In highly infected fields, carrot lossescould be as high as 50 per cent as a consequence of the disease sclerotinia. Itis important therefore to monitor the crop carefully in order to assess diseaserisk and be in a position to plan a cost-effective preventative disease controlprogramme. The BASF and ADAS sponsored sclerotiniamonitoring system for carrot growers started in early June and growers andadvisors can log-on every week for the following 10 weeks to the carrot Sclerotinia Monitoring link on www.agriCentre.basf.co.uk.
Dr Caroline Youngfrom ADAS explains that carrot crops are usually vulnerable to infection by sclerotinia from June through toSeptember, when the resting bodies or sclerotia germinate. Sclerotia germinate when the soil is moistand soil temperatures are 10C and above. At this point in the year, soil temperatures have been exceeded andthe soil is plenty moist enough.
Currently the risk of sclerotinia infection is lowbut this could change soon. The risk of Sclerotiniais based on three factors firstly are there sufficient sclerotia at the soil surface of thecarrot field? Secondly is there sufficient moisture and soil warmth toencourage germination, which results in airborne spore inoculum? And thirdly isthe crop at a vulnerable stage? Carrot plants that are young and vigorous andwhere there is still good air movement around the bed are not so vulnerable toSclerotinia. The most vulnerable stage is when leaves start to senesce at thebase of the plant.
The ADAS/BASF Sclerotinia monitoring systemmonitors sclerotial germination each week from June through to August. Itindicates the extent of disease risk and the need for fungicide treatment.
Growers should checkthe published information on the AgriCentre web site weekly and be ready totake the right action at the right time.
Most carrot growers start their fungicide applications, before the canopycloses over, so that they are able to get their fungicide right down to thebase of the plants. The approach is mainly protectant as, once Sclerotinia is in the crop, it isalmost impossible to get on top of. The Sclerotinia fungus invades diseased, senescing or decaying leaves of the carrotcrop, particularly those in contact with the soil. Roots are infectedvia the leaf petioles and through the crown. So starting the control programmebefore the canopy closes over is advisable.
DrYoung reports that some carrot crops planted early this year are surprisinglyadvanced already. Normally canopyclosure takes place from mid-June to late July, depending on sowing date andgrowing conditions. Most growers would go for a three spray fungicideprogramme, with the sclerotinia-targeted sprays starting in June or July. Whatyou need to do is the cover the plant with a fungicide before it gets toodifficult to do so and alternate different chemistries as a resistancestrategy.
Robert Storer, FieldVegetable Product Manager for BASF, agrees that monitoring, timing andfungicide application are important for the successful prevention of Sclerotinia in carrots. Signum is a protectant fungicide and itshould be placed at the base of the plant where the infection starts and whereold leaves come into contact with the ground. The first Signum spray isrecommended before the crop canopy has closed over and the ground is stillvisible. A closed canopy shields the base of the plant from fungicide sprays. Itshould be alternated in a programme with other fungicides with an alternativemode of action as part of an anti-resistance strategy and it can be appliedtwice to any crop.
Signum is fullyapproved in carrots and provides broad spectrum disease control of sclerotinia, alternaria and Powdery Mildew. The physiological, AgCelence,benefits of Signum are not insignificant and play an important role inmaintaining the canopy greener for longer, says Rob Storer.
DrYoung also points out that avoiding fields with infected soil for a period of 3to 5 years would be helpful. This is particularly so when crops are on grownon rented land. You need to know past cropping to implement an effectiverotation.