lesions, surrounded by black speckling, can appear. Under humid conditions, white spore masses can also form. Under conducive conditions, the disease can spread to and distort pods. These may turn brown and shatter prematurely.

Life cycle

The fungus enters its sexual stage on crop debris. Small structures (less than 1mm in size), called apothecia, develop and release airborne spores (ascospores). These can be blown for several miles and infect young oilseed rape crops in the autumn. On living plant tissue, asexual spores (conidiospores) are produced. On leaves, very small white spots (spore masses) can be seen. Spores are spread by rain and, thus, can only move relatively small distances. Consequently, the disease tends to develop in patches. As leaves senesce during stem extension, the resulting crop debris allows the pathogen to enter the sexual stage again. This can produce a further flush of wind-dispersed spores. Pathogens that produce more than one infection cycle per crop cycle, such as LLS, are called polycyclic diseases.

Importance

Historically, LLS was most serious in Scotland and the north of England. However, the importance has increased in recent years throughout England. LLS incidence has been very high across the UK over the last decade (2008–18).

Risk factors

  • Crop residues are a source of inoculum
  • The amount of stem and pod infection in the summer and summer temperatures are important factors in the risk to following crops
  • There is some evidence for transfer of LLS from the surface of contaminated seed. Therefore, the use of home-saved seed from heavily infected crops is not advised
  • Varieties with lower disease resistance ratings are more susceptible (especially those with a resistance rating under 6)
  • Late-emerging crops may be less severely affected than earlier sowings
  • Cold winters slow plant growth but the pathogen remains active. Severely affected plants may be killed during the winter and any surviving plants may become stunted or distorted.
  • Well-timed sprays can significantly reduce disease risk. Poor control will be observed in situations where crops are treated late (when the disease is well established)

Management

Varietal resistance

Disease ratings for light leaf spot are published within the AHDB Recommended Lists.

Light leaf spot forecast

Issued each autumn by AHDB, the preliminary LLS forecast shows the proportion of the oilseed rape crop (disease resistance rating of 5) estimated to have more than 25% of plants affected by LLS in the spring for the current season.

The forecast uses previous season pod incidence data and deviation from the 30-year mean summer (July and August) temperature. It also uses historic average winter rainfall data. In spring, the forecast is updated to reflect deviation in actual winter rainfall data from the 30-year mean.

The forecasts should only be used as a guide to risk levels. As local risk varies, crops should be inspected regularly on a field-by-field basis. To provide the strongest risk assessment, plant samples should be put in polythene bags and kept at 10 to 15 degrees C for around five days to bring out symptoms.

Chemical control

Where LLS occurs regularly and phoma sprays have not given adequate control, an autumn fungicide should be considered in November or early December. There is no treatment threshold before stem extension. An application of a suitable fungicide should be considered as soon as LLS is detected in the field. Varieties with LLS resistance ratings below 6 should be the priority for treatment.

A second spray may be required in February or March, if LLS symptoms are found. Unlike phoma, LLS is still a threat to yield in the spring and losses could be 30% if all plants are affected at early stem extension. At early stem extension, 15 per cent of plants affected is associated with a five per cent yield loss.

Triazole fungicides are available for LLS control. However, there have been reports of decreased azole sensitivity in UK LLS populations. Although this has not been confirmed and the impact on field efficacy not quantified, it is a cause for concern. Guidance published by the Fungicide Resistance Action Group should be followed.

Non-azoles are available for LLS control. The alternative modes of action are included in AHDB fungicide performance trials and show good activity against LLS. Therefore, It is recommended that a range of products that represent different modes of action groups are used throughout the fungicide programme. This should include timings where LLS is not the main target but is likely to be present. Due to continuous changes in fungicide availability and efficacy, people should read the detailed information on AHDB’s fungicide performance pages for the latest situation.

In terms of product dose, fungicide performance trials show that optimum dose is highly site and situation specific and dependant on variety resistance rating, crop growth and disease pressure.

Product choice will also be influenced by requirements for phoma activity and/or plant growth regulation of large plants and label restrictions.

Further information

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