**3. Global challenges**

Based on the available information, experiences of classical genetic improvement for the development of cultivars tolerant or resistant to diseases, pests, abiotic factors and recently to improve the nutraceutical quality of the fruit, will be addressed. An important aspect is that, for each goal of incorporating tolerance or resistance to a certain problem, the required sequence is to search for sources of genetic resistance [19], or of the richness of nutraceutical compounds [3], and then transfer it to the new cultivars. The commercial strawberry is octoploid, and wild plant populations of the 26 species known to date are found in nature [20], including their ancestors, and the newly discovered *F*. *emeiensis* [21]. The availability and use of this genetic wealth in the formation of cultivars were derived from the international literature. The ease of use of these genes depends on the chromosomal level of the species, the nuclear and cytoplasmic genetic compatibility between them, the type of inheritance of the resistance (qualitative or quantitative) and the availability of an appropriate technique to identify the resistant individuals.

#### **3.1 Disease resistance**

The development of cultivars with tolerance or resistance to certain diseases has been an approach of limited use in strawberries. Root and crown diseases are the group of parasites that cause the most economic damage. In the last century and the current one, the presence of at least seven important diseases has been reported: *Phytophthora fragariae*, *Verticillium* spp., *Phytophthora cactorum* [15]. In the latter, *Colletotrichum* spp. [18], *Fusarium oxysporum* f. sp. *fragariae* in Chile [22]; China [23]; Spain [24]; California, USA [25]; Iraq [26]; Serbia [27]; Turkey [28]; Iran [29]; Bangladesh [30]; Ecuador [31]; *Macrophomina phaseolina* in Florida, USA [32]; Israel [33]; California, USA [34]; Spain [35]; Argentina [36]; Iran [37]; Australia [38]; Chile [39]; Tunisia [40]; Italy [41]; and probably *Neopestalotiopsis* spp., emerging parasite whose presence has been reported in 17 countries during this century in Brazil [42], Egypt [43]; Morocco [44]; Spain [45]; Iran [46]; Vietnam [47]; Belgium [48]; Argentina [49]; India [50]; Korea [51]; Uruguay [52]; Italy [53]; Mexico [54]; China [55]; Ecuador [56]; Finland [57]; Taiwan [58], and that it could acquire global importance if the damage persists in future years. It is important to highlight that *Colletotrichum* spp. [59], *Fusarium oxysporum* f. sp. *fragariae* (FOF), *Macrophomina phaseolina* [60–62], and *Neopestalotiopsis* spp. [63–65] were reported since the twentieth century, but only FOF was important in Australia [66], Japan [67], Argentina [68], Korea [69], and Mexico [70].

Genetic resistance in strawberries has a history dating back to the last century, and valuable experiences that confirm the goodness of this strategy. In this sense, the United States Department of Agriculture, released a multitude of cultivars resistant to various races of *P. fragariae*, adapted to the cold climate of various countries in the world, whose commercial use and/or as sources of resistance, were used to obtain cultivars in other countries. The resistance genes used by the USDA were transferred from *F. chiloensis* clones Yaquina and Del Norte; and *Fragaria* x *ananassa* through the cultivars 'Md 683' and 'Aberdeen' (**Table 1**) [71, 72]. In populations of *F. chiloensis* from California, USA there is also resistance to this fungus [73, 74].

Other diseases of the twentieth century that justified the development of resistant cultivars were *Verticillium* spp., and *P. cactorum*. For the first disease, in the last century sources of genetic resistance were detected in *Fragaria* x *ananassa* [75], and *F. chiloensis* [76], but more recent studies also found resistance in *F. virginiana* and three diploid species [77]. The same situation occurred for *P. cactorum*, where resistance genes have been tracked back in commercial cultivars from the USA and Germany [78–81], and in two diploid species [82] (**Table 1**).

Root diseases that acquired global importance from the XXI century, have been the subject of research that allowed to strengthen the efforts made regionally during the twentieth century, such was the case of anthracnose. The disease can be caused by the species *C. fragariae*, (CF), *C. gloesporioides* (CG) and *C. acutatum* (CA), also affecting all organs, among them flower and fruit rot [18]. The following cultivars resistant to CF were developed: 'Florida Belle' [83]; to CF and CG 'Treasure' [84]; and resistant to CF and CA 'Pelican' [18] and at least one race of CG [85]; and resistant to CA, 'Flavorfest' [86]. Subsequent studies identified sources of resistance to CG in octoploid species [86, 87].

