With much of the country being hitwith heavy showers over the last few days growers are being urged to bevigilant for Phoma. Thats theadvice of ADAS plant pathologist Dr Faye Ritchie.
She cautions that even with the firstfew weeks of September being mainly dry it wont have slowed spore production.At Boxworth, Cambridgeshire we had fifteen days with rainfall in August alone.Like many parts of the country further wet weather in the last week has meantweve now had over 20 days with rain since 1st August, with furtherrainfall expected this is likely to encourage spore release. We could see Phoma leaf spotting in oilseed rapecrops within the next 2 to 3 weeks if this continues, although this will dependon variety and geographical location.
She points out that smaller, laterdrilled crops are more at risk from Phomaas the distance for the disease to travel from infected leaves down the petioleto the stem is shorter. However, growers should start to assess the threat bychecking more established plants first. Leaf spotting is often more visible onearlier sown crops due to their larger leaf area. However, their longerpetioles often prevent or delay all-important stem infection and these cropstend to be less at risk, notes Dr Ritchie.
Dr Neal Evans from WIN agrees andsays that the date when thresholds are reached could vary considerably.Theres no doubt that the dry spell will delay thresholds being reached aftera pretty wet August. However, many eastern areas have been hampered by seafrets since. With near 100% humidity, stubbles will have remained wet a keyfactor in spore production, he cautions.
He too is worried by the risk tosmaller plants and believes the cabbage stem flea beetle issue hasnt helped.Theres no doubt that this seasons beetle problem has slowed cropdevelopment. It is likely to increase the risk of infected stems, he warns.
But Phoma isnt their only concern. Following last seasonsexceptional Light Leaf Spot pressure Dr Ritchie points out it will form thebedrock for infection this season. Weve now seen an increase in Light LeafSpot in oilseed rape crops over the past six years, with the disease affectingcrops in the south as well as the north. Light Leaf Spot spores also originatefrom stubbles and crop debris with spores released in the autumn. OSR beinggrown in close proximity to last years crops are likely to be at higher risk.And with many crops sown in good time this year, they are likely to be exposedto the disease, she notes.
Bayer CropSciencesTim Nicholsonreminds growers that few actives are effective against both diseases. LightLeaf Spot has been spread south through the intensity of rape, recentfavourable weather conditions and the lack of varietal resistance. It has alsobeen given a helping hand by the fact that autumn disease control has beentypically focused on Phoma.
If we are to manage Light Leaf Spotmore effectively then it is vital that Phomasprays also provide effective protection against Light Leaf Spot as well.Despite the arrival of new modes of action, prothioconazole remains thestandard against both diseases and with a half rate of Proline275(Prothioconazole) to target Phoma typicallycosting 14/ha its the most economic option to start an autumn disease controlprogramme for the two diseases, he stresses.
Dr Evans also feels growers shouldconsider split treatments. With more resistant varieties weve slowly beengetting on top of Phoma,however the opposite is true with Light Leaf Spot. Here the lack of varietalresistance in conjunction with fungicide applications targeting Phoma has allowed the disease tospread. A single azole application may manage a Phoma threat, especially in a dry autumn, but it is often tooearly to protect against Light Leaf Spot.
A second pass does place morepressure on farm resources but the second application typically is more suitedto Light Leaf Spot. As Light Leaf Spot is now probably the greatest threat formost, if not all OSR crops, then split treatments are a sensible precaution,he concludes.