Extended OSR disease exposure possible, growers warned

Light leaf spot.

What this season has shown us is the extremes with our current weather and a cold spell shouldn’t be ruled out. As a result, growers are being advised to stay vigilant as should this occur crops could be exposed to disease for longer.

In Aberdeenshire, many farmers took the opportunity of early August rain and drilled at the start of the month. So far this is paying off. With fair weather, warm soils and good seed beds, agronomist Ben Lowe says some crops reached the one true leaf stage in 10 days and look particularly healthy and green for this time of the season.

He believes these early-drilled crops have every possibility of ‘growing away’ from pest and disease threats, but Mr Lowe sounds a note of caution.  A cold spell could see growth grind to a halt and, with crops exposed for longer, disease pressure could build.

In his area, light leaf spot (LLS) is the key threat – although a ‘southern summer’ could lead to an increased phoma threat. Mr Lowe has used Bayer’s Spotcheck service in the past, and is likely to do  so again this season. It is worth noting it has recorded the disease in the most northerly of Scottish regions.

He appreciates that, if crops grow away from disease, it does bring the question of an autumn fungicide into call; but his view is, with the potential impact of LLS, the price of rapeseed and that growers are likely to be making a pass with a stem weevil treatment or Kerb (propyzamide) the inclusion of a fungicide is added peace of mind. “Currently there is plenty of potential in August-drilled crops and rapeseed prices are likely to remain buoyant. Even a small uplift in yield is likely to pay handsomely.”

Fungicide options

There are number of autumn fungicide options but he cautions not all are effective against both LLS and phoma.

The most potent LLS options are either bixafen or fluopyram with prothioconazole, which is also a potent combination against Phoma too. It is an autumn option for Mr Lowe if disease pressure is particularly high. Otherwise, Proline (prothioconazole), Savanna (tebuconazole) or a combination of the two are more likely.

At flowering, Mr Lowe is almost certain to go down the bixafen or fluopyram route plus prothioconazole to tackle sclerotinia.

That disease control strategy is unlikely to change for later drilled crops, although Mr Lowe will make a call nearer the time. “We could have two very different crops this autumn, but again the rapeseed price means they are worth looking after,” he notes.

Twin challenge

A little further south in North Yorkshire, AICC agronomist Patrick Stephenson has the twin challenges of LLS and phoma. He considers LLS still the greater threat in most seasons but, like Mr Lowe, believes phoma could be as prevalent this year.

It is a rain-driven disease, and ADAS guidance is that phoma thresholds are typically reached after 20 rain events after drilling. And, with crop going into the ground in early August, Mr Stephenson suggests this could be reached earlier than the typical late October/early November period, so he advises growers to remain cautious.

He points out the disease favours warm weather; something many areas experienced at the start of September. Research has shown the time of peak airborne spore release (ascospores) from infected stubble is during warm and humid weather. These then land on the leaves of the newly-emerged crop. Some good news is that phoma is a monocyclic disease so a well-timed fungicide preventative application should protect plants.

Mr Stephenson, too, will treat crops with an autumn fungicide, and typically looks for actives that are effective against both diseases.

But he warns that timing is important as there are no real curative options. “You need to spray when thresholds are reached. No fungicide is fully curative.”

LLS can be more challenging. Mr Stephenson describes it as a bit of a mystery, and it can be hard to spot early on. “It can be very patchy in fields, and the spotting hard to see; sometimes the disease is almost symptomless.”

He too has used Bayer’s SpotCheck service to corroborate what he believes might be happening in a field. Again, he stresses the importance of timing. “You have to spray when symptoms appear. If it takes hold the damage is done.”

SpotCheck has another purpose for him, and that is justifying treatments. He is looking to use any fungicide as appropriately as possible, matching rates to reflect varietal resilience and disease severity.

Mr Stephenson also is mindful of resistance management. “Prothioconazole suits all OSR disease timings so it is used extensively in WOSR. The addition of bixafen (Aviator) is certainly welcome, bringing another mode of action,” he notes.

But he says disease management has to start with variety selection, and he is pleased that a number of the top yielding varieties also have good LLS or phoma resilience and sometimes both.

Of those, LG Aviron is rated as an 8 and 7 respectively, Auckland and Aurelia get a 7 for both diseases. DK Exstar is a little better, with 8 for LLS and phoma.

Hard to spot

ADAS is partnering Bayer with the SpotCheck service, and ADAS plant pathologist Philip Walker notes that the disease is very difficult to spot in the early stages of infection. “In its early phase, LLS expresses itself as small white dots scattered across the top or underside of the leaf, where the fungus is erupting from the leaf surface. They can look like tiny grains of sugar but without a hand lens or microscope they are difficult to spot.”

Milder winters is one reason why LLS has continued to be a problem in some seasons, despite reduced OSR area. ADAS research has shown that LLS spores need 17 hours of leaf wetness for leaf infection to occur, so the warmer and wetter it is the more quickly the disease will cycle. In ideal conditions the latent period could be as short as 17 days.

And whilst the winter months might not see optimum conditions, the disease will cycle in temperatures as low as 4°C, so it can remain active during the winter.