Integrating cultural and chemical control in carrots

With Sclerotinia being such a widespread and serious disease of carrots, growers are using complementary ways of reducing disease risk by integrating cultural and chemical control methods, according to consultant Howard Hinds of Howard Hinds Crop Consultancy.

"Although we have excellent chemistry to control Sclerotinia in carrots, including products such as Signum and Compass, the disease is so destructive that we need to do everything we can to minimise disease pressure as well as optimising the effectiveness of the tools we have, throughout the crop’s life," says Howard.

He has been working in conjunction with BASF on a new canopy clipping technique which involves clipping foliage between the rows, allowing more air movement in the canopy and decreasing disease pressure. "The main UK carrot variety Nairobi is very prone to canopy collapse and lodged foliage will develop tissue rot when it contacts the ground. At this stage it also becomes very susceptible to Sclerotinia infection. We have been looking at developing machinery that can clip off the canopy. Initially we used an adapted flail, but Wroot Water, the irrigation company, has now designed a single bed clipper which uses a round disc cutting into the soil at the edge of the bed. Assessments of Sclerotina infection in early September and October show that clipping can reduce infection by between 61% and 94% in the September assessment."

Howard explains that this technique, although very promising, has to be part of an integrated approach. "As you can’t clip early, you need to control Sclerotinia with an effective fungicide programme starting early in the season, before the canopy closes over."

Howard continues to explain that Sclerotinia produces spores from soil-borne bodies underneath the crop canopy and these infect senescing plant tissue near or on the soil surface. "From here the disease infects stems and leaves and eventually the root crowns, resulting in core rots. So it is important that the first fungicide spray such as Signum is applied early on, well before the foliage has a chance to close over and starts to senesce. Once the disease is in the crop, it is impossible to get rid of it."

"Signum is one of the strongest materials we have on Sclerotinia and Alternaria and it needs to be used at its best timing, which is early in the programme. It needs to be integrated into the programme with other chemistry in order to comply with label recommendations and also to minimise any risk of disease resistance. Many chemicals contain strobilurins, so special care is needed to make sure this chemistry is not over-used across a programme which can be up to six sprays. The use of a fungicide such as Compass which is non-strob is a useful alternative to consider," advises Howard.

Robert Storer, Field Vegetable Product Manager for BASF agrees that timing is key to the successful control of Sclerotinia in carrots. "Signum is a protectant and systemic fungicide, recommended in carrots for the control of Sclerotinia, Alternaria and Powdery Mildew. The first Signum spray should be applied before the crop has closed over completely and the ground is still visible and it needs to be placed at the base of the plant where the infection starts. Sclerotinia spreads when the old leaves come into contact with the ground and the closed canopy also creates a microclimate favourable to further disease development."

Predicting disease is also an important part of any integrated management approach and BASF sponsor a Sclerotinia monitoring system to help carrot growers assess disease risk. "This monitoring system predicts when to apply the first fungicide treatment and helps in planning an effective control programme. Starting in June, weekly results are published on the Carrot Sclerotinia Monitoring link on www.agriCentre.basf.co.uk. The service is based on monitoring the germination pattern of sclerotia. Sclerotia germinate usually when soil is moist and temperatures are between 5°-25°C, with optimum temperatures being 15°-25°C," explains Robert.