The first Sclerotinia spore germination in carrots was identified in Norfolk last week (23 June), although continued dry weather has minimised the risk of disease development, according to John Birkenshaw of ADAS. However, with maincrop carrots progressing well, crops at risk of infection should receive a fungicide treatment before the canopy closes in, he advises.
The wet, warm micro-climate beneath the crop canopy can prove sufficient for Sclerotinia and Alternaria to develop, particularly in irrigated crops or sheltered and low-lying areas of fields. "Growers must inspect crops closely at the base of leaves and stems, for any early signs of infection," warns Mr Birkenshaw. "Once disease gets into the crop, it is difficult and far more expensive to fix.
"If Alternaria is already active, dose rates must be kept high to achieve control. These treatments should also give some protection against Sclerotinia."
Syngenta Technical Manager, Jon Ogborn, reports Syngenta trials have shown that the new carrot fungicide, Amistar Top, provides high levels of Alternaria control. The pre-mix addition of difenoconazole with azoxystrobin, which provides a two-pronged attack, adds significant extra benefit over straight Amistar and other fungicides. In addition, with both fungicides having different modes of action, Amistar Top is also a valuable aid to resistance management.
In the trials Amistar Top reduced Alternaria infection from over 46% in untreated plots, to just 11%, with a 36% increase in control over tebuconazole. In ADAS trials, the pre-mixed Amistar Top achieved a 40% increase in Alternaria control over straight Amistar. Amistar-based programmes have also given good reductions of Sclerotinia in HDC funded trials.
"Application timing is key for Alternaria and Sclerotinia control," adds Mr Ogborn. "For earlier applications growers need to target spray application at the foliage - ideally using air-induction nozzles, such as the Amistar nozzle, and applying at a water volume of 100 l/ha to maximise leaf coverage. As the season progresses, switching to 65º-80º vertical flat fan nozzles and increasing water volume will help spray penetration and enhance basal Sclerotinia control."
With the hot, dry weather Powdery Mildew is also a concern for carrot growers, reports Mr Ogborn, but Amistar Top application for Alternaria also has label approval for Powdery Mildew and will provide a good level of control.
"Having the triazole pre-mixed with Amistar provides valuable extra curative activity on any disease in the crop, as well as preventing further spread onto new growth. It also gives in-built resistance management and simplifies the spraying operation."
Cutworm numbers build
Beware for high numbers of cutworm currently being caught in monitoring traps, warns ADAS. Warm conditions have triggered the development of Turnip Moth eggs and larvae, with insecticide treatment now predicted for early July in southern and eastern England, unless carrot, potato sugar beet and other susceptible crops receive at least 10mm of irrigation, or heavy rain within the next few days.
Latest catches of adult Turnip Moths by HDC at Wellesbourne - which gives an indication of cutworm activity - have risen from less than two per day at the beginning of June, to 15 a day over the past week. The minimal rainfall in June has, for the most part, being insignificant in reducing the cutworm threat.
Jon Ogborn advises Hallmark Zeon is recommended for cutworm control at 75ml/ha, applied according to egg hatch and peak activity forecasts, and repeated 10 to 14 days later if damage warnings persist.
