Oilseed rape yields can be raised by almost 0.25t/ha by encouraging crops to stay greener during the two months after the mid-flowering stage.

Fungicide sprays used at flowering can shows this type of yield increase even in the absence of visible signs of disease in oilseed rape crops.

Work by Julie Smith, plant pathologist at crop consultants Adas, has shown the use of an SDHI-strobilurin product can extend greening, and could also help with potential problems of resistance building up to fungicides.

“Yield responses were seen in low or nil disease years that could not be explained by disease control,” she told a recent briefing.

See also: Oilseed rape growers put on light leaf spot disease alert

SDHI and strobilurin fungicides have been shown to have greening effects in addition to disease control, whereas the popular azoles have little greening effect.

She explained that a mid-flowering spray of the SDHI boscalid and strobilurin dimoxystrobin gave a 0.23t/ha yield boost across eight Adas sites during 2014 and 2015.

This yield advantage she concluded came from keeping the crop canopy as green as possible from mid-flowering to the green/brown seed stage, which she refers to as Healthy Area Duration (HAD).

Green canopy

The effect was to increase the size and the duration of the green crop canopy from mid-flowering, typically at the end-April, to late June and this led to higher yields by allowing greater photosynthesis.

“The canopy is able to work harder and more efficiently to transform light energy into dry matter,” she added.

This boscalid-strobilurin mix, launched last year by BASF as Pictor, is aimed primarily as a sclerotinia-prevention spray, so any extra yield effect will be useful for growers struggling with low rapeseed prices.

The fungicide product, as well as giving increased leaf greenness, also increased the water efficiency of the plants and delayed leaf senescence, she added.

“We need to stop blasting oilseed rape with azoles all the time, and need to formulate an anti-resistance strategy”
Julie Smith, Adas

“The Pictor had a positive effect on the green leaf area and this led to the increased yield,” Ms Smith said.

Clare Tucker, BASF’s oilseed rape expert, said the details of the field trials by Adas showed positive greening and physiological effects which led to the higher yields.

She added the product, which typically costs about £30-£35/ha, could produce £60-£70/ha of extra rapeseed at current prices, according to the trial results.

Fungicide resistance

In addition, avoiding an azole fungicide at the flowering stage may help with preventing the build-up of resistance to fungicides, as azoles are often used earlier in the season to control light leaf spot.

“We need to stop blasting oilseed rape with azoles all the time, and need to formulate an anti-resistance strategy,” Ms Smith said.

Light leaf spot is the most damaging disease of oilseed rape, and growers may have used azoles fungicides, such as prothioconazole, tebuconazole and metconazole, as many as three times in a growing season before flowering.

“To have a non-azole option is very critical. It makes sense to use an alternative mode of action at flowering to those used against light leaf spot earlier in the season,” she said.

There has been no real hard evidence of a decline in the efficacy of azoles against light leaf spot, but as they have shown a decline in efficacy against septoria in wheat it makes sense to have an anti-resistance strategy, Ms Smith added.