The genetics and gene action conferring downy mildew resistance was revealed among the populations derived from Indian slicing cucumbers, from the parental source comprised of resistant parent IIHR-438 and moderately resistant parent IIHR-433 and two susceptible parents Swarna Agethi and IIHR-431. The two cross Swarna Agethi × IIHR-438 and IIHR-431 × IIHR-433 combination was made and develop six generations (P1, P2, F1, F2, BC1 and BC2) and parents, filial, backcross populations was deployed for genetics. The Mendelian segregation in F2 and back cross suggested that resistance is governed by two pair of recessive genes i.e. inhibitory recessive epistasis (3 resistant: 13 susceptible) in both the cross. IIHR-438 possess higher degree of resistance and epistatic interaction (additive × dominance) was greater importance than main effect. These genetic finding on downy mildew resistance infers to advance the reciprocal recurrent selection breeding method followed by pedigree selection for breeding of downy mildew resistance cucumbers effectively.
Research Article
Genetic analysis of downy mildew resistance in Indian slicing cucumber (Cucumis sativus L.)
https://doi.org/10.21203/rs.3.rs-3897107/v1
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Cucumber
Downy mildew
Genetics
Resistance
In India, cucumbers (Cucumis sativus L.) are among the most widely grown and commercially significant summer vegetables. It's a perfect summer vegetable crop that's mostly grown for its delicious, soft cucumber slices, which are great in salads, pickles, desserts, and cooked vegetables (Tatlioglu,1993). India produced 1.09 million tonnes (mt) of cucumbers overall on 17,000 hectares of land (NHB, 2021). Cucumber production was 2.17 mha worldwide, yielding 83.60 mt annually (FAO, 2021). Commercially grown cucumber cultivars and hybrids are vulnerable to various diseases, one of which being downy mildew, a serious foliar disease found on every continent in the Northern and Southern hemispheres (Call et al., 2012; Lebeda and Cohen, 2011). Although the disease's occurrence on cucumbers was noted as early as the 19th century, it wasn't discovered to be a major cause of disease incidence on an economically important scale until the mid-1980s (Colucci et al., 2006). The disease downy mildew is caused by the oomycete pathogen Pseudoperonospora cubensis (Berk. & Curt.) Rostov., it has wide host range of more than 60 vegetable species belongs to the 20 genera in the Cucurbitaceae family (Lebeda, 1992; Lebeda and Cohen, 2011). Six pathotypes of Pseudoperonospora cubensis (Berk. & Curt.) Rostov. have been reported across the worldwide based on their compatibility with specific host range of genus Cucurbitaceae (Cohen et al., 2003).
Downy mildew epidemics affecting the genus Cucumis have been reported in more than 70 countries globally, with output losses estimated to be between 60 and 70 per cent (Palti, 1974; Cohen, 1981), and in India the yield loss reported was 70–75 per cent (Bommesh et al., 2018). Where,leaf moisture content and daytime temperatures of 25 to 30°C and nighttime temperatures of 10 to 15°C are favorable factors for the occurrence of downy mildew (Palti and Cohen, 1980), whereas the incubation period under natural (field) settings varies from 4 to 12 days, depending on the host genotype and climate (Lebeda, 1997), lesions on the upper side of the leaf and finally angular leaf patches covering the entire plant are the symptoms. For this disease, there are no consistent sources of resistance in the commercial cultivars of cucumbers that are now available, nor have there been any established racial interactions. (Lebeda et. al., 2006).
It has been suggested that there are many inheritance patterns for downy mildew disease resistance includes three recessive genes (Doruchowski and Lakowska-Ryk, 1992, Call, 2012, Kozik et al., 2013), three partially dominant genes, an interaction between recessive resistance genes and dominant susceptible genes (Badr and Mohamed, 1998; El-Hafaz et al., 1990), one or two incompletely dominant genes (Petrov et al., 2000), a single recessive gene (Angelov, 1994, Bhutia, 2015), and multiple recessive genes (Criswell et al., 2010). Several researchers have indicated that many genes with a single factor influence control the genetics of downy mildew resistance. Conflicting results with respect to inheritance of downy mildew resistance in cucumber were likely due to the factors like multiple pathotypes and races of pathogen, environmental conditions, source of resistance and different mechanisms of resistance may have different inheritance patterns (Criswell et. al., 2010). One such method that sheds light on the kind and scope of the relevant gene effects is generation mean analysis (Hayman, 1958).