The same happened for FOF*,* resistant cultivars were detected in Korea [69], Japan [88]. Genetic resistance to FOF strains was detected in Mexico in cultivars from the United States and, also in *F. chiloensis* ssp. *pacifica* [89, 90] (**Figure 1**). The 'Ventana' cv is FOF resistant in California, USA [91]. In Mexico, there are selections in an intermediate stage of advance, which carry genes from both the cultivated species and *F. chiloensis* for resistance to FOF, adapted to the tropical high-altitude climate in Mexico [92] (**Table 1**; **Figure 2**). Recent investigations detected genotypes resistant to *Macrophomina phaseolina* in Australia and Egypt, although some are out of date cultivars [93, 94].


*The Genetic Diversity of Strawberry Species, the Underutilized Gene Pool and the Need… DOI: http://dx.doi.org/10.5772/intechopen.102962*

#### **Table 1.**

*Sources of disease resistance in F. x ananassa and other species of wild strawberry.*

Other diseases that attack foliage, flower, and fruit, caused mainly by fungi and a bacterium, are documented in **Table 1**. The damage due to *Botrytis cinerea* is globally important [15], and there have been detected although commercial cultivars are only moderately susceptible [95, 96], and apparently, the diversity and genetic variation for tolerance or resistance to the fungus is absent in the commercial species, which has

#### **Figure 1.**

*Root system from clones of Fragaria chiloensis ssp. pacifica showing different degrees of resistance, 60 days after being inoculated with fusarium oxysporum f. sp. fragariae.*

#### **Figure 2.**

*Comparison of an experimental clone carrying genes of F. chiloensis resistant to fusarium oxysporum f. sp. fragariae, and the viral complex of Irapuato, Gto. (central furrow), and two susceptible genotypes (left and right furrows).*

*The Genetic Diversity of Strawberry Species, the Underutilized Gene Pool and the Need… DOI: http://dx.doi.org/10.5772/intechopen.102962*

been an impediment to develop tolerant cultivars. However, in this century sources of genetic resistance were identified in progenitor species of cultivated strawberry and in a diploid species [97, 98] (**Table 1**).

The case of the bacterium *Xanthomonas fragariae* is quite similar to that of *Botrytis*, except that until recently resistance was detected in *F. virginiana*, since no sources of resistance were found in the commercial species [99, 100]. Additionally, evidence of immunity genes found in *F. moschata* [100–102], and resistance genes in the diploids *F. nilgerrensis* and *F. vesca* f. *alba,* and 'Illa Martin' and in the diploid *F. pentaphylla* [102], but not in *F. nilgerrensis*, *F. daltoniana*, *F. iinumae*, *F. vesca*, *F. viridis*, *F. gracilis*; *F. nubicola* and *F. orientalis* [101].

For other foliage diseases such as powdery mildew [103, 104], *Mycosphaerella fragariae* and *Diplocarpon earliana* [11, 105]; since the previous century, resistance genes were found in cultivated species and in *F. virginiana*, and tolerant or resistant cultivars were developed. Subsequently, knowledge has been enriched with the detection of resistance genes in other octoploid and diploid species [106–108] (**Table 1**).

A scientifically important and economically transcendental case, for the strawberry industry in California, during the twentieth century, was the practical demonstration that genetic tolerance was the best alternative to avoid economic losses, caused by the yellowing viral complex [109]. Around 1945, the University of California released the cultivars 'Shasta' and 'Lassen', tolerant to the viral complex. This event marked the beginning of the Era of the formation of cultivars with high yield potential and sensory quality of strawberry, adapted to the subtropical climate of California. The tolerance genes introduced in these cultivars were derived from a cultivar called 'Ettersburg 121', which had within its ancestors' genes from *Fragaria chiloensis* of the central coast of California, USA (**Table 1**) [73, 74]. In Irapuato, Gto., Mexico, there are advanced clones with tolerance to the local viral complex, which carry genes of *F. chiloensis* ssp. *pacifica* from California USA, [92] (**Figure 3**).

#### **3.2 Pest resistance**

Pests of greatest global importance and causing major economic damage, are the two-spotted spider mite (*Tetranychus urticae*), lygus bug (*L. lineolaris*, *L. hesperus*) and possibly other genus of bugs, in addition to thrips (*Frankliniella occidentalis*). The aphid *Chaetosiphon* spp. is a pest that transmits at least five viruses and for that reason, it is also important [15].

The genetic improvement in strawberries for tolerance or resistance to some of these pests was almost null in the previous century, for several reasons. Partly because of the availability of synthetic pesticides, which at first allowed easy control. Also due to the technical difficulty, time invested and economic cost of maintaining a genetic improvement program to achieve this objective, and in another, because there was a lot of pressure to develop cultivars with high yield potential and good sensory quality, even if they were susceptible to the most important pests of the crop.

