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High sclerotinia petal test results from two ADAS sites are a warning of potential high pressure from the disease. 95% of petals tested positive at an ADAS Herefordshire trial site with a similar result also obtained at a second site in Devon.

ADAS plant pathologist Julie Smith says it doesn’t necessarily mean an outbreak will occur but is a little concerned. “When you get a petal test of over 90% at two sites it shows there is potential for a severe epidemic in parts of the country. Ascospores need a mean temperature of 7 °C or above for 24 hours with 80% humidity to germinate. Those areas with high inoculum levels only need the weather conditions to trigger the disease,” she warns.

Temperature and moisture are key factors that drive the disease and Ms Smith acknowledges that both sites are particularly high risk. The disease is spread when petals carrying spores fall and then stick to leaf surfaces. The pathogen invades the leaf, moves through the petiole and colonises the stem. “Not only does humidity and moisture trigger the disease it’s also critical that petals stick to leaves and axils. But predicting petal stick events is difficult given localised weather conditions. Warm showery weather from mid flower onwards is perfect for Sclerotinia,” she adds.

Fortunately she believes forward crops might have missed the brunt of the infection phase because of the cool April temperatures. “Those crops that flowered early have probably missed the most favourable conditions. It is crops at mid flower now that are particularly vulnerable.”

AICC agronomist David Lines of Sunnyridge Agronomy agrees and says his immediate concern is cooler weather extending flowering. “Most of my rape area received a Proline275 (prothioconazole) + Amistar (azoxystrobin) mix at the early flowering stage. For those crops that flowered early I was hoping that this would suffice. However, with flowering likely to be prolonged I think this is now unlikely. I think two sprays could be needed for backward crops too as flowering could be reached at the height of the infection phase.”

But Sclerotinia isn’t his only concern as light leaf spot (LLS) still threatens some crops. “Some of the more susceptible varieties are still troubled with the disease, especially where earlier spray timings have been compromised. It shows how important varietal resistance is.

“What’s now important is to try and halt the spread of LLS further into the canopy and pods. We are fortunate that we have a number of Sclerotinia actives to select from but with those varieties still vulnerable to the disease second sprays will need to include prothioconazole to provide additional LLS control,” he concludes.