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Clean start for OSR

Agronomy Issues
Combining OSR
Prolonged dry weather at harvest could slow the development of phoma cankers for spore release this season, but will also hold up new crop establishment

This harvest iOSR growers reported crops were being cut in very dry conditions and onto dry surfaces, which could slow down the priming of cankers for spore release.

Any prolonged period without rainfall may mean that new crop infection would be later, suggested Syngenta iOSR Field Technical Manager, Georgina Wood.

The flip side being that extremely dry soils would delay planting and emergence of new season OSR, so plants may then be smaller and more susceptible to Phoma infection when it does inevitably strike. 

“Phoma leaf spotting can develop very quickly when conditions are favourable,” Georgina warned. ”In trials, it has been seen to go from below threshold 10%, to over 70% of plants infected within  a week to 10 days.”

Regular field walking is essential to monitor infection, especially on small leaved or susceptible varieties. Last year’s early infection meant crops were reported above the 10% of plants infected threshold even before phoma forecasts were being issued, for example. 

Georgina Wood with Phoma stem canker

Caption: Average phoma stem canker resistance ratings are now even lower on the latest additions to the Recommended List highlighted Georgina Wood

Rating resistance

Georgina advocated that varietal resistance can play a role in practical autumn management of the disease. Whilst a high resistance rating does not prevent infection, it appears to have a positive impact in slowing the development of lesions on the leaf and, crucially, the speed at which disease moves to the stem to give more flexibility in treatment timing. That is especially useful on small leaved plants in early infection seasons.

New AHDB advice has indicated that growers should manage varieties with an RL stem canker rating below 8 as more susceptible and treat at a threshold of 10-20% of plants infected, whilst those of 8 and above could be considered less susceptible and have a threshold above 20%. On this year’s RL only two varieties, DK Secret and Aquila, are rated at 8 or above – with two varieties at 3 and seven at 4.

Furthermore, the average stem canker rating of the five new varieties added to the list this year - at 4.6 - is lower than the 5.2 overall average of the 2018/19 RL - so the situation is not getting any better and fungicide programmes remain the core solution.

In practice, Georgina suggested growers could consider sowing varieties with lower disease ratings first and in better fields, where they could be expected to develop bigger leaves earlier, before infection hits. Early hybrid vigour may also prove useful in getting plants to bigger leaf stages faster.  

Trial results

Results of multiple ADAS phoma trials over recent seasons have shown that, where canker index was moderate to severe, yield response averaged 0.4 t/ha across treatments and application rates – worth around £120 a hectare.  The trials have shown best yield responses with two-spray programmes.

Early Phoma leaf spotting on OSR

Caption: Early Phoma leaf spotting can quickly develop on small leaves, especially in wet, warm weather. Growers should be on the lookout for both Leptosphaeria maculans and L. biglobosa Phoma symptoms

ADAS Plant Pathologist, Philip Walker, recalled: “Last year’s phoma epidemic developed rapidly in early October, and as result many T1 sprays were applied after the threshold and in a highly curative situation, where phoma leaf spot incidence was above 60%. Despite this the T1 fungicide applications substantially decreased phoma leaf spot on all varieties regardless of their resistance rating.

“The T2 fungicide applications which were applied when re-infection was observed, at 10-20% phoma incidence, produced further reductions in phoma leaf spot and would have been important in reducing the development of phoma stem canker, especially in the high risk year.”

Light Leaf Spot timing

With Light Leaf Spot forecasts based primarily on levels of disease infection in the spring, iOSR growers and agronomists could see early warnings for this season’s crops in the autumn.

However, experience from last year, when year leaf sampling through the autumn revealed little or no Light Leaf Spot infection - even when leaves were placed in conditions highly conducive to the pathogen - again indicated infection timing was actually far later in the season.

Light Leaf Spot

Caption: Light Leaf Spot was late to arrive again last season- not really taking hold to any extent until into the New Year. However, widespread infection in the spring could trigger warnings for this year earlier than actual infection timing.

“The low autumn pressure could have been for a number of reasons, with field conditions less favourable to the pathogen,” pointed out Georgina Wood.

“Furthermore, the resistance of varieties being grown may have been higher, along with the more robust and repeated Phoma fungicide treatments, applied earlier in the autumn, also having a beneficial effect in delaying the onset of Light Leaf Spot until well after Christmas.”

However, she reported Syngenta trials on the iOSR Focus Site and Innovation Centres did pick up infection in the spring, which developed significantly into signs of the disease pre-harvest and spotting on the pods. “It reinforced the advice we have seen in recent years, to focus strongly on Plover timing for phoma in the autumn, and to retain the best LLS fungicides for when they will have the best effects in the post-Christmas and earliest spring timings.”

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