While light leaf spot has traditionally been considered the more significant fungal disease threat to Scottish oilseed rape crops, shifting weather and disease patterns mean phoma stem canker – calculated to cost UK OSR growers £60-100m annually in lost yield – has crept up the order of concern.

Breakdowns in resistance among varieties with good phoma scores but dependence on one particular gene means new routes to management may be needed.

That is the background against which breeder, LSPB, is encouraging growers to spread risk by considering the addition of varieties containing a new phoma-resistance gene to their existing variety choices. Selected from turnip rape, the RlmS gene provides a new varietal resistance alternative to the Rlm7 gene found in many established OSR varieties with phoma resistance.

“Oilseed rape has faced big difficulties in recent years and phoma has perhaps been pushed down the list beneath challenges such as flea beetle and pigeons,” suggested Chris Guest, LSPB's managing director.

“Whilst it’s not a significant issue in Scotland, the virulence of the phoma pathogen is increasing in some other UK regions. “Control of leptosphaeria maculans is dependent on two resistance types.

"Major gene-mediated qualitative resistance prevents colonisation of the leaf, and the visible signs of infection on its surface. Minor gene-mediated quantitative resistance prevents or slow growth along the petiole to the stem and within the stem, reducing canker severity and the within-stem, infection signs.

"Leaf spotting which affects photosynthesis is the outward sign early on, but cutting into stems at a later stage will show the extent of the disease, which at its most severe can result in the plant losing all ability for translocation of water and nutrients through the xylem and phloem

“Pathogen population monitoring has already shown an increase in isolates virulent to Rlm7, with 25% of tested phoma isolates in 2018 virulent against Rlm7. Mean phoma scores of varieties containing this gene have consistently dropped over the past six years, from 8.4 to 7.6, a worrying trend which suggests Rlm7 is in danger of being overcome, which is already the case in countries such as France, where 90 per cent of the crop is affected," he added.

"A focus only on OSR varieties containing this gene for phoma resistance will select strongly for virulent isolates, so broad genetic diversity when making variety choices will help to manage phoma’s impact.”

Read more: Don’t overlook the damage caused by stem-based diseases in oilseed rape

There are two ways in which phoma-resistant varieties can be developed, explained Mr Guest. Quantitative resistance is based on several genes with minor effects. Selected via field observation, it results in a durable resistance, and while it does not stop infection, it reduces its impact.

Then, there is R-gene related resistance, which is based on a single gene with a major effect, usually introduced from wild relatives such as turnip rape, and selected via cotyledon testing and molecular markers. Its resistance prevents infection from becoming established, in the beginning, although can potentially be overcome by the fungus in the long term.

“There is currently a heavy reliance on Rlm7 resistance to stem canker among the varieties on the Recommended List, with current ratings for varieties which incorporate the gene already down to 7.5. For growers seeking to spread risk, our first RlmS variety, the hybrid Respect, was added to the Recommended List last year, and we now have a second RlmS hybrid, Flemming, on the RL for 2022-23,” said Mr Guest.

Said to have a vigorous growth habit both in winter and spring, Flemming’s strength against phoma underlines the value of the adult stem-based resistance that results from RlmS, rather than Rlm7, he pointed out. It also offers turnip yellows virus resistance.

“We have two further RL candidate varieties featuring the RlmS gene. Murray and Vegas both offer high gross output both in the east/west region and in the UK, plus strong light leaf spot resistance, good oil content and vigorous growth in both autumn and spring.

“Varieties with the Rlm7 gene remain in our portfolio, but we are excited by the ability to bring the two types together to help growers and the wider industry manage phoma.”