Information on oilseed rape growth stage, sclerotinia pressure and weather-based infection risk can help reduce sprays and improve spray timing, according to the latest AHDB research. Charlotte Cunningham reports.

With the crop about to enter its critical flowering period, AHDB has announced it will run its sclerotinia infection risk alerts service again this season to help growers better target sprays.

AHDB has also issued the spring update of the light leaf spot forecast, as well as predictive information on aphid activity (first flights and abundance).

Sclerotinia forecasts

Each spring, warm (>10°C), moist soils cause Sclerotinia sclerotiorum to develop brown spore-releasing structures on the soil. Carried in the wind, the spores can land on OSR, feed on petals and germinate.

Even where spores are present and food is sufficient, conducive weather is required for infection to occur. OSR is at the greatest risk of infection when relative humidity and air temperatures are high (>80% and >7°C, respectively) for more than 23 consecutive hours.

The ADAS-led project provided infection-risk information for 15 commercial sites, three times each week (2015–18), during the main risk periods. Located across England and Scotland, some sites included monitoring of airborne spores and petal inoculum levels. The researchers found that decisions based on inoculum levels and weather-based infection risk resulted in 26% fewer crops needing treatment.

Three of the sites included a fungicide timing trial. Under relatively high disease pressures, fungicides applied in response to alerts resulted in an average yield response of 0.3 t/ha (compared with the untreated control). Where sprays were made routinely at early flower (in the absence of alerts), average yield increases were reduced to 0.22 t/ha.

Risk alerts service

Catherine Harries, who manages disease research at AHDB, said: “In the absence of sclerotinia inoculum, oilseed rape is not at risk from infection. Where inoculum is found, weather largely drives infection risk and this is where our alerts service is particularly useful.”

The UK areas where weather conditions are currently, or are forecast to be, suitable for the sclerotinia pathogen to infect crops are highlighted by the risk alerts service.

Typically, the optimum time for a single spray is just before mid-flowering on the main raceme and, since fungicides have protectant activity, should be applied prior to an infection risk alert. Persistence of full-dose fungicides is approximately three weeks. If a spray is made earlier, or if the flowering period is extended, a second spray may be required under conducive infection conditions.

Access the sclerotinia infection risk alerts during the main flowering period at ahdb.org.uk/sclerotinia

Light leaf spot and aphid forecasts

AHDB’s light leaf spot spring forecast shows that the disease risk to OSR has increased over most regions, since the autumn preliminary forecast. This is due to the wetter than average winter for most parts of the UK. The exception is eastern Scotland, which has reduced slightly due to drier weather than normal. The regional disease forecast highlights the proportion of OSR crops with a disease resistance rating of 5 predicted to have more than 25% of plants affected by the spring.

With January–February air temperatures above the 30-year average, aphids are anticipated to take flight relatively early this spring. In fact, the AHDB Aphid News service predicts that aphids will fly about 1–3 weeks earlier in Scotland and Northern England and about four weeks earlier over much of the rest of England.