Edinburgh Napier University

  Crop yield decline is increasingly associated with the intensive practices of modern agriculture. It affects a wide range of crops, including Coriander (Coriandrum sativum L.), the UK’s most economically important herb. The crop suffers from a particularly severe form of decline, which can reduce yields by 50%. Unlike other widely grown crops, the growth of coriander in the UK has not been optimised, and growers use highly variable practices. The main aim of this study was to investigate crop and soil management techniques which could reduce coriander yield decline: e.g., different depths of tillage, various sowing densities, and the desiccation or sterilisation of crop soils. Glasshouse pot trials were used to assess the efficacy of these practices at reducing yield decline in successive coriander crops. Results showed reduced levels of decline when soils were: harrowed (compared to unharrowed), and sown at a ‘medium’ density (compared to a relatively low or high density). Coriander grown for a second cycle under a set of ‘optimum growth’ conditions still experienced some decline, suggesting a level of microbial involvement.

To investigate the potential involvement of soil microbes, soil desiccation and soil sterilisation were assessed as soil management techniques. Desiccation of crop soils after one cycle of crop growth prevented yield decline in a subsequent crop. Additionally, sterilisation of field soils (showing severe decline symptoms) produced 50% greater yields per pot and 70% larger plants, compared to a crop grown in nonsterilised field soils. MinION nanopore sequencing (16S and ITS barcode approach) was used to facilitate a microbial community study. Identifications were made for fungal and bacterial taxa of rhizosphere and bulk soils in a grower’s field soils and in soils from the glasshouse desiccation experiment. Results showed a defined shift in fungal taxa between healthy and yield decline samples. Overall results indicated a multifactorial problem, with the likely involvement of deleterious soil microorganisms. The next stages of investigation should be to assess the efficacy of a set of management strategies and optimised growth parameters in a field trial environment. Greater replication and further study are needed to elucidate the microbiological mechanisms of coriander yield decline, including potentially identifying specific associated microorganisms.