Since, downy mildew is the most destructive disease in Indian cucumber growing locations, it is imperative to develop varieties and hybrids that are resistant to the disease. At ICAR-IIHR in Bengaluru, India, efforts are being made in this regard to create cucumber hybrids and cultivars that are resistant to downy mildew. There are 41 cucumber germplasm and advanced breeding lines were assessed in 2018 both in natural and artificial epiphytic conditions (using the detached leaf test method). Among them genotype IIHR-438 has shown resistance reaction and genotype IIHR-433 had moderately resistant. In order to comprehend the genetics of resistance in these lines, the current experiment was conducted.
2.1. Plant population development
The experiment was conducted at Vegetable Block of ICAR-Indian Institute of Horticultural Research, Bengaluru. The materials for the study comprised of a downy mildew resistant inbred lines IIHR-438 and moderately resistant IIHR-433. The Swarna Agethi is popular variety with high fruit quality in India but susceptible to downy mildew disease. The cross combination of Swarna Agethi (Susceptible, S) × IIHR-438 (Resistant, R), and IIHR-431(S) × IIHR-433 (Moderately resistant, MR) are made to develop the population for six generation mean analysis. The four progeny are F1, F2, BC1 (Back cross with resistant parent) & BC2 (Back cross with susceptible parent) along with parental lines (P1 and P2) and Susceptible check variety Pusa Uday were evaluated for disease reaction from August to October of the year 2020. The evaluation trial was laid out in a randomized complete block design with three replications. Each replication of P1, P2 and F1 consisted of 10 plants; BC1 and BC2 consisted of 15 plants each; and F2 consisted of 75 plants. All package of practices were followed to raise the good crop and proper care has been taken throughout the experiment
2.2. Disease scoring method
In response to downy mildew, a natural epiphytotic screening was conducted from August to October of 2020, a period of ideal weather for the disease's spread. Based on a 0–9 scale assessment of the severity of the disease's symptoms (Fig. 1) on the leaves at the top, median, and base of each individual plant, the incidence of the disease was recorded (Bommesh et. al., 2018). Susceptible check to ensure that the disease spread uniformly, Pusa Uday (S) was raised after every five rows in addition to along the border. A month following planting, disease scoring was initiated and continued every week for seven weeks, or until plant growth stopped. The mean percentage disease index (PDI) is computed using the following formula based on the individual plant data on leaf area damaged in all replications that were recorded
According to a scale developed by Bommesh et al. (2018), the disease reaction of a genotype was categorized into four groups based on PDI: resistant (R) 0–20%, moderately resistant (MR) 21–40%, susceptible (S) 41–60%, and highly susceptible (HS) > 60%.
2.3. Area under disease progression curve (AUDPC)
According to Jeger and Viljanen-Rollinson (2001), the AUDPC was calculated using the following formula based on the disease scores
$$\text{Area under disease progression curve}\text{ (}\text{AUDPC}\text{)}\text{=}{\sum }_{\text{i=1}}^{\text{n=1}}\left[\left\{\frac{{\text{X}}_{\text{i+1}}{\text{+ X}}_{\text{i+1}}}{\text{2}}\right\}x\left\{{\text{t}}_{\text{i}+1}-{\text{t}}_{\text{i}}\right\}\right]$$
Where,
Xi= disease index expressed as a portion at ith observation
ti = age of the plant at ith observation
n = total number of observations
2.4. Apparent rate of infection (AIR)
The rate at which an epidemic spread is gauged by the apparent rate of disease progression (r). Using the Van der Plank (1968) formula, disease incidence data collected at weekly intervals for seven weeks was utilized to determine the apparent infection rate.
r = 2.3/t2 − t1 {log (X2 (1 − X1 )/ X1 (1 − X2 ) )}
Where,
r is the apparent infection rate in non-logarithmic phase
X1 is the disease index at time t1
X2 is the disease index at subsequent week time t2
2.5. Classical Mendelian segregation pattern of resistance
In order to integrate the detected segregants for downy mildew resistance into a traditional Mendelian model, various Mendelian ratios were explored in this experiment. Based on the PDI ± SE of resistant and susceptible parents, individual plants in F2 and backcross progeny were divided into two major phenotypic classes: resistant and susceptible, based on their response to disease. As recommended by Panse and Sukhatme (1985), the total number of plants pertaining to the various classes was tallied and then subjected to chi-square analysis for goodness of fit to different classical Mendelian ratios.
