How to determine sclerotinia disease risk in oilseed rape

A fundamental concept in plant pathology is the disease triangle. It consists of three elements – the pathogen, the host and the environment. This article, by Robert Saville, AHDB Crop Protection Scientist, gets straight to the point(s) on how this three-sided shape relates to sclerotinia control in oilseed rape (OSR).

In the case of sclerotinia of OSR (the host), the pathogen is Sclerotinia sclerotiorum. These two sides of the triangle are relatively simple to explain – unlike the environment.

At this time of year, when moist soils warm, sclerotinia resting bodies (known as sclerotia) germinate and release windblown ascospores. The odds appear to be stacked against the pathogen. The relatively few spores that land on OSR flowers are lucky – because the petals provide essential nutrients for spore germination. For infection to occur, petals still need to fall onto the canopy and experience suitable conditions. In fact, infection requires long periods (>23 hours) of high humidity (>80%) and air temperatures above 7°C. Only then will the pathogen infect the crop.

Once infected, the pathogen spreads to the leaves and the stem. Eventually, this can result in lesions that girdle the stem and cause premature ripening and lodging. Sclerotia form within the stem and roots of infected crops, which are returned to the soil at harvest to kick off the disease cycle in subsequent years.

Our sclerotinia infection risk tool exploits the understanding of the three elements required for infection – and, in particular, gets to grips with the tricky environmental angle.

The tool uses current and historic weather data to provide an area forecast of how favourable the environmental conditions are for infection over the next 48-hour period. The map-based view uses a traffic-light system to highlight sites at ‘low risk’, ‘near-miss’ or ‘high risk’ of infection.

In addition to weather data, the tool also gives an indication of inoculum levels, based on spore numbers recorded in a UK network of six spore traps (managed by Rothamsted Research).

If your crop is in flower (growth stage 60–69), spores are being detected in the spore trap network (or via local petal tests) and the forecast is showing high-risk infection conditions within the next 48 hours, then a protective spray should be considered.

So far, spring has been relatively cool and the thresholds for infection have seldom been met. However, keep a close eye on the tool as things change quickly. In some years/areas, crops race through the susceptible growth stages quickly, helping to avoid the need for a second spray – and, potentially, the requirement for a sclerotinia spray altogether (especially when spore pressure is extremely low and infection conditions are unconducive).

Although forecasts are never perfect, an analysis of the 2020 season shows that sclerotinia forecast alerts were around 96% accurate, with only around 2% false negatives.

So what is the point? Understanding each element, or point, of the disease triangle and having easy-to-interpret information at hand will support your spray decisions. This will potentially save you money (through reduced sprays) and increase the efficacy of any treatment (through better-timed sprays).