Autumn Disease Update

Autumn disease update

Chris Charnock, Syngenta OSR Technical Manager

Phoma spotting There is a high risk that phoma will hit establishing crops early this year; the wet soils and repeated showers are likely to trigger the release of infective spores from the trash of last season’s crop – and there were certainly plenty of stem cankers visible on stems at harvest.

It is worth keeping a close eye on any volunteer oilseed rape seedlings in fields after this year’s harvest; they are likely to be the first to show signs as they are closest to the source of infection and will have been up for the longest.

Also, any crops in fields adjacent to where oilseed rape was grown over the past season will be more exposed to spores, especially if they are to the east and the prevailing winds are from the west. Phoma spores are relatively heavy, so neighbouring fields will be under greater exposure, unless we have particularly strong winds.

And if you haven’t yet cultivated in the trash and stubble from this year’s oilseed rape harvest, it is worth having a rummage around for any stem cankers; if they are easy to find it’s a good indication the phoma risk will be high.

High levels of Phoma stem canker (left) on last season’s stems could result in early leaf spots on this year’s crop (right).

It will be particularly important to be vigilant for phoma infection on late-sown crops this season, since early attacks on small leaves will spread down to the stems quicker than on larger leaves. Early infections are far more damaging in terms of final yield loss.

Be ready to treat with Plover as soon as thresholds are reached (10 - 20% of plants showing phoma symptoms). Timing with Phoma applications is crucial, so don’t compromise phoma sprays to fit in with herbicide applications.

Strong varietal resistance can help slow down the progress of infection, but crops are still likely to need control - especially from early infections. Scottish growers are also being advised to be more aware of Phoma infection, since it appears to be affecting crops further north. Later-sown crops that are smaller going into the autumn and winter are especially vulnerable.

Downy Mildew risk In warm, damp seedbed conditions there is always a risk of Downy Mildew attack, which can result in newly emerged plants dying off. Later-planted crops are more susceptible to damage and losses, which could create problems for growers this season.

Downy Mildew attack can slow down emergence and establishment, especially in late-planted crops.

Most infections occur on the cotyledons, and if growth is slow the seedlings will struggle to get going. At best, establishment will be delayed and autumn growth checked, but in many instances an outbreak of Downy Mildew can lead to patchy crops. Any infection in the crop over the autumn will also increase the chance of Downy Mildew developing in the spring prior to stem extension, leading to early loss of lower leaves that could hit yields.

Clearly crops sown at reduced seed rates have less flexibility to compensate for high levels of seedling losses. That will have made seed treatment with Cruiser OSR even more imperative; protecting against the autumn Downy Mildew infection, to assure vigorous early growth and stronger establishment.

Rain spread of Light Leaf Spot Light Leaf Spot was also widespread in last season’s crop, which means we can expect further infections this autumn and winter. Primary infection occurs from windblown spores, but once it is in the crop spread will be perpetuated by wet weather and spore splash during increasingly prevalent heavy rain showers; patches of disease from the initial infection points are common to see.

Light Leaf Spot has historically been seen as an issue in Scotland, but has been spreading south in recent years. As is the case with Phoma, varietal resistance to Light Leaf Spot can slow progress of the disease, to buy growers valuable extra time and flexibility with fungicide timing. But if weather conditions remain conducive to disease - as we have seen with warm, wet conditions well into December in recent seasons – then a two or three spray programme to tackle both diseases will pay dividends.

Avoiding growth checks With this year’s late sowing and slow establishment of many crops, avoiding any further growth check going into the autumn could prove essential in achieving the ground coverage to minimise pigeon feeding and potential winter damage.

Removing volunteer cereals will be essential to give oilseed rape seedlings the light for strong early growth, and reduce competition for available moisture or nutrients.

Providing sufficient nitrogen to support strong early growth is important, especially this year where, in many instances, a lot of green straw was incorporated into seedbeds and may have locked up available nitrogen reserves. A number of growers involved in the Driving OSR Yields initiative report giving crops extra N in the autumn to compensate.

Furthermore, fungicides that risk checking growth should be avoided in late-sown or backward crops.

Disease lookout We are asking all the Syngenta Area Managers to be especially vigilant for first signs of phoma spots or other oilseed rape diseases, and report back any initial infections they see or hear about when talking to growers. The growers involved with Driving OSR Yields initiative will also be on the lookout, as well as trying to pick out any differences between varieties and establishment techniques.

We will use the Driving OSR Yields website to keep you up to date with any disease reports as quickly as possible.