A yield-sapping fungus is becoming more common – making it important to select oilseed rape varieties carefully.
Verticillium stem stripe can cause yield reductions of 30% or even more. With no fungicide treatments available, other ways must be found to control the soil borne fungus, says Michael Shuldham of NPZ UK.
“The key to managing risk is crop hygiene, optimised establishment conditions and wider rotations,” he says. In addition, the AHDB recommended list shows significant differences in the level of resistance between varieties.
Disease dynamics
“Verticillium stem stripe is less well known than phoma or light leaf spot as a threat to oilseed rape yields. But it is increasingly prevalent, so it is worth detailing the dynamics of the disease’s progress through the season.”
The disease-infection cycle starts with microsclerotia. Root infection usually takes place during the seedling stage at a relatively wide temperature range, explains Mr Shuldham.
The fungus colonises the vascular tissue and upper plant during stem extension. Leaf yellowing may occur from April onwards. Then yellow vertical stripes, followed by brown stripes, occur on stems towards the end of June and early July.
As ripening progresses, stripes extend along the full length of the plant from soil level into the upper branches. Usually whole plants are affected, but symptoms are occasionally limited to a few branches.
Scraping infected stem surfaces can reveal grey discolouration of the vascular tissue beneath the stripe. This becomes more obvious as outer fleshy tissues of the stem dehydrate during ripening.
During this final phase, the stem begins to shred with masses of minute microsclerotia visible in the tissue. Affected roots are grey in colour. Severely affected plants ripen prematurely with canopy collapse and seed shedding possible.
Symptoms are also visible after harvest on stubbles and crop debris, says Mr Shuldham. Disease levels in NPZ trials are increasing year on year, he adds.
“This year we were seeing symptoms before the Cereals event in June, which was a first. Susceptible varieties were ripening up to two weeks early, reducing thousand grain weight and yield.”