Dominant Variance Has an Important Role in Downy Mildew Resistance in Cucumber Jamal-Ali Olfati 1* , Habibollah Samizadeh 2 , Gholam-Ali Peyvast 1 , S. Akbar Khodaparast 3 , and Babak Rabiei 2 1 Department of Horticultural Science, University of Guilan, Rasht, Iran 2 Department of Agronomy, University of Guilan, Rasht, Iran 3 Department of Plant Pathology, University of Guilan, Rasht, Iran *Corresponding author: jamalaliolfati@gmail.com Received February 23, 2010 / Accepted April 15, 2011 Korean Society for Horticultural Science and Springer 2011 Abstract. Downy mildew inflicts severe damage on cucurbits in humid areas of production throughout the world. The genetics of resistance to downy mildew (Pseudoperonospora cubensis) in the cucumber (Cucumis sativus L.) was studied by the means of a half diallel table between 6 inbred lines. Decomposition of dominance variation indicated that the dominance effect was not unidirectional and that dominant genes were not uniformly distributed among the parents. These facts were confirmed and using a further detailed Hayman’s graphical analysis. Narrow-sense heritability estimates were 0.42 so the breeders are able to use selection for this trait. In other hands the heterosis in some crosses was high so breeders are able to use specific cross to produce hybrid with high level resistance. Additional key words: Cucumis sativus, diallel, hymen analysis, inheritance, Pseudoperonospora cubensis Hort. Environ. Biotechnol. 52(4):422-426. 2011. DOI 10.1007/s13580-011-0049-0 Research Report Cucurbit downy mildew is an obligate parasite, with the rare exception of oospore production that can only survive and reproduce on living host tissue. To minimize losses from Pseudoperonospora cubensis it is advisable to plant resistant varieties, usage of biocontrol agents or use of protective fungicides. Determining the inheritance of cucumber ( Cucumis sativus L.) resistance to downy mildew ( Pseudoperonospora cubensis) has been the subject of research for the past 70 years. There are several proposed inheritance patterns for resistance to downy mildew as follows: three recessive genes (Doruchowski and Lakowska-Ryk, 1992; Shimizu et al., 1963); three partially dominant genes (Pershin et al., 1988); interaction between dominant susceptible and recessive resistance genes (Badr and Mohamed, 1998; El-Hafaz et al., 1990); one or two in- completely dominant genes (Petrov et al., 2000); and finally, a single recessive gene (Angelov, 1994; Fanourakis and Simon, 1987; Van Vliet and Meysing, 1974, 1976). Conflicting results regarding the expression and inheritance of downy mildew resistance in cucumber is likely due to four main factors. First, the pathogen is highly variable and populations have not been sufficiently studied to have a full understanding of virulence factors (Lebeda and Urban, 2004). Multiple pathotypes and races have been identified (Lebeda and Widrlechner, 2003) in some cases more than one path- otype in a geographical region has been determined (Lebeda and Urban, 2004). Different races have been reported (Angelov et al., 2000; Epps and Barnes, 1952; Hughes and Van Haltern, 1952; Shetty et al., 2002) and there are likely different genes involved in resistance to different races, if a gene for gene interaction exists. Environment is the second factor that plays a role in pathogen virulence. Temperature, humidity, rainfall and inoculums concentration all influence the severity of cucumber downy mildew (Cohen, 1977). Interactions among pathogen, host and environment are complex and not easily elucidated. A third factor is the differing mechanisms of resistance. Different mechanisms of resistance have been proposed (Angelov and Krasteva, 2000; Baines, 1991; Barnes and Epps, 1950, 1954; Palti and Cohen, 1980; Tarakanov et al., 1988). Through previously mentioned inheritance studies, a number of mechanisms of resistance for cucumber downy mildew were examined. For rating the resistance of cucumber to downy mildew pathogen, Doruchowski and Lakowska-Ryk (1992) used necrotic lesions, Van Vliet and Meysing (1974, 1976) and El Hafaz et al. (1990) used sporulation intensity, Fanourakis and Simon (1987) used incidence of chlorotic