Carrot crops face a high risk of Cavity Spot attack this season, with wet soil conditions a precursor for a high incidence of infection by the damaging pathogen. The good news for growers, is that the current high soil moisture levels will assist activity from SL567A fungicide treatments.

Pythium violae, the main causal pathogen of Cavity Spot in the UK, needs sufficient moisture to become active in the soil, warns Syngenta Field Technical Manager, Ian Holmes. Speaking to growers at a Syngenta Vegetable Field Trial Site in Lincolnshire this week (9 May 2012), he reported that the vast majority of the UK main crop carrot crop is expected to once again be sown with the variety, Nairobi. “It remains the most commercially attractive variety, but it has a low tolerance to Cavity Spot, so growers have to be especially aware of the risks.”

Whilst extending the rotation, shortening the growing season and sowing more resistant varieties can all reduce the incidence of Cavity Spot, he highlighted commercial pressures for quality roots and the demand for late-lifted or stored crop means fungicide control remains imperative. After an economically challenging season for carrot growers last year, root quality has been paramount to achieve viable prices, particularly in late lifted crops from under straw. Mr Holmes advised that growers currently only have one chance to tackle Cavity Spot – with SL567A during the early phases of establishment. This is the timing when the Pythium violae pathogen is believed to be attacking the crop roots. “Treatment with SL567A offers the best protection against infection of newly developing roots, and the potential to harvest high quality clean roots later in the year.” Trials have consistently shown the best results have been from application at the one to two-leaf stage of the crop. “Although overwintered crops often suffer late attacks, treatment in the spring is still the most effective way to minimise damage, helping prevent initial Pythium infection, as well as limiting infection of secondary pathogens that can enter the root through Cavity Spot lesions.”

Mr Holmes noted that both earlier and later applications have been tested, but neither have shown the same levels of efficacy and robust protection as the crucial establishment timing.

Application advice
In a dry establishment period Mr Holmes would advocate irrigation prior to SL567A application, to raise soil moisture and optimise fungicide activity. Spraying can then be undertaken with a water volume of 200 l/ha, which is faster and more efficient than applying in a very high water volume - which in reality would not provide sufficient moisture to make a difference.

He added that carrot growers can take advantage of the Syngenta Vegetable Nozzle to target application at the soil surface for Cavity Spot pathogen control. This will reduce any risk of drift, gives greater flexibility in boom height compared to standard nozzles and the high velocity spray pattern can help achieve good penetration of any cover crop canopy to reach the soil surface and seedling carrots.