Phoma Stem Canker in oilseed rape is a disease that costs European producers £100 million a year though lost yield and the costs involved in trying to control it.
Phoma is caused by two similar co-existing pathogens Leptosphaeria maculans and Leptosphaeria biglobosa, of which Leptosphaeria maculans is now causing the most damage in Europe. All monogenic resistances so far work only on Leptosphaeria maculans, while there are no qualitative resistances for Leptosphaeria biglobosa available. As monocyclic disease, Leptosphaeria maculans goes through one infection cycle per season. But spores from old OSR residues can be released over several months. Epidemics are started by airborne ascospores produced on crop residues, stubble, infected stems and roots that are then dispersed by the wind. These spores infect the leaves of young susceptible plants and produce phoma leaf spots. From the leaf, the fungi then grow along the petiole into the plant stem.
Phoma then invades and kills plant cell tissue, resulting in the formation of leaf spots and stem cankers and the cycle repeats. Resistance to phoma results in better and longer lasting plant health with better lodging resistance as well as a longer and undisturbed assimilation process that supports growth through tough climate conditions such as early summer drought. It also underpins optimum harvest date and can lead to higher yields as well as reducing the general level of phoma spores in the field. RLM7 has been the main genetic disease resistance mechanism for many years and has been very effective, but there is growing evidence that its effectiveness is reducing. In DSV trials in Dyngby in Denmark, for example, on a scale of 0 - 10 with 0 being excellent resistance with no phoma spotting in evidence and 10 being high susceptibility to Phoma, varieties without any qualitative resistance showed an average score of 7.5 with a range of 6.5 to 8.5.
Varieties with the RLM7 gene showed a wider range of phoma susceptibility scores - from 4 to 9 - but of real concern is that the average was the same 7.5 as varieties with no disease resistance at all. When you consider 53% of oilseed rape varieties on the current 2023/24 AHDB Recommended List for the UK rely on RLM7 as the mainstay of their protection against this yield-robbing disease, clearly this is a very worrying situation. There is an urgent need for new genes to be developed if we are to realise the same, and hopefully better, resistance to phoma that growers have relied upon over the last few years. RLMS offers some hope but a new type of phoma resistance would add a much-required additional layer of security for the future. Phoma Blocker from DSV is just that. At the heart of Phoma Blocker is LepR1 - a completely new type of genetic mechanism for phoma resistance in Europe that works by constructing a strong wall of callose and lignin around the infection preventing disease spreading in the plant.
Whilst LepR1 by itself has been shown to have the best resistance to the most common phoma strains in field trials across Europe, DSV believes it is most effective when used in conjunction with other disease resistance traits. Such an approach not only produces optimum levels of protection for growers, it also helps by extending the longevity of existing genetics. DSV Phoma Blocker is made up of LepR1 with RLM7 and this combination not only showed the narrowest narrow range of susceptibilities to phoma in the Dyngby trials, it also did so with an average of score of just 2.3 - compared to the 7.5 for RLM7 by itself. This is easily visible in field trials with Phoma Blocker, where the difference in levels of protection with the new disease resistance compared to some of the most popular varieties with RLM7 alone are very marked. But the real advantage of the new approach is that DSV now uniquely owns three different phoma resistance mechanisms - RLM7, RLMS and now LepR1 - that can be rotated in the field to minimise major breakdown of any one type of resistance.
By being able to do this, DSV has taken phoma resistance in oilseed rape several stages further on.
DSV already has Phoma Blocker varieties moving through the UK testing process and it fully anticipates the first of these will be commercially available in 2024 after completing NL2 testing this year.