2.6. Castle Wright estimation
The method provided by Castle and Wright (1921) was used to estimate the minimum number of effective genes contributing to the difference in quantitative trait between two inbred lines.
N = D2/ 8(VF2-VF1)
Where,
N = minimum number of gene pairs by which the parental lines differ,
D = Mean of larger parent - Mean of smaller parent,
VF2 = Variance of F2 population and
VF1 = Variance of F1 population.
2.7. Nature and magnitude of gene effects for downy mildew resistance
The generation mean analysis was carried out by the plant-specific PDI values. The scaling test that Hayman and Mather (1955) and Mather (1949) recommended was carried out. To estimate the various genetic components, a six-parameter model as proposed by Hayman (1958) and Jinks and Jones (1958) was used. Windostat Version 9.2 from Indostat Services, Hyderabad, India, was used for statistical analysis.
3.1 Mean performance of different generations
The sporangia inoculum in the host was maintained by planting test entries alongside the susceptible (Pusa uday) rows. Significant differences between the generations for both crosses are revealed by the analytical variance.
In the Swarna Agethi× IIHR-438 cross, the populations namely P1 (Swarna Agethi) (57.9), F1 (Swarna Agethi× IIHR-438) (54.4), F2 (45.1), BC1 (F1 × Swarna Agethi) (53.4), and check variety Pusa Uday (55.2) are shown susceptible reaction to downy mildew disease. On the other hand, P2 (IIHR-438) (9.6) showed a resistant reaction, whereas BC2 (F1 ×IIHR-438) (38.4) recorded a moderately resistant response (Table.1). In the second cross combination between IIHR-431(S) × IIHR-433 (MR), the highest mean PDI was found in BC1(IIHR-431 × IIHR-433) × IIHR-431) (62.12), then in F1(IIHR-431 × IIHR-433) (61.25), P1 (IIHR-431) (59.92), and finally Pusa uday (55.20). Among the generations, the P2 had the lowest mean PDI (23.40) with moderate resistance reaction, and they are all substantially distinct from one another (Table 2).
| Generations | Disease reaction | Mean PDI | AUDPC | Apparent infection rate (r) per unit per day | |||||
|---|---|---|---|---|---|---|---|---|---|
| 1 to 2 Week | 2 to 3 Week | 3 to 4 Week | 4 to 5 Week | 5 to 6 Week | Average ‘r’ | ||||
| P1 | S | 57.9(49.5) | 2450.91 | 0.85 (1.36) | 1.06 (1.43) | 1.15 (1.44) | 1.25 (1.50) | 1.29 (1.51) | 1.12 (1.45) |
| P2 | R | 9.6 (18.0) | 351.05 | 0 (1.00) | 0 (1.00) | 0.39 (1.18) | 0.43 (1.19) | 0.91 (1.38) | 0.34 (1.16) |
| F1 | S | 54.5 (47.5) | 2291.46 | 0.79 (1.34) | 0.94 (1.39) | 1.11 (1.45) | 1.26 (1.50) | 1.29 (1.51) | 1.07 (1.44) |
| F2 | S | 45.1 (42.2) | 1867.59 | 0.72 (1.31) | 0.86 (1.36) | 1.01 (1.41) | 1.18 (1.47) | 1.26 (1.50) | 1.00 (1.42) |
| BC1 | S | 53.4 (46.9) | 2229.68 | 0.85 (1.36) | 0.98 (1.41) | 1.08 (1.44) | 1.22 (1.49) | 1.29 (1.51) | 1.084 (1.44) |
| BC2 | MR | 38.4(38.3) | 1577.94 | 0.67 (1.29) | 0.82 (1.34) | 0.94 (1.39) | 1.11 (1.45) | 1.22 (1.49) | 0.952 (1.40) |
| Pusa Uday | S | 55.2(48.0) | 2570.09 | 0.7 (1.30) | 0.84 (1.35) | 1.16 (1.47) | 1.26 (1.50) | 1.31 (1.52) | 1.054 (1.43) |
| F value | 712.61** | 814.79** | 5636.17** | 991.49** | 507.69** | 508.26** | 110.54** | 438.7** | |
| SE (m) | 0.41 | 26.83 | 0.002 | 0.005 | 0.013 | 0.005 | 0.015 | 0.005 | |
| CD@5% | 1.29 | 82.66 | 0.005 | 0.015 | 0.041 | 0.015 | 0.015 | 0.015 | |
| CV (%) | 1.73 | 2.44 | 0.23 | 0.614 | 2.325 | 0.59 | 0.542 | 0.618 | |
