Stem health drives OSR yield

ADAS trials underline DK hybrids’ strengths

Observations gleaned from many years of variety trials suggest poor stem health in the final month of the season may be the explanation, believes David Leaper, Agrii oilseed rape seed specialist. For many growers and their advisers, foliar disease is the focus of crop protection strategies. This is perhaps understandable, as disease is easy to identify and measure, while the large canopy provides a big target to apply fungicides. Promoting stem health is intrinsically more complicated, and has more to do with variety selection, suggests Mr Leaper. The two issues, he says, should be seen in unison.

“Disease on the canopy later in the season can reduce photosynthesis, which ultimately leads to lower yield. The same is true for stembased disease, but with the added threat to the plant’s vascular system. In this respect, stem-based diseases are like a hand around your throat.”

The importance of stem health is an emerging concept, but has come to the fore because of the contrasts that exist between varieties in the final weeks of the season – the impact is seen at harvest. “Varieties that look brilliant suddenly lose their potential post-flowering. In some situations, Verticillium stem stripe (Verticillium longisporum) is to blame, but closer inspection and analysis reveals a complex of stem-based diseases are often responsible,” Mr Leaper says

Yield benefit from cleaner stems That disease should spread through a crop towards the end of its life is hardly surprising. Rather, the concern is how it serves to promote premature senescence and, in some varieties, weaker pods. “We’ve seen how light leaf spot later in the season promotes early senescence, which leads to smaller grains and reduced yield. Elgar suffered especially badly one year from this, and we are seeing reports in other popular varieties,” Mr Leaper says. The manifestations of poorer stem health tend to vary with the season, which adds another consideration. “In some seasons, and at some sites, there is a greater incidence of Verticillium, while in other years there is more stem blackening, which suggests a complex of diseases involving phoma stem canker and light leaf spot,” he says. What is highly evident, however, is the relationship between those with cleaner stems at the final inspection and yield at harvest. “We see varieties such as Turing, Vegas, Murray, DK Exsteel, and InV1035, none of which claim Turnip Yellows Virus (TuYV) resistance, have much cleaner stems, and a gross output above the mean of controls,” Mr Leaper says. “With Verticillium stem stripe, some varieties suffer less yield loss, despite being badly affected, because they have other traits that counter the negatives, such as early maturity and pod-shatter resistance. We saw this many years ago, notably with DK Excalibur.” There will be some growers who see the notion of good stem health as a negative, as it has been linked with later maturity, but this is not necessarily the case, believes Mr Leaper

“Maturity is pre-determined by variety genetics. We shouldn’t be using disease as an opportunity to bring harvest date forward. Instead, we should focus on high gross output varieties with good stem health, and DK Exsteel is an excellent example of this,” he says (see chart below). Complex interactions For Faye Ritchie, ADAS plant pathologist, stem health is part of the wider plant health conundrum. Many factors influence whether stems are green at the end of the season, not least the disease environment on the farm. In some regions, clubroot will be important, and while not a stem disease, affected plants will be more susceptible to disease. “Stem health is a complex issue, and it’s difficult to make it the overriding factor determining variety choice. It pays to know the ranking of disease issues that are present on your farm. In some situations, Verticillium stem stripe will be a significant threat to performance, in which case stem health indicators should be a consideration when shortlisting varieties,” Dr Ritchie says. Generally, plant health is not about controlling any one disease above all others, but about understanding how variety resistance and other traits can be used in combination to limit the impact of disease and other factors on yield. “It owes much to choosing the right variety for the situation and then managing it well. There are indications that Verticillium is associated with yield loss, but it is not alone.

“For Verticillium, most varieties fall somewhere between ‘susceptible’ to ‘moderately resistant’. If this disease is not important to you, focus on varieties with good Phoma or light leaf spot control depending on local risk,” Dr Ritchie says.

All five Dekalb varieties included in the latest ADAS trials have shown good resistance or tolerance to Verticillium, in a season where infection levels on the Cambridgeshire site – chosen for its history of problems from the disease – have been relatively high. This is the third successive year the Verticillium indexes of both DK Excited and DK Imprint CL have been much lower, and their yields far higher, than the susceptible control variety, and on a par with the intermediate standard, confirming their resilience to this damaging disease. Furthermore, DK Expectation has shown similar levels of Verticillium resistance or tolerance for the two years it has been included in the trial work. DK Exposé and V367OL – tested for the first time in 2021/22 – are also demonstrating good genetic strength.

With three-year Verticillium indexes of just under 44 on the 0-100 ADAS scale, DK Excited and DK Imprint CL are consistently showing infection levels of only around half the susceptible control,” notes Bayer trials manager, Richard Williams. “At the same time, their average seed yields are more than 1 t/ha ahead of it.

DK Expectation has recorded almost identical disease and yield performance advantages over the susceptible control in its two years of testing, V367OL doing the same in its first year, and DK Exposé out-yielding the control by just over 2 t/ha.”