**2.7 Downy mildew**

*Cucumber Economic Values and Its Cultivation and Breeding*

**2.5 Inheritance for resistance to diseases**

genes and environment aspects are required.

**2.6 Powdery mildew**

number of fruit per plant are directly related to yield. The other traits like shape index (length: diameter and length/stalk ratio), fruit skin (size of spines, dull or uniform fruit color) and fruit bitterness decides the market value of fruits which are important particularly for slicing cucumber [51]. A single recessive gene controls fragrance in cucumber [52], fruit color traits were controlled by two major genes, single recessive gene, (*w*), was identified that controls white immature fruit color [53], single recessive gene controlled the inheritance of quantity of beta-carotene [54]. Green flesh color is one of the most important and commercial trait of cucumber fruit was controlled by a major effect QTL [55]. The bent characters of the fruit were quantitative inheritance controlled by multiple genes and major genes [56]. Twin fused fruits were controlled by single recessive gene [57]. Two loci controlling fruit bitterness in cucumber [24–26]. The QTLs were also identified in cucumber for several economic traits including fruit spine, skin colors, fruit netting, fruit size, hollow size, flesh thickness variation [58], fruit carpel number, sex expression, fruit length, fruit diameter, fruit shape, fruit number, and powdery mildew resistance

Several diseases infect cucumber which results in huge yield losses in production. The common disease infecting cucumber is powdery mildew, downy mildew, anthracnose, scab, angular leaf spot and virus diseases (cucumber mosaic virus (CMV), papaya ring spot virus, (PRSV), zucchini yellows mosaic virus). The management of these diseases increase the cost of production because of increasing use of chemicals for control of diseases which also has adverse implication on environment and ecology. The development of resistant sources and breeding of resistant varieties are economical, healthier and eco-friendly approach. There are different approaches for breeding resistance against different kinds of stresses which involve both conventional and modern tools. Achieving the goal of development of a resistant variety also needs attention on the durability of resistance for which the knowledge of resistance on the inheritance, expression and interaction with related

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

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and region.

[28, 29].

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 from PI 197088 and PI 330628.
