**2.6 Powdery mildew**

It is one of the most severe foliar diseases in cucumbers but the inheritance of its resistance still remains unclear which is the major bottle-neck in the development of improved cucumber cultivars. Several fungicides are available to control this disease but their application increases environmental burdens and financial pressures on growers [59]. Therefore, development of the suitable genotypes resistance to powdery mildew is need of the hours. Several resistant cucumber cultivars have been identified, are only resistant to powdery mildew, such as PI 250147 [60], S06 [61], and SSSL0.7 [62]. The resistance in the Puerto Rico 37 variety was controlled by multiple recessive genes [63], PM resistance in the PI 197087 variety was controlled by 1–2 major and 1–2 minor genes [64], one and two recessive genes in PI 2008151 and Natsufushinari, respectively [65], major recessive gene (s) and a dominant gene (R) and a dominant suppressor gene (I) in P1212233 and P123514, [66, 67]. The genetic loci conferring resistance to PM have been gradually identified *pm-1, pm-2, pm-3* and *pm-h* loci [60]. A dominantly inherited PM resistance gene, *Pm1.1* present in Chinese Long line Jin5–508, which has been mapped. Most resistant cucumber cultivars become susceptible to PM at low temperatures. Collectively, the inheritance of cucumber PM resistance is complex and affected by temperature and region.

**205**

*Classical Genetics and Traditional Breeding in Cucumber (*Cucumis sativus *L.)*

Understanding the inheritance of downy mildew is fundamental to successful cucumber breeding programs. Downy mildew resistance is controlled by a single recessive gene [68, 69], pair of dominant and recessive interacting genes [70], two recessive genes in PI 197088 and a single recessive gene in 'Poinsett' [71], three recessive genes, *dm-1*, *dm-2* and *dm-3* [72], two incompletely dominant genes [73], inheritance for downey mildew resistance is quantitative [24–26, 74], number of genes, dominant, partially dominant, or recessive are responsible for resistance [75], multiple recessive genes [76]. The cucumber accession PI 197087 from India and its derivatives like Gy14 are resistant to downy mildew that is conferred by *dm-1.* The *dm-1* conferred DM resistant was less effective since 2004 when new DM pathogen strains emerged. The different results for inheritance of downy mildew resistance may be due to some factors like pathogen variability, environmental factors (temperature, relative humidity, inoculum movement etc.), mechanism of resistance and source of resistance. Recently, two major-effect QTLs for resistance to DM strain (*dm4.1* and *dm5.2*) were identified

Chemical treatments against cucumber mosaic virus (CMV) are not effective; therefore genetic resistance is considered the primary line of defense. Genetic analysis revealed that CMV resistance in cucumber is quantitatively inherited [77] where as in *Cucumis sativus* var. *hardwickii* (wild relative) is controlled by single recessive gene [78]. The inheritance of resistance to zucchini yellow mosaic virus in cucumber is controlled by single recessive gene (). The genes resistant to viruses has been mapped on chromosome 6 including *psrv* for PSRV, *wmv* for WMV and *cmv6.1* for CMV [77, 79, 80]. Recently, cucurbit aphid-borne yellows virus (CABYV) has

The development of high yielding cultivars with better fruit quality is the major goal of cucumber breeding programs across the globe. Due to the introgression of gynoecious sex form in the background of different cucumber market classes, the development of varieties with earliness characters along with high female: male flowers ratio is also an important objective for cucumber breeding. Therefore, rapid advancement has been made in the development of gynoecious and parthenocarpic cucumber lines/varieties suitable for green house cultivation. Cucumber breeders have developed several genetic stocks for pest and disease resistance. The introgression of pest and disease resistance gene from available sources should be the prime goal for most of the cucumber breeding programmes in future for the development of multiple disease resistance (MDR) varieties including virus resistance. The various fruit quality traits have been selected in cucumber depending on regional preference of the consumers. These quality traits include fruit size, shape (length and diameter), color (light green, dark green), shelf life, seediness and nutritional quality. Cucumber breeders have combined several sex expression and fruit quality genes to develop desirable genotypes. Many pickling cucumber hybrids were developed by crossing gynoecious lines with monoecious lines. These hybrids are predominantly gynoecious in nature which provides pollens for fruit set which enhances

*DOI: http://dx.doi.org/10.5772/intechopen.97593*

**2.7 Downy mildew**

from PI 197088 and PI 330628.

**3. Traditional breeding**

been reported in cucumber from India [81].

**3.1 Traditional breeding objectives and achievements**

**2.8 Virus diseases**

*Classical Genetics and Traditional Breeding in Cucumber (*Cucumis sativus *L.) DOI: http://dx.doi.org/10.5772/intechopen.97593*
