UK:High potential for Sclerotinia threat this season

Date:04-15-2011

Many fields that grew rape in 2007 or 2008 have been cropped with oilseed rape this year and very high levels of disease then will mean the potential innoculum source could be very high now, warns ProCam UK Agronomy Manager, Nick Myers.

"We know that you only need very few sclerotia in the soil to become an epidemic and we know that these resting bodies remain viable for a number of years. So inevitably many crops are potentially at high risk this season," he warns.

Sclerotinia erodes yields by causing premature senescence, reducing seed weight and causing early seed loss. According to Dr. Peter Gladders of ADAS yield loss is 50% of the plants infected and with 20% infection being commonplace, this is an expensive potential loss.

Nick Myers says that lessons learnt on preventing Sclerotinia mean that we are now in a better position to prevent this damaging disease.

"One valuable lesson is that rainfall in April is no great indicator of disease incidence. In 2008 many areas had low rainfall but experienced the worst Sclerotinia for many years.

"So even in a dry year this disease can be a problem. It appears that heavy dew or crop wetness is sufficient to get the soil surface moist and to encourage germination."

Another lesson is that the current fungicides, although effective, are largely protectant, so they need to be applied before disease is apparent, he points out.

"We have also learnt that most crops need more than a single treatment during flowering. The best fungicides we have can give protection against Sclerotinia for a three week period. If flowering is protracted, infection events can occur outside this period of cover.

"Reliable disease control can only be achieved by a two spray programme that will protect the crop for the six week flowering period."

Last year, which was relatively dry, crops that received just one mid-flowering spray had noticeable levels of Sclerotinia in the crop, Nick points out.

"We have also found that using a growth regulating fungicide such as metconazole (Juventus) at yellow bud to early flowering in the programme not only helped with canopy management but started off the Sclerotinia control.

"Metconazole helps to reduce lodging, evens up the canopy, synchronises flowering and improves branching so that light penetration can be optimises. It also protects against early infection of the flowers by Sclerotinia spores for a week or two."

This spray should be followed at mid flowering by a more robust fungicide treatment with specific activity on Sclerotinia such as Filan (boscalid) or Galileo (picoxystrobin). Thiophanate-methyl (Topsin) has also given good results, he says.

"What you are trying to do is to keep the canopy green and protect the crop from disease for as long as possible at the start of the yield building phase."

Potential crop values this year mean that the risk of getting it wrong could allow the disease to impact severely on the bottom line.

"A comprehensive fungicide programme to protect the crop from this damaging disease makes economic sense in most years and particularly so this year with the inherently high commodity prices."