Article taken from Farmers Guide, November 2017.
We've been hearing reports of early phoma infection in oilseed rape crops and in some instances these breached threshold levels well ahead of the usual mid to late October spray timing.
In small crops, the threshold is 10 per cent plants infected, and for larger crops or for varieties with a phoma rating of 8 or more it's 20 per cent.
Crops that exceeded these levels have been treated with a holding spray of Difcor (difenoconazole) at 0.25-litres/ha, or Metal (metconazole) at the same rate where some PGR activity was needed.
These crops probably won't need spraying again until early to mid-November, when the same thresholds should be adhered to.
Other crops should be closely monitored for phoma from now on. Close attention will be needed, especially on small plants or varieties with a resistance score of 6 or less, as this is shaping up to be the worst season for several years.
The weather has been relatively warm and there has been a regular supply of moisture in most areas, ideal for the production and spread of the airborne ascospores from infected stubbles.
These land on young, susceptible leaves and develop into the classic tawny coloured leaf spots. Once established, the fungus grows into the leaf, through the petiole and into the stem.
Small plants are at higher risk as the fungus has less distance to travel to reach the stems.
Here, the fungus forms cankers later in the season that restrict water and nutrient transport. Serious yield loss can result, especially in severe cases where bad lodging and even stem breakages can occur.
I'll be using Frelizon (penthiopyrad + picoxystrobin) at 0.5-litres/ha on all crops, including those that had the holding spray. However, this won't be available next year and stocks are already short, but Recital (fluopyram + prothioconazole) at the same rate will do a good job.
Fungicide should be applied as soon as possible once thresholds have been breached. This application will also provide some early light leaf spot control. With the onset of cooler winter days, this spray should last through until early spring. If light leaf spot is seen before then, be prepared to retreat with Praline (prothioconazole) or Recital from February onwards.
Virus vectors
Peach potato aphids have been observed in some crops. These are vectors of turnip yellows virus so susceptible varieties will require treatment with Biscaya {thiacloprid) at 0.3-litres/ha.
It is well worth adding TerraSorb to one of these applications at 1.5-2.0-litres/ha. This aminoacid based biostimulant plus trace elements helps boost root and shoot growth and is especially useful on smaller crops to help get them ready for winter.
Kerb (propyzamide) offers a great opportunity to hit black-grass hard again {Centurion Max has already worked well this year), with a different mode of action from cereal herbicides. It will need to be applied once soil temperatures fall to 10°C or lower at 30cm depth, probably a good month away yet. Astro-Kerb, which also contains aminopyralid, is a good choice where broad-leaved weeds are a problem.
Finally, it is worth mentioning that aphids are very active in cereals this autumn. Crops not treated with Redigo Deter (prothioconazole + clothianidin) should be sprayed with a pyrethroid as soon as the crop has one leaf, to prevent transmission of barley yellow dwarf virus.
Crops that did receive the seed treatment will need checking four to six weeks after emergence. It is worth remembering that although every night of frost will reduce an aphid population by half, it will take a good week of frosts before crops are clear.
Esme Shephard is an agronomist with Agrovista, based in Wiltshire (esme.shephard@ agrovista.co.uk).