Soft fruit is one of the most buoyant sectors of UK commercial horticulture. Retail strawberry sales were £385M in 2007 and UK growers supply fruit to meet the demand during the peak months from May to October. East Malling Research (EMR) has consulted representatives of the strawberry industry about the challenges that are facing them to achieve sustainability while remaining commercially competitive. The important considerations include the impact of climate change, reduced availability of water, consumer and retailer pressure for pesticide reduction and the need to reduce the environmental footprint of strawberry production. All these factors relate directly to Defra priorities within the policy for Sustainable Farming and Food Science. This project is jointly funded by Defra and the UK strawberry industry and will develop a scientifically based breeding programme to deliver improved cultivars that will help growers to address all these issues.
The project has three scientific objectives that are independent of each other but all feed into a fourth objective, which is the development of improved cultivars.
Florence, bred at EMR, is a productive, robust cultivar with multiple disease resistance. It is well suited to low input growing systems without polytunnels and with reduced pesticide applications, thereby reducing the environmental impact of production. It has been an important cultivar for UK production in July but in recent years it has lost popularity with the major retailers due to its dark colour and problems with inadequate firmness if harvested in hot weather. In Defra project HH3201SSF plants were regenerated from somatic leaf tissue to produce a population with genetic changes from the standard Florence. Eleven ‘somaclones’ were selected that showed potentially beneficial changes in fruit colour and/or firmness. These will now be rigorously evaluated to determine if they show genuine, stable improvements in quality traits but also to ensure that there have been no concomitant deleterious changes in yield, fruit size or disease resistance. Three field trials at EMR will identify the best clones to test on commercial farms and also to use as parents for further breeding.
Climate change predictions are for an increase in average summer temperatures in southern England. This will cause problems when growing everbearing strawberry cultivars, which crop from July to October. All current everbearing cultivars are closely related and suffer from thermodormancy, resulting in a serious dip in production following a period of high temperatures. For example, in 2006 UK everbearer yields were around 30% below average due to a hot period during the summer. We have obtained everbearing lines that are unrelated to modern cultivars, including older cultivars (bred before 1980) and accessions of the wild species Fragaria virginiana. Their response to high temperatures will be tested by using controlled environment cabinets to expose potted plants to three different temperature regimes during the period June-August. Lines that are less prone to thermodormancy than the standard will then be used as parents in a systematic crossing programme. Seedling progenies will be screened for absence of thermodormancy by growing in the field under plastic tunnels and reducing venting to increase temperatures. Breeding lines will be selected where thermodormancy is absent or weak and then used as parents to produce a second seedling population. This will be screened to identify 5-10 elite lines which do not show thermormancy in response to high temperatures.
Cold-stored strawberry plants are used for programmed production by planting from May onwards. Harvesting commences approximately two months after planting and this is known as the 60-day system. This system is relatively inefficient, due to low yields per hectare, and consequently has a negative environmental impact. Furthermore, these plants are then typically kept for a second year, when their cropping season cannot be manipulated and this can result in short periods of overproduction during June. There is a need for improved cultivars that are more productive in the 60-day system and have naturally different cropping seasons in year 2. Our previous research has shown that productivity can be significantly increased by growing genotypes that produce multiple inflorescences (flowering stems) per crown. Experiments in 2007 showed that some lines with this characteristic initiated flowers after planting, in long days. This unusual behaviour could have been influenced by cool summer temperatures and will be confirmed by repeating experiments in different seasons and comparing performance from different planting dates. Inheritance will be investigated by selecting five genotypes with different 60-day behaviour and crossing them in all combinations. Daughter plants from the progeny will be evaluated in the 60-day system to provide data for a quantitative genetic analysis.
The above objectives will provide genetic information and germplasm that will feed into the breeding programme. Annually, around 13,000 seedlings will be raised and evaluated by a team of experienced breeders. Approximately 1% will be selected for preliminary trials, where they will be compared to standard commercial cultivars. The best selections will then progress to trials on commercial farms, organised by the industry partners, and also be tested for resistance to four fungal diseases: powdery mildew (Podosphaera aphanis), black spot (Colletotrichum acutatum), wilt (Verticillium dahliae) and crown rot (Phytophthora cactorum). The programme objectives are to develop June-bearing cultivars that are well-adapted to the 60-day system and everbearing cultivars that are productive during July and August. Within these underpinning objectives we will focus on improving fruit size, eating quality and shelf life, reducing waste, improving disease resistance and water use efficiency, and developing robust cultivars for low input systems. Collectively these objectives address the common theme of reducing the environmental impact of UK strawberry production.