OSR stem sclerotia pinpoint pathogen risk
Inspection of this season’s oilseed rape crops for signs of sclerotinia attack could give a valuable insight into risks of infection pressure next year. The presence of sclerotia in stems at harvest would indicate an ongoing return of pathogen to the soil.
Monitoring oilseed rape crops in development trials of a new Syngenta sclerotinia fungicide, Syngenta Technical Manager, Georgina Wood (above), highlighted infection can be seen with the early die back of stems (below), which appear bleached white, when uninfected ripening stems look more brownish in colour.
“Early die-back can occur for a number of reasons, including verticillium wilt or phoma,” she pointed out.
“However, if you split open the stem with a knife, sclerotinia can be easily differentiated by the black sclerotia – which look a bit like mouse droppings.”
Where stems are still green, sclerotinia infection can appear as a white mycelial fuzz (below, right), particularly around leaf axial joints where infection typically initiates.
“This spring conditions were particularly dry and cool soil temperatures at the onset of flowering, which suggested infection was likely to be relatively low,” recalled Georgina.
“However, that also meant that the flowering period was extended for a protracted period, during which time there were some heavy rains and higher temperatures to trigger sclerotia germination in the soil.”
That typically coincided with petal fall, which is the highest risk timing for sclerotinia infection (above, left), she warned.
“Sclerotinia is inherently a relatively weak pathogen, but if spores land on petals before they fall and decompose, it provides a food source for disease to develop. Where those infected petals stick on wet oilseed rape leaves or wedged into leaf axials, the disease can grow and get into the plant, to complete its life cycle.”
The development of sclerotia in the stem and resulting early die back can lead to large yield losses in high infection seasons, as well as perpetuating potential issues each year.
Georgina Wood advocated growers can map in incidence of sclerotinia identified in the field using the new Syngenta Protector digital agronomy tool. That will help to build a picture of potential issues and risks year on year, to enable more proactive future agronomy decisions.