With much of the country being hit with heavy showers over the last few days growers are being urged to be vigilant for Phoma. That’s the advice of ADAS plant pathologist Dr Faye Ritchie.

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She points out that smaller, later drilled crops are more at risk from Phoma as the distance for the disease to travel from infected leaves down the petiole to the stem is shorter. However, growers should start to assess the threat by checking more established plants first. “Leaf spotting is often more visible on earlier sown crops due to their larger leaf area. However, their longer petioles often prevent or delay all-important stem infection and these crops tend to be less at risk,” notes Dr Ritchie.

Dr Neal Evans from WIN agrees and says that the date when thresholds are reached could vary considerably. “There’s no doubt that the dry spell will delay thresholds being reached after a pretty wet August. However, many eastern areas have been hampered by sea frets since. With near 100% humidity, stubbles will have remained wet – a key factor in spore production,” he cautions.

He too is worried by the risk to smaller plants and believes the cabbage stem flea beetle issue hasn’t helped. “There’s no doubt that this season’s beetle problem has slowed crop development. It is likely to increase the risk of infected stems,” he warns.

But Phoma isn’t their only concern. Following last season’s exceptional Light Leaf Spot pressure Dr Ritchie points out it will form the ‘bedrock’ for infection this season. We’ve now seen an increase in Light Leaf Spot in oilseed rape crops over the past six years, with the disease affecting crops in the south as well as the north. Light Leaf Spot spores also originate from stubbles and crop debris with spores released in the autumn. OSR being grown in close proximity to last year’s crops are likely to be at higher risk. And with many crops sown in good time this year, they are likely to be exposed to the disease,” she notes.

Bayer CropScience’sTim Nicholson reminds growers that few actives are effective against both diseases. “Light Leaf Spot has been spread south through the intensity of rape, recent favourable weather conditions and the lack of varietal resistance. It has also been given a ‘helping hand’ by the fact that autumn disease control has been typically focused on Phoma.

“If we are to manage Light Leaf Spot more effectively then it is vital that Phoma sprays also provide effective protection against Light Leaf Spot as well. Despite the arrival of new modes of action, prothioconazole remains the standard against both diseases and with a half rate of Proline²⁷⁵ (Prothioconazole) to target Phoma typically costing £14/ha it’s the most economic option to start an autumn disease control programme for the two diseases,” he stresses.

Dr Evans also feels growers should consider split treatments. “With more resistant varieties we’ve slowly been getting on top of Phoma, however the opposite is true with Light Leaf Spot. Here the lack of varietal resistance in conjunction with fungicide applications targeting Phoma has allowed the disease to spread. A single azole application may manage a Phoma threat, especially in a dry autumn, but it is often too early to protect against Light Leaf Spot.

“A second pass does place more pressure on farm resources but the second application typically is more suited to Light Leaf Spot. As Light Leaf Spot is now probably the greatest threat for most, if not all OSR crops, then split treatments are a sensible precaution,” he concludes.

The first phoma forecast should be available later this week; http://www.rothamsted.ac.uk/phoma-leaf-spot-forecast