Despite this unfavorable environment, there were pioneering scientists in spider mite and aphid resistance research. By far, the two-spotted spider mite has always been the main pest of strawberries and for this reason, the first studies evaluated the reaction of cultivars of the time to the mite. Experience showed that it developed larger populations on certain genotypes, which confirmed the presence of genetic variation in the host, with various degrees of damage, from tolerant to susceptible [110].

#### **Figure 3.**

*Comparison between a resistant clone (upper furrow), and a susceptible one to the viral complex present in Irapuato's region.*

A survey with a greater number of cultivars and clones of octoploid species, allowed us to locate sources of resistance in the cultivated species, in *F. chiloensis* from North America [111, 112] and from South America and in *F. virginiana* [113]. However, an important aspect was that in the wild clones of both octoploid species, a higher level of resistance was identified than in the cultivars. In addition, and very important, is that some *F. chiloensis* clones, that were resistant to the spider mite, were also resistant to the aphid *C. fragaefolii*, which is a vector of at least five economically important viruses (**Table 2**).

For the other pests of global importance, genetic variation is generally mentioned at the cultivar level, and this is the case of *Lygus* spp. [114] and *Frankliniella occidentalis* [115], although the information available for both pests is still very limited, and many aspects of the host–parasite relationship are unknown.

An outstanding case is a problem with the oriental fly *Drosophila suzukii*, a pest that has spread throughout the main strawberry-producing countries, which parasitizes other small fruits as well, in which the damage could be considered in the future. A study carried out in Germany, with octoploid and diploid genotypes, identified that in the diploid *F. vesca* certain clones had a low mosquito emergence [17] (**Table 2**).

#### **3.3 Outstanding characters of adaptation and resistance to abiotic factors**

In this section, a series of agronomic attributes are presented and discussed, which allow the plant a better adaptation to the environment and/or mitigate its adverse effect on it and eventually result in higher productivity and quality of the fruit and therefore are attributed with high economic importance.

*The Genetic Diversity of Strawberry Species, the Underutilized Gene Pool and the Need… DOI: http://dx.doi.org/10.5772/intechopen.102962*


#### **Table 2.**

*Sources of resistance to pests of global importance in F. x ananassa and other species of wild strawberry.*

Among the 11 listed attributes, those related to wide adaptation, low chilling requirements, resistance to low temperatures and versatility for different photoperiod regimes, have had primary importance with the evolution under cultivation of the octoploid strawberry and consequently with the already cited adaptation to environments as contrasting as the duration of the photoperiod, temperature regime, coldchilling needs, rainfall, soil texture, etc. [11, 74, 116–120].

Previous characters present in one, or both, octoploid parental species of cultivated strawberry were surely transferred to it during the synthesis of both species in Europe in the seventeenth century, as well as with the introduction of these ancient European cultivars into the USA, and its numerous introgressions of *F. chiloensis* and *F. virginiana* genes, by amateur breeders in the United States during the eighteenth century, which originated a multitude of cultivars more rustic and adaptable to the environments of that country, where the availability at first of short-day cultivars, over the years and due to this introgression, originated long-day octoploid cultivars [11] and later, as explained below, the day-neutral cultivars. Other sources of cyclical flowering have been documented in *F. virginiana* [118, 119].

One of the classic examples of the impact on the strawberry industry is the incorporation of genes that confer the day-neutral character and allow continuous flowering in the subtropical environment. The original source of the day-neutral character was found in *F. virginiana* ssp. *glauca,* in a plant collected in Utah [74]. These genes were introduced through the cultivar 'Shasta' and by means of a carefully modified backcross program, after three cycles of crossing and selection, the first cultivars with the day-neutral trait were released in 1979. In contemporary times most day-neutral cultivars carry those genes transferred by Bringhurst [4, 9].

Resistance of the plant and its different organs to low temperatures is another attribute, which allows minimizing the damage with temperatures below 0°C and is crucial to mitigate the damage of these organs. In the tropical climate at altitudes above 1500 meters above sea level, temperatures below the mentioned threshold can cause large yield losses during autumn and winter in cultivation without a macro-tunnel. However, the fore effect is probably maximized by the sudden increase in temperature up to 25°C in three hours, so this wide thermal oscillation could be the cause of the damage to the plant, flower, and fruit. Among the genotypes sown in Mexico, it has been


#### **Table 3.**

*Genetic diversity for traits associated with abiotic factors of global importance in F. x ananassa and other species of wild strawberry.*

observed that the most susceptible to this thermal shock are the day-neutral cultivars of California, compared to the short-day cultivars of California and Florida, respectively.

There are reports about sources of resistance to low temperatures in the progenitor species of the cultivated strawberry [6, 11], and also in some diplo, tetra and
