Sclerotinia is potentially one of the biggest yield robbing diseases in winter oilseed rape and in this article, Geoff Hailstone, technical business manager for Scotland at UPL discusses controlling this disease and protecting yield by preventing infection in flowering OSR.

It's a disease that can go unnoticed until July. The first sign of disease is often empty pods on bleached stems and if they are cracked open you can find white fungal growth and black fungal bodies inside.

But now is the time to think about how to prevent this yield robbing situation before the disease takes hold. As temperatures rise, with soils still moist from the wet winter, ground conditions favour the emergence of small pale mushroom-like fruiting bodies, which trigger the start of new Sclerotinia infections.

These emerge from black sclerotia, resting bodies which may have been dormant in the soil for many years. The fruiting bodies release spores which are carried in the wind to the upper canopy of the rape crop, where they germinate, feed on the flowers and pollen, then drop with the petals infecting the stems below.

The disease completes its life cycle by growing new sclerotia inside the stem, blocking the flow of water and nutrients to the developing pods and then dropping back to the soil at harvest to survive as resting bodies through the rotation – potentially putting other crops such as potatoes, peas, beans, carrots and onions at risk.

Two key factors need to come together for the disease to cause potential yield – the presence of sclerotia in the soil and favourable weather conditions. The perfect storm is a history of problems, or a rotation with other host crops such as potatoes, and weather giving greater than 80% relative humidity and warmer than 7°C for 23 consecutive hours.

In a high disease year, a robust fungicide programme can deliver a yield response as high as 2 t/ha where 100% control is achieved, and 1 t/ha if control is at just 50%. The good news is that OSR crop also gives the best opportunity in the rotation to lower the number of sclerotia surviving in the soil.

In theory, a single spray could give sufficient control if timed when the main raceme is at mid-flower and just before an infection period. However, in most cases fungicidal activity is unlikely to persist for more than three weeks, whilst the flowering period can extend much longer in cool weather, so even a perfectly timed full fungicide application is likely to run out of steam towards the end of flowering, just when petal fall and disease transmission is imminent.

Oilseed rape fungicides are all protectants so need to be applied before infection takes place. Sclerotinia risk reports and alerts based on sclerotia numbers, inoculum on petals and the weather can help in predicting the threat, but the best results still come from a preventative application before the risk charts show amber or red.

A two-spray programme is normal practice in high-risk areas like Scotland – particularly in the North-east of the country – but also anywhere that has a history of sclerotinia and the possibility of wet weather at flowering.

When choosing fungicides, it is important to manage disease resistance as well as controlling the pathogen. Strains of sclerotinia with decreased sensitivity to SDHIs have been detected in France for a number of years, demonstrating the importance of following resistance management guidelines and using fungicides with different modes of action.

Evito T is in only its second year of use following registration by UPL last spring. It is a combination of fluoxastrobin – an advanced strobilurin (Qol) – and tebuconazole. Both have activity on sclerotinia, with the combined formulation working better than either active used alone. Fluoxastrobin is a ‘next generation’ strobulurin.

It is fully systemic and has excellent rainfastness, with as much as 85% of the fungicide present only 15 minutes after rainfall, allowing more windows for application in the showery weather conducive to disease. Its systemicity also ensures even distribution through the plant and movement of the chemistry into the stems – crucial given the route of infection.

The product can be applied from early to late flowering (but not before May 1) with the best results coming from early to mid-flower. The formulation at the full dose of 0.8 l/ha gives best control and longer persistence.

Even at this late stage, a final consideration is light leaf spot (LLS). Levels of this disease vary around the country but are high in areas where earlier sprays were missed because of adverse weather and ground conditions.

More resistant varieties, such as Alizze, Aspire, Aurelia, Barbados and Nikita, may be getting their first spray at yellow bud or early flowering, whereas those more susceptible, like Anastasia, Crome and Crocodile, may need LLS control topping up.

In both scenarios, protecting the pods and upper canopy from LLS is something which should be considered. The tebuconazole in Evito T will give a level of efficacy against this disease when sprayed at early flowering.


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