In carrots Sclerotinia monitoring adds to successful disease control
The BASF and ADAS sponsored Sclerotinia monitoring system for carrot growers starts in early June and growers and advisors are invited to log-on every week to the carrot Sclerotinia Monitoring link on www.agriCentre.basf.co.uk in order to assess their disease risk and plan a cost-effective preventative disease control programme.
Dr. Peter Gladders from ADAS Boxworth explains that carrot crops are usually vulnerable to infection by Sclerotinia from June through to October, when the resting bodies or sclerotia germinate. "Sclerotia germinate when the soil is moist and soil temperatures are 10°C and above. So far this year soil temperatures have been exceeded, but the Eastern part of the UK in particular has experienced very dry conditions. Looking at oilseed rape where sclerotia germination of Sclerotinia has been monitored since mid-March, there have been low levels of sclerotia germination in the East, but more in the wetter West and North. In carrots where irrigation is common, the drought will be less significant, but we could still do with some rainfall to help early crop establishment."
"As last year, we may be in for a low incidence year if rainfall remains low. The risk of Sclerotinia requires two parts of the equation – is there Sclerotinia in the soil of the carrot field (we know that inoculum levels generally are high after Sclerotinia epidemics in oilseed rape in 2007 and 2008) and secondly is there sufficient moisture, which is more questionable, even with irrigation? All the more reason to monitor your disease risk."
The ADAS and BASF Sclerotinia monitoring system monitors patterns of sclerotial germination each week for ten weeks from June through to August. It indicates the extent of disease risk and the need for fungicide treatment. Growers should check the published information on the AgriCentre web site weekly and be ready to take the right action at the right time.
"Despite the dry weather I would still advise carrot growers to start their fungicide applications early, before the canopy closes over, as once Sclerotinia is in the crop it is almost impossible to get on top of it. The Sclerotinia fungus invades diseased, senescing or decaying leaves of the carrot crop, particularly those in contact with the soil. Roots are infected via the leaf petioles and through the crown. So starting the control programme before the canopy closes over is advisable."
Dr. Gladders reports that some carrot crops were planted early this year, but most crops still have low ground cover. "Irrigation could stimulate further sclerotia germination and the fungicide programme may need to start in early sown crops in the next three or four weeks. If it continues to be dry, then growers may want to consider extending the spray intervals to 14 days, but if we do get some moisture they should stick with the normal 10-14 day intervals."
Robert Storer, Field Vegetable Product Manager for BASF, agrees that monitoring, timing and spray intervals are all important for the successful prevention of Sclerotinia in carrots. "Signum is a protectant fungicide and it should be placed at the base of the plant where infection starts and where old leaves come into contact with the ground. The first Signum spray is recommended before the crop canopy has closed over and the ground is still visible. A closed canopy shields the base of the plant from fungicide sprays."
"Signum is fully approved in carrots and provides broad spectrum disease control of Sclerotinia, Alternaria and Powdery Mildew. Trials data show that Signum also gives good control of diseases responsible for the loss of yield and quality in carrots such as Cercospora, Black Liquorice Rot and Acrothecium. If the drought continues, it may be other diseases such as Powdery Mildew and the physiological benefits of Signum that may warrant its application this season," says Rob Storer.
He reports that in ADAS trials Signum gave 58% control of Sclerotinia, compared with just 5% for azoxystrobin and 28% for tebuconazole. "For Alternaria, Signum also gave the highest level of control, reducing infection levels to 19% from 55% in the untreated, with azoxystrobin reducing infection down to 30% and tebuconazole to 28%. Where Powdery Mildew was the target disease, the untreated suffered 62% infection but Signum reduced this to just 3%, compared with both azoxystrobin and difenoconazole resulting in 10% infection."
A protectant and systemic fungicide, Signum contains 267g/kg boscalid and 67g/kg pyraclostrobin, formulated as a water dispersible granule. It is recommended in carrots as a protectant spray or at the first signs of disease for the control of Sclerotinia, Alternaria and Powdery Mildew at a dose rate of 0.75 to 1 kg/ha in 200-900 litres of water. Two applications can be made to the crop. It is subject to LERAP B and has a harvest interval of 14 days in carrots. It should be alternated in a programme with other fungicides with an alternative mode of action as part of an anti-resistance strategy.
