Horticulture: Development of Fusarium wilt resistance in celery and understanding of pathogen virulence factors to underpin UK and EU biosecurity

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The Fusarium oxysporum (Fo) fungal species complex includes more than 100 different pathogenic formae speciales (f.spp.) adapted to specific hosts, including important crops such as celery, lettuce, tomato and herbs such as coriander. Fo pathogens continue to spread globally with some responsible for major pandemics, most notably Fo f.sp. cubense causing Panama disease on banana. The impact of Fo pathogens is often substantial with complete crop loss not unusual, and this has been exacerbated by an absence of biosecurity measures and the lack of foresight in developing and deploying plant resistance in advance of new Fo f.spp. and races arriving from other regions. Fo f.sp. apii (Foa) causes Fusarium wilt of celery and is the most important and significant disease affecting production. Infection occurs through the roots and is initiated by long-lived spores in the soil. The plant's vascular system becomes invaded leading to stunting, yellowing and wilting. Control of Foa is very challenging with high losses as fungicides and rotations are ineffective; hence the development of plant resistance is a priority. However, like some other Fo f.spp., new Foa races have evolved which overcome resistance. Four races of Foa exist with the recently emerged race 4 the most damaging in the absence of resistance in commercial celery. Furthermore, unlike other Foa races, Foa4 can also cause disease on coriander. There is therefore an urgent need to develop high level resistance to this disease in celery and assess impact on coriander in the UK. Currently Foa mainly affects celery production in the USA and China but there is a very high risk of the pathogen spreading to major crop and seed production areas in Europe such as Spain, Italy and the UK where conditions are already suitable, but which will become more conducive with climate change. Celery is a major crop worldwide and is an important part of a healthy diet as it is high in fibre, minerals and vitamins. The biggest producers of celery are the USA and Spain but there is also substantial production in the UK. Significantly, UK-based Gs Growers who are the biggest producer of celery in Europe grow a large proportion of their crop in Spain for all year-round supply. The crop is also critical for project partner Tozer Seeds as they are one of the major celery breeders and seed suppliers worldwide. It is therefore very important that celery crops in Europe and the UK are protected against future arrival of Foa4.The main aim of this project is to address the challenge of Fusarium wilt by identifying new sources of resistance to Foa4 in a diverse set of celery types from Tozer Seeds. In addition, we will also assess the response of selected commercial coriander lines to the pathogen. As the genomes of the celery diversity set have just been sequenced, we will also identify parts of the genome and variations in gene sequences associated with this resistance in order to develop DNA markers. These can significantly speed up breeding new varieties as resistance is easily identified by DNA analysis without the need for a plant test. Lastly, we will also carry out genome sequencing of Foa isolates from different races as well as an isolate of Fo from coriander to identify gene candidates involved in pathogenicity. This will highlight differences between Foa races but also allow specific diagnostic targets to be identified for development of rapid DNA-based tests for the pathogen. These could then be employed as a biosecurity measure for detection of Foa4 outbreaks in the field or the presence of the pathogen in imported celery plants or seed.Overall, the project will provide the first important steps for breeding celery cultivars resistant to Fusarium wilt as well as a rapid specific test for Foa4 and other races. These outcomes will therefore benefit celery breeders, producers and consumers as well as plant health agencies by mitigating the biosecurity risk of the pathogen.