The biology, epidemiology and control of Sclerotinia sclerotiorum on carrots in north east Scotland
The fungal plant pathogen Sclerotinia sclerotiorum (Lib.) de Bary is responsible for disease in a wide range of crops throughout the world. Sclerotinia disease, or cottony soft rot, in carrot (Daucus carota L.) crops is generally a post-harvest problem during storage. Research has, therefore, focused in this area. However, little attention has been paid to foliar and root crown damage caused by S. sclerotiorum during crop development, a problem prevalent in crops in the North East of Scotland. The conventional method for controlling S. sclerotiorum in the developing crop is fungicide application, which often gives only partial control. There is no recognised effective control method in organic systems. This study investigated a diverse array of potential control methods, involving combinations of cultural, biological, environmental and forecasting methods, all lying within the confines of organic regulations. A number of germination characteristics of the S. sclerotiorum sub-population in question were studied. Steam sterilisation of soil was then investigated as a possible replacement for methyl bromide fumigation. Steam was shown to have potential control properties regarding sclerotia of S. sclerotiorum, if combined with manipulation of soil matric potential. Commercially available biological control agents, soil amendments and conventional fungicides were compared in the search for the most effective control for Sclerotinia. The product found to be most effective in this group, and acceptable for use in organic systems, Coniothyrium minitans, was further investigated in combination with organically produced compost and readily soluble nitrogen fertiliser. C. minitans provided a level of control in both instances. The application of high levels of nitrogen fertiliser encouraged severe infections, as did excessive irrigation of crops. A number of methods and practices were unsuccessfully applied, or suggested by the studies, that can reduce the incidence of disease caused by S. sclerotiorum in carrot crops in North East Scotland, and are acceptable to both conventional and organic systems.