Strawberries. James F Hancock
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Genetic engineering and gene editing
References
INDEX
This book is meant to be an overview of all aspects of strawberry science and culture. It is targeted to strawberry researchers and students of horticulture but should be of interest to all horticulturalists and strawberry growers who want to be updated on the science and history behind strawberry growing. The second edition is in many ways a complete overhaul of the previous edition. It has been 20 years since the first one was published, and a lot has happened since then.
Chapter 1 addresses the taxonomy and evolution of the strawberry. After years of mystery, the species origins of the octoploid dessert strawberry have finally been completely elucidated through modern genomic approaches. We also have a much greater understanding about how sex evolved in the octoploid strawberry.
Chapter 2 provides a history of how the strawberry was domesticated. The second edition still relies heavily on George Darrow’s The Strawberry. History, Breeding and Physiology and A History of the Strawberry by S. Wilhelm and J.A. Sagen, but it delves much more deeply into how North American breeding programmes diversified over time. There is also a new section on the impact of California cultivars on European breeding.
Chapter 3 is a summary of the major cultural systems employed across the world and their component parts. The ‘fine tuning’ of cultural methods continues to be an intense area of investigation and updates on what is now being done are provided. A new history on strawberry culture in North America is also included.
Chapter 4 is an overview of the worldwide strawberry industry, including cultural methods and the major varieties grown. Among the greatest changes over the last 20 years have been a major shift from open to protected culture in Europe and Asia, and the scramble to find a replacement for methyl bromide fumigation. I thank David Simpson and Bruno Mezzetti, who provided much information on the cultivar situation in Europe.
Chapter 5 addresses strawberry anatomy and developmental physiology. A considerable amount of new information has been generated on the temperature and photoperiod control of flowering in the strawberry, particularly for the so-called day-neutral or remontant types. There has also been much new information published on the underlying genetic regulation of strawberry development.
Chapter 6 highlights research work on the fruiting and postharvest physiology of the strawberry and includes a new section on strawberry nutrition. In the last two decades, a veritable flood of genomic information has emerged that is providing a much greater understanding of how strawberry development is regulated. The strawberry has become the model system for deciduous, perennial plants.
Chapter 7 deals with the diseases and pests of strawberry. The most common problems are highlighted with a discussion of their symptoms, biology and control methods. A couple of major problems have emerged in the last decade that have become nightmares. Spotted wing drosophila has fully escaped from south-eastern Asia and is now a major worldwide pest of all soft fruits. With the abandonment of methyl bromide fumigation, charcoal rot has become a big problem in California and Florida and is now threatening strawberries in all warm climates.
Chapter 8 reviews strawberry breeding and genetic research, an area where tremendous progress continues to be made. Breeding activity across the world has increased dramatically over the last two decades, particularly in Mediterranean and other sub-tropical environments. Strawberry breeders have also begun to widely use molecular approaches in their programmes, including marker-assisted and genomic selection.
Overall, the aim of this book has been to bring together and summarize all the available information on strawberry history, physiology, genetics and culture. I hope that I have successfully filtered through the voluminous literature that now exists and have at least succeeded in hitting the high points.
Numerous species of strawberries are found in the temperate zones of the world. Only a few have contributed directly to the ancestry of the cultivated types, but all are an important component of our natural environment. The strawberry belongs to the family Rosaceae in the genus Fragaria. Its closest relatives are Duchesnea Smith and Potentilla L.
Species are found at six ploidy levels in Fragaria (Table 1.1; Fig. 1.1). The most widely distributed native species, Fragaria vesca, has 14 chromosomes and is considered to be a diploid. The most commonly cultivated strawberry, Fragaria × ananassa, is an octoploid with 56 chromosomes. Interploid crosses are often quite difficult, but species with the same ploidy level can often be successfully crossed. In fact, F. × ananassa is a hybrid of two New World species, Fragaria chiloensis (L.) Duch. and Fragaria virginiana Duch. (see below).
Table 1.1. Wild strawberries of the world and their fruiting characteristics. (Adapted from Staudt, 2008 and Liston et al., 2014.)
Fig. 1.1. Geographic distribution of Fragaria species based on their clade and ploidy. (From Liston et al., 2014.)
There are 13 diploid and 12 polyploid species of Fragaria now recognized (Table 1.1). Although a large number of the strawberry species are perfect flowered, several have separate genders. Some are dioecious and are composed of pistillate plants that produce no viable pollen and function only as females, and some are staminate male plants that produce no fruit and serve only as a source of pollen (Fig. 1.2). The perfect-flowered types vary in their out-crossing rates from self-incompatible to compatible (Table 1.1). Isozyme inheritance data have indicated that California F. vesca is predominantly a selfing species (Arulsekar and Bringhurst, 1981), although occasional females are found in European populations (Staudt, 1989; Irkaeva et al., 1993; Irkaeva and Ankudinova, 1994). Ahokas (1995) has identified at least two different self-incompatible genotypes of Fragaria viridis in Finland.
Fig. 1.2. Morphological diversity of Fragaria: (A) staminate flower of F. chiloensis; (B) pistillate flower of F. chiloensis; (C) sympodial stolons