| *Note. In mean PDI, the Values in the parentheses are arc sign transformed.In Apparent infection rate, the values in the parentheses are square root transformed. S, Susceptible; R, resistant; MR, moderately resistant; Susceptible parents Swarna Agethi, resistant parent IIHR-438, F1 (Swarna Agethi × IIHR-438) F2, | |||||||||
| BC1 (F1× Swarna Agethi) and BC2 (F1× IIHR-438) | |||||||||
| Treatments | Disease reaction | Mean PDI | AUDPC | Apparent infection rate (r) per unit per day | |||||
|---|---|---|---|---|---|---|---|---|---|
| 1 to 2 Week | 2 to 3 Week | 3 to 4 Week | 4 to 5 Week | 5 to 6 Week | Average ‘r’ | ||||
| P1 | S | 59.92 (50.70) | 2099.67 | 0.99 (1.41) | 1.13 (1.46) | 1.21 (1.48) | 1.26 (1.50) | 1.3 (1.52) | 1.18 (1.47) |
| P2 | MR | 23.4 (28.91) | 767.01 | 0.55 (1.24) | 0.69 (1.30) | 0.79 (1.33) | 1.05 (1.43) | 1.15 (1.46) | 0.84 (1.35) |
| F1 | S | 61.25 (51.48) | 2162.54 | 0.96 (1.40) | 1.12 (1.45) | 1.23 (1.49) | 1.28 (1.51) | 1.3 (1.52) | 1.18 (1.47) |
| F2 | S | 47.65 (43.63) | 1659.49 | 0.8 (1.32) | 1.02 (1.42) | 1.17 (1.47) | 1.22 (1.49) | 1.27 (1.50) | 1.1 (1.45) |
| BC1 | S | 62.12 (51.99) | 1933.02 | 0.95 (1.39) | 1.07 (1.44) | 1.18 (1.47) | 1.25 (1.50) | 1.29 (1.51) | 1.15 (1.46) |
| BC2 | S | 44 (41.53) | 1529.55 | 0.83 (1.35) | 1.01 (1.42) | 1.11 (1.45) | 1.19 (1.48) | 1.26 (1.50) | 1.08 (1.44) |
| Pusa uday | S | 55.2 (47.96) | 2570.09 | 0.7 (1.30) | 0.84 (1.39) | 1.16 (1.46) | 1.26 (1.50) | 1.31 (1.52) | 1.05 (1.43) |
| F value | 296.83** | 1321** | 155.5** | 136.53** | 184.86** | 24.37** | 14.44** | 61.2** | |
| SE (m) | 0.47 | 15.84 | 0.005 | 0.005 | 0.004 | 0.005 | 0.005 | 0.005 | |
| CD@5% | 1.48 | 49.36 | 0.015 | 0.016 | 0.012 | 0.017 | 0.015 | 0.017 | |
| CV (%) | 1.82 | 1.51 | 0.621 | 0.615 | 0.464 | 0.63 | 0.563 | 0.65 | |
| *Note. In mean PDI, the Values in the parentheses are arc sign transformed. In Apparent infection rate, the values in the parentheses are square root transformed. S, Susceptible; R, resistant; MR, moderately resistant; Susceptible parents IIHR-431, resistant parent IIHR-433, F1 (IIHR-431× IIHR-433), F2, BC1 (F1× IIHR-431) and BC2 (F1× IIHR-433) populations of the cross combinations IIHR-431× IIHR-433. | |||||||||
| Cross | F2 | Test cross with resistant parent | Test cross with susceptible parent | Minimum number of effective genes# | |||||
| 1:3 | 1:15 | 3:13 | 1:1 | 1:3 | 1:1 | 1:3 | |||
| Swarna Agethi× IIHR-438 | x2 | 3.88** | 64.62** | 0.04 | 0.56 | 22.40** | 45.00** | 15.00** | 2.1 |
| p | 0.05 | < 0.01 | 0.80–0.85 | 0.45–0.50 | < 0.01 | < 0.01 | < 0.01 | ||
| IIHR-431 × IIHR-433 | x2 | 3.55 | 68.00** | 0.096 | 1.25 | 5.34** | 39.00** | 13.00** | 2.4 |
| p | 0.05 | < 0.01 | 0.75–0.80 | 0.25–0.30 | < 0.01 | < 0.01 | < 0.01 | ||
| #Castle and Wright (1921) **p = 0.01, | |||||||||
In the Swarna Agethi × IIHR-438 cross, the AUDPC ranged from 351.05 to 2450.91 in different generations, susceptible check variety Pusa Uday recorded highest value of 2570.09. Among the generations P1 (2450.91) recorded highest AUDPC, with F1 (Swarna Agethi × IIHR-438) (2291.46), F2 (1867.59) and BC1 (F1 × Swarna Agethi) (2229.68). The P2 (IIHR-438) exhibited lowest AUDPC (351.05) followed by BC2 (F1 × IIHR-438) (1577.94) generations are significantly differing from each other. In the second cross of (IIHR-431×IIHR-433) moderately resistant parent IIHR-433 recorded least AUDPC value (767.01) and other parent (IIHR-433) had AUDPC of 2099.67, but check variety had highest value of 2570.09. Among their progenies, F1 (IIHR-431 × IIHR-433) possessed highest AUDPC (2162.54) followed by BC1 (F1 ×IIHR-431) (1933.02), BC2 (F1 ×IIHR-433) (1529.55) and F2 (1659.49).
