Major continental losses to stem canker in 2010/11 suggest many oilseed rape crops across the country could be harbouring a hidden disease time-bomb this season even though high levels of phoma leaf spotting may not have been obvious, warns Dekalb North West Europe technical manager, Kuldip Mudhar.
Just like this season in the UK, winter OSR in Denmark and Germany, in particular, went in late under difficult, wet conditions in autumn 2010, he points out. And similarly too, the early phoma threat was high.
Interestingly, few signs of major leaf infection were evident there in the autumn and early winter. But by late-flowering a large number of susceptible crops were suffering widespread cankering, leading to serious stem damage and yield losses. In Danish farm trials, for instance, the average yield of varieties with a phoma stem canker resistance of 5.0 or less was more than 15% below that of varieties with a rating of 8.0 or more.
This underlines just how much stem canker damage a small amount of phoma can cause susceptible varieties under the right conditions primarily a combination small plants and early infection.
We know low levels of phoma leaf spotting can be hard to quantify on small plants, explains Mr Mudhar. At the same time, short leaf petioles and slow-growing plants mean the fungal mycelium has a much shorter distance to travel to reach its destination in the stem.
Add to this the fact that over half the varieties on last years Recommended List had phoma stem canker resistance ratings of less than 5.0 and the extent to which autumn spray programmes were restricted by the weather and ground conditions and we are clearly treading a tightrope with the disease in this seasons crop.
The scale of the problem is impossible to quantify at this stage, since no symptoms are visible while the fungus grows from the leaf spot, down the petiole and establishes itself in the stem.
However, research shows that under the mild conditions most of the UK had up to mid-January the mycelium can grow as much as 5mm per day. And even at 3-5oC its able to move at 1mm/day. So the open early winter will have given the fungus plenty of time to penetrate to the stems of this years particularly small plants, giving real cause for concern.
While the scale of the early cankering and stem collapse responsible for the greatest losses will become apparent as crops move into flowering, Kuldip Mudhar stresses that much of the damage from the disease results from fungal growth compromising water and nutrient uptake. This means that even later developing and less obvious cankering is likely to have a significant impact on performance; especially in thinner stemmed crops and with later-maturing varieties.
Yield losses from phoma stem canker in susceptible varieties that result in early crop senescence, poor pod fill or both frequently go undiagnosed, he observes. Either that or they are attributed to other diseases or climatic factors.
If the recent Danish and German experience is anything to go by, the true impact of the disease could become all too evident to many UK growers this season. The major wake-up call it gave their growers was clear in a huge swing towards resistant varieties in the past year. In Denmark, indeed, the proportion of plantings in resistant varieties grew from less than 30% in 2011/12 to almost 70% in 2012/13.
Specialist single gene RLM7 resistance is proving remarkably durable in preventing phoma infections, reports Mr Mudhar. Combined with high levels of polygenic resistance and key plant physiological characters like bigger leaves, longer petioles, faster leaf development and stem extension to slow fungal spread and restrict stem canker-forming ability, this provides the best possible insurance against losses.
A late winter spray in February ahead of stem extension may be advisable with susceptible varieties this season, he adds.