Department of Plant and Soil Science, University of Aberdeen, St. Machar Drive, Aberdeen

Background and objectives
Carrots commercially produced in Scotland are susceptible to attack from the fungus Sclerotinia selerotiorum, especially in organic production systems where fungicide cannot be used. S. sclerotiorum survives in the soil in the form of sclerotia, which have the capacity to germinate successfully after up to 7 years dormancy. In suitable conditions, prolonged periods of high soil matric potential and temperatures of 16-21C, selerotia near or on the soil surface germinate carpogenically to form apothecia. The resulting ascospores released attack aerial plant tissues. Infection progresses from the lamina into the base of older petioles of the carrot, and can therefore avoid being removed with the foliage during harvest. The fungus can then invade via the crown, causing crown and later root rots. It is therefore essential to determine the seasonal timing and duration of conditions required for successful carpogenic germination, sporulation, germination of the ascospores and successful establishment within the carrot tissue, when attempting to develop preventative strategies for Sclerotinia control based on biological control in organic production.

Materials and methods
S. sclerotiorum sclerotia are produced in vitro and exposed to a range of temperatures, matric potentials and relative humidifies in soil microcosm-based assays. Based on this information, controlled field studies into the efficacy of biocontrol agents are undertaken. Here, soil conditions are manipulated using soil management techniques, such as variable irrigation regimes and covering soil with straw or plastic sheeting. A weather station network allows continuous monitoring of climatic and soil conditions throughout the study and in a set of commercial carrot fields throughout Scotland.

Results from initial microcosm and field studies will be discussed.