Causes of yield decline: Yield decline (or ‘soil sickness’) is an increasing global problem and has been observed in a wide range of crops including oil seed rape, potatoes and wheat (Bennet et al., 2011). Yield decline is a complex issue and several factors have been suggested as a cause including soil microbial populations (e.g. build up of specific pathogens or deleterious rhizosphere populations), plant autotoxicity and soil compaction (Bennet et al., 2011). In the case of coriander, there is no published information on the cause of yield decline and growers have not specifically observed any potential pathogens. Plant growth is stunted and yields are reduced, but this does not seem to be related to root infection. In the specific case of early season cold, wet soils Pythium root infection has been observed but this is not considered to be related to the ‘normal’ yield decline phenomenon (Gibbs, pers comm). A detailed literature search on coriander has demonstrated that this particular crop is understudied, Given this background we envisage that any information derived from this project would be of benefit to the growers.
Previous unpublished findings : Our previously AHDB funded research on coriander yield decline (CP 117, 2014-2016) has shown that the phenomenon can be reproduced under controlled greenhouse conditions using different coriander varieties and soil types. We also believe that root autotoxicity is not the main cause of yield decline based on the results of an experiment where coriander roots were buried in fresh soil and a coriander crop grown in this soil : no yield decline was observed. Detailed analysis of soil microbial communities using next generation DNA sequencing methods appears to show that no single microbe (bacterial and/or fungal) appears to cause yield decline and that different soil communities (from different soils) can produce the same plant response i.e. decrease in above ground biomass. One hypothesis from these observations would be that coriander growth under current horticultural production conditions (minimum tillage) results in the formation and persistence of a soil community that has the ability (overall function) to reduce coriander yield. Therefore it is possible that a perturbation of this community e.g. by deeper soil tilling after coriander growth, soil amendment, desiccation or biofumigation may prevent or reduce yield decline.Given this background, the aim of this project will be to examine potential soil management strategies that change the soil microbial community present in the hope of reducing the yield decline effect. This will be a ‘proof of concept’ approach to demonstrate that methods capable of changing the soil microbial community are able to prevent or reduce coriander yield decline.