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Phoma Agronomy Update 2017

Syngenta Field Technical Manager, James Southgate, looks at what has been happening on the company’s iOSR Innovation Centre, and the implications for Phoma control this season.

Key concerns:
•    Phoma has arrived far earlier this season
•    Wet weather continues to increase pressure
•    Crops are still small when being infected
•    Autumn workload can compromise spray timing
•    Growers’ concerns when LLS will hit?

Why has Phoma arrived so early this year?

Phoma relies on wet weather to prime cankers on last year's crop stems to produce spores. Wet weather through early August means that has already been reached in most areas this season, which is significantly earlier than normal, and certainly sooner than we have experienced for many years.

Once cankers are primed, spores are released after each rain event - and this year that has been very frequent. Spores are spread in the wind, and then germinate and infect the leaf, developing into the characteristic leaf spots.

Typically that takes two to three weeks, but it can be faster in warm weather, which we have seen with the earlier infections becoming visible as leaf spots far sooner this season.

What is the effect of seeing lesions on small crop leaves?

Smaller leaved crops need to be treated as a priority, since it takes less time for the leaf spot infection to spread down the leaf and petiole to reach the stem. Once infection reaches and gets into the stem, fungicide treatment has little or no effect in preventing the development of cankers.

This season the issue of smaller leaves is particularly serious, since most crops were still at a smaller growth stage when early infection hit. Furthermore, in the current warm conditions, disease development is faster and movement down the leaf is quicker.

 In some instances, growers had also delayed drilling, to get the maximum benefit from the excellent stale seedbed conditions that encouraged black-grass and other grass weeds to chit, but that has meant OSR seedlings are still small.

The better news is that growing conditions are still good in most areas, so OSR crops are developing quickly and larger leaves should be less vulnerable when it comes to repeat infection and secondary Phoma treatments.

Can we treat these small leaved crops?

As highlighted it is even more important to treat small leaved crops quickly, to stop disease development, but it is equally important not to check the plant’s growth at this stage.

Plover has been shown in trials to be especially suitable for early application as it is excellent in preventing Phoma developing and has been shown to have little or no effect on crop growth with treatment from two-true leaf stage onwards. The PLOVER label states from the four-true leaf stage.

That’s particularly beneficial where crops have been patchy to establish - which had been the case in some knobbly seedbeds this year – and there are some small seedlings alongside larger plants showing the effects of Phoma leaf spotting that need treatment.

Do we need curative control on the Phoma lesions?

Plover is exceptionally good at stopping Phoma from developing and preventing spores landing on the leaf. Best results are achieved with preventative applications, triggered by treatment thresholds when you know that levels of infection are in the crop.

The conventional treatment thresholds are when 10% of plants show signs of leaf spotting, but proactively that needs to be considered alongside risk factors to prioritise which crops to treat and when. 

If treatment has been delayed, and Phoma is rife and active, you can use Plover at the full rate of 0.5 l/ha and that will give greater curative activity. However, two applications at 0.25 l/ha, typically at a fortnightly interval, will give better preventative control and protection over the season.

When reassessing the crop to check the need for follow-up application, look for lesions that have the active black picnidia. Older lesions that have dried up after treatment tend to go necrotic brown.

What should be my priority now for Phoma treatment?

Firstly, crops that are showing the highest level of leaf spotting and have already reached the treatment threshold of 10% of plants infected. 

Secondly, target smaller leaved crops where disease will develop faster. You should also look at the variety resistance to Phoma, and treat fields with greater susceptibility first.

Also, look at weather conditions and temperatures; you need to act faster in warmer weather to stop rapid spread, but some frosty mornings will slow down development. 

Fields that are close to last year’s OSR crop will be exposed to greater spore pressure and risk. Phoma spores are very widespread, but the greatest concentration is within 500 meters of a previous OSR stubble – especially if that crop showed signs of Phoma cankers and there’s any stubble or trash on the surface.

How does varietal resistance help?

Varietal resistance generally reduces the speed of spread of infection on the leaf, and results in smaller cankers on the stem - with less effect on yield.

In practice that can allow greater flexibility in timings for autumn phoma fungicide applications, which can be valuable in busy work schedules and enable growers to target more susceptible crops first.

Furthermore, in a low pressure or late infection season, it may mean that a one-spray strategy could suffice on varieties with a resistance rating of six or more, when a variety with lower resistance would typically require two sprays. However, in an early infection season, such as we are experiencing this year, uncontrolled canker infections will have longer to develop and even resistant varieties could suffer greater losses unless leaf spots are treated.

It's also fair to say that variety breeders have focussed attention on Light Leaf Spot resistance over recent years, and that many new varieties have limited or less robust Phoma resistance. Over the past few years, around 85% of the UK OSR area has been sown with varieties with relatively low Phoma resistance rating of 6 or less.

What about Light Leaf Spot now?

Light Leaf Spot (LLS) has been increasing in prominence in recent seasons across the UK; it was previously considered more of a problem in northern England and Scotland. That maybe because crops treated twice for Phoma in the autumn in the south were also being protected from LLS infection; any reduction in Phoma treatment could be allowing LLS to develop further?

ADAS assessments of OSR reference crops for the Phoma Alert last season highlighted that LLS didn’t start to come into crops last year until later in the autumn - well after Phoma infection thresholds had been triggered.

Previous research by ADAS had shown that the most effective timing for LLS treatment had been with a post-Christmas fungicide application. 

The implication would be that Phoma should be the primary objective for early season fungicide treatments, with a view to increasing LLS control levels towards the end of the autumn or into winter applications. 

We know that Plover does have useful activity in preventing LLS, so the crop is being protected against early infection along with the Phoma control. Furthermore, with later or repeat Plover applications, we would advocate adding in tebuconazole as a bolster to LLS control.   

With this Plover + tebuconazole mix it is providing extremely cost effective and reliable disease protection through the autumn and early winter. It also leaves the option for prothioconazole to be used at its most appropriate timing for LLS control in the spring. Furthermore, it doesn’t introduce a strobilurin in the autumn, when we want to really protect and utilise that chemistry in the early summer, for Sclerotinia control and green leaf retention.

One spray or two for Phoma this season?

The levels of Phoma leaf spotting already seen in some crops this season does mean an early start to the autumn fungicide programme. With the continued wet weather being experienced, we can expect further spore release and predict secondary infection.

Unless conditions dry up and it turns cold very soon, in all probability 2017 will be a two-spray season. Historically we have seen good economic yield response from a two-spray Plover programme in most seasons – and especially in high pressure years.

Crops do need to be monitored and treated at the appropriate timing when Phoma leaf spotting is evident, but for most – if not already sprayed - that will mean imminently with a first spray and then watching carefully for a follow-up treatment.