In contrast to last autumn’s exceptional phoma pressure – which started early and went on for months – this season’s dry conditions could slow infection.
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 new crop infection would be later, advised 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 the 10% threshold, to more than 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.
Georgina advocated that varietal resistance can play a role in practical autumn management of the disease. While 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 a Recommended List (RL) stem canker rating below 8 as more susceptible and treat at a threshold of 10-20% of plants infected, while 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 as low as 3 and six 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.
Results of multiple ADAS phoma trials over recent seasons have shown that, where the canker index was moderate to severe, yield response averaged 0.4 tonnes/hectare across treatments and application rates – worth about £120/ha. The trials have shown best yield responses with twospray programmes.
ADAS plant pathologist Philip Walker says: “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,” he adds.
This harvest Berkshire iOSR grower, Joe Dilibero, left his winter barley stubble longer than normal, using a trick other Syngenta iOSR group growers believe could help reduce flea beetle movement and damage to emerging OSR seedlings. After the long and extremely hot period, Joe is hoping flea beetle pressure may be reduced, especially with one 130ha block going into an area with no recent OSR history.
Joe believes that, for both winter barley and oilseed rape, hybrid crop vigour gives plants greater capability and reliability to cope with increasingly difficult challenges from pests, diseases and weather extremes.
Flea beetle is reportedly now more active in the north and west, growers are being warned
There is more movement of cabbage stem flea beetle (CSFB) into the west and north of the UK, so growers who have not had an issue before should be vigilant, according to Syngenta insect pest specialist Dr Max Newbert.
Sowing OSR earlier will allow bigger plants to cope with adult feeding damage, reports Max.
“But larger numbers of larvae will build up over the longer autumn egg laying period. Drilling the last week of August seems to be the best balance between adult damage and larvae numbers,” he adds.
Initial work by ADAS and AHDB suggests that leaving significant areas of volunteer OSR in the ground until September can attract CSFB away from freshly-drilled OSR in nearby fields, resulting in lower damage in the crop. Subsequent control of the volunteers will stop the life cycle and limit further movement of the adult CSFB, to reduce pressure on-farm.
With light leaf spot forecasts based primarily on levels of disease infection in spring, iOSR growers and agronomists could see early warnings for this season’s crops in autumn.
However, experience from last year, when leaf sampling through 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.
Georgina Wood says: “The low autumn pressure could have been for a number of reasons, with field conditions less favourable to the pathogen.
“Furthermore, the resistance of varieties being grown may have been higher, along with the more robust and repeated phoma fungicide treatments, applied earlier in 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 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 autumn, and to retain the best light leaf spot fungicides for when they will have the best effects in the post Christmas and earliest spring timings.”