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Light leaf spot

Course: Oilseed rape diseases | Last Updates: 12th October 2015

 
Julie Smith
Adas Rosemaund, plant pathologist
Biography >>
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Light leaf spot is a threat to oilseed rape crops across the UK with its ability to stunt plants and cause premature pod ripening, halving yields where disease pressure is severe.

Recent epidemics have caught growers out, not least because many popular varieties have relatively poor natural resistance to the pathogen coupled with the fact that control is a challenge. Typically, about 0.75t/ha has been lost in each of the past two seasons.

The pathogen

Like septoria of wheat, light leaf spot (pyrenopeziza brassicae) is polycyclic, meaning that several infection cycles can occur in a season. The fungus survives between crops on infected crop debris and produces fruiting bodies called apothecia. Wind dispersed ascospores are released from mature apothecia during the autumn and initiate an epidemic in newly emerged oilseed rape crops.

In such a way, new fields are infected. Evidence suggests that ascospores may be dispersed several miles from the initial source, and it is common to see patchy distributions across fields.

Light leaf spot is usually asymptomless during the autumn and the first symptoms to appear on the leaves, often during late November/early December, may easily be missed. Conidia (asexual spores) are produced in these leaf lesions and the pathogen is subsequently spread by rainsplash. The distance of spore dispersal via rainsplash is limited and new infections develop on neighbouring plants, close to the initial foci.

Image of a leaf with light leaf spot
The first symptoms on leaves often appear in late November/early December

The latent period – the phase after infection by spores and before symptoms show – is typically anything from 14 to 30 days. At 4C it tends to be nearer 30 days and at 15C, the former.
Above 20C, development of the pathogen is inhibited. 

Vertical spread of the disease occurs via rainsplash and conidia may be transferred upwards onto newly emerging leaves, stems and pods.

The growing points can become infected during the rosette stage, leading to stunting and distortion.

Plants affected by disease in the autumn are more susceptible to frost damage, and extensive plant loss may occur during a hard winter if the crop is severely infected.

A second flush of ascospores may be produced from apothecia that form on dead leaves in the late spring, which can contribute to disease development in the upper canopy and pods.

Symptoms

Symptoms of spotting – difficult to see on wet leaves – appear from late autumn. Last season they showed from January in England and from March in Scotland.

Light leaf spot might be confused with other diseases, fertiliser scorch or frost damage. The pathogen is quickly and easily diagnosed if leaves are incubated by placing them in polythene bags and storing them at room temperature for three to four days.

Pale green lesions appear during November/December. These later take on a mealy appearance without a defined edge. Tiny spore droplets (acervuli) are produced, usually around the outer edge of the lesion, often on both leaf surfaces. The spore droplets are white and look a little like grains of salt.

Severe infections can cause leaves to become twisted and distorted. Stems and pods may also become infected and the resulting lesions appear brown or purplish with associated black speckling around the edge.

Stem showing signs of light leaf spot
Stems can become infected with resulting lesions appearing brown or purplish with associated black speckling around the edge. (Picture: Adas)

Not restricted to the base of the stem, lesions may reach several centimetres in length and in severe cases they split vertically along their length. Pod symptoms range from fine speckling to stunted brown patches that ripen prematurely.

Incidence

During the past two seasons, Fera’s CropMonitor survey has detected light leaf spot in more than 85% of English oilseed rape crops; the average over the past decade is just 54%. Last year, 44% of plants within affected fields were infected, compared with an average of 19% over the previous decade.

 

Although light leaf spot has historically been more severe in the North, the disease is now as prevalent in oilseed rape crops in the South. It is unclear why, though theories abound.

The weather in recent seasons has been conducive, current varieties on the AHDB’s East-West Recommended List are relatively susceptible, and typically growers have not been able to spray at the optimum timings. There are also concerns over variable sensitivity to azole fungicides, though there is no evidence of fungicide breakdown in the field.

Many of the varieties grown in the South have good genetic resistance to phoma stem canker, but not necessarily to light leaf spot. Growers may unintentionally have been ignoring light leaf spot as they target phoma leaf spot in the autumn.

The first phoma spray should protect against light leaf spot, but offers a limited period of protection, so unless there is a follow-up spray, the disease can develop.

Risk assessment

The AHDB light leaf spot forecast, available to growers via Rothamsted Research’s website, is useful. It has a traffic light warning system based on a calculated plant threshold of 25% plants infected within a field. The model is based primarily on previous light leaf spot infection and winter rain days.

The forecast doesn’t yet have an early warning component marking the onset of an epidemic, which is important to prevent the disease taking a hold. Trials investigating early detection techniques will help scientists to develop this feature.

Useful in-season regional information about light leaf spot levels can also be found on the CropMonitor website.

Control

Combining genetic resistance with well-timed fungicide applications is the best approach to controlling light leaf spot. Avoid varieties with ratings of below 6 where the risk is considered high. Also, ploughing infected oilseed rape stubbles and delaying sowing may help.

Unfortunately, varietal resistance is limited. The AHDB Recommended List 2015-16 for the East/West has just two varieties with a rating of 7 and the rest score 6 or below.

The varieties are Harness and a clubroot resistant restored hybrid called Cracker, but reports from Scotland suggest that light leaf spot resistance in Cracker might be breaking down in some regions. However, there are some potentially good candidate varieties coming through the system with ratings of 8.

Chemical control relies mainly on azole fungicides, which also control phoma. There is no threshold for treatment of light leaf spot– general advice is to spray as soon as you see symptoms, to protect newly expanding leaf tissue and growing points.

Oilseed rape being sprayed in the autumn.
A key factor in the development of light leaf spot is poor fungicide timing.

There is a good yield response to two well-timed protectant treatments in AHDB-funded fungicide performance trials. Ideally, target the disease in November, but monitor and treat as and when you consider it necessary, depending on the risk, the weather and symptom development.

A key factor in disease development is poor fungicide timing. Treatments cannot eradicate lesions and offer only around three weeks’ protection. Do not wait until stem extension to apply the second application. You need to monitor crops regularly from January onwards. A further application may be required to protect the pods if the disease is still active at flowering.

Use a fungicide based on either prothioconazole or tebuconazole, or a mixture of the two.

Straight prothioconazole product, Proline, has performed best in Scotland over many seasons, whereas the tebuconazole mix, Prosaro, often has a slight yield advantage in England, thought to be linked to its growth regulatory activity. Similarly, applying straight tebuconazole (such as Orius) or tebuconazole with prochloraz (such as Orius P) can lead to a yield benefit in English crops due to coincidental regulation of large canopies.

There is no evidence that full label rates are statistically better than half rates in terms of disease control under moderate disease pressure where timing is optimal.

Under high disease pressure or to aid canopy management, three-quarter to full rates could be appropriate.

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