In the Swarna Agethi × IIHR-433 cross, the highest average apparent infection rate (AIR) was observed in the susceptible parent Swarna Agethi (1.12) followed by F1 (Swarna Agethi × IIHR-433)(1.07), BC1 (F1×Swarna Agethi )(1.08) and check variety Pusa Uday (1.05) population and least was recorded in resistant parent IIHR-438 (0.34). The high rate of infection was observed at second week (1.06) in Swarna Agethi, and it further reached to 1.29 (AIR) during fifth to sixth week of observation and same trend of infection was observed in F1 (Swarna Agethi × IIHR-433) and BC1 (F1×Swarna Agethi). Among the back cross populations BC2 (F1 × IIHR-438) recorded less average AIR of 0.95. The IIHR-438 recorded least pathogen infection rate till third to fourth week (0.39) of disease scoring recorded, but it was reached to 0.91 at final observation (sixth week) with average AIR was 0.34.
In the IIHR-431 × IIHR-433 cross, the parent IIHR-431 had the highest average AIR (1.18). The high infection rate was noticed in the second week and increased to 1.30 (AIR) in the sixth and seventh weeks of observation. The F1 population attained an average AIR of 1.18 throughout the same week intervals, while the check variety recorded the highest AIR of 1.31 during the fifth and sixth week, with an average AIR of 1.30 (AIR). The IIHR-433 showed the lowest apparent infection rate up until the third or fourth week of illness scoring; however, by the sixth week of the final observation, it had increased to 1.15 with an average of 0.84. The average AIR for the back cross population BC1 and BC2 were 1.08 and 1.15, respectively.
3.2. Classical Mendelian segregation pattern of resistance
Employing traditional Mendelian paradigm, the genetics of downy mildew resistance was studied, limiting the plants to two classes of disease response: susceptible (susceptible and highly susceptible reaction) and resistant (resistant and moderately resistant reaction) progeny of the two crosses in order to determine how well they fit different traditional Mendelian ratios (Table.3). In Swarna Agethi × IIHR-438 cross, the number of resistant and susceptible plants found in F2 was 43 and 182, respectively. Among the various Mendelian ratios examined, the F2 population separated into a 3:13 ratio (resistant: susceptible) (χ2 = 0.04; p = 0.80), suggesting that in this cross, resistance to downy mildew is conferred by two pairs of recessive alleles. The back cross investigation provided additional validation for the inheritance pattern. The segregation ratio in the test cross with the resistant parent had the best match, with a ratio of 1:1 (χ2 = 0.56; p = 0.45–0.50). All of the measured ratios were discovered in the test cross with susceptible parent was no goodness of fit.
In the cross IIHR-431 × IIHR-433, the resistant and susceptible plants observed in F2 were 44 and 181 respectively. Of various Mendelian ratios tested F2 population segregated in 3 (resistant): 13 (susceptible) (χ2 = 0.09; p = 0.75), indicating that two pair of recessive genes confers resistance to downy mildew in this cross. However, in test cross with resistant parent, the segregation ratio had the best fit with 1:1 (χ2 = 1.25; p = 0.25–0.30) not with 1:3. Both the 1:1 and 1:3 ratios evaluated in the test cross with sensitive parent were determined to be significant, indicating no goodness of fit. Consequently, the data do not support the notion that straight forward Mendelian segregation occurred in this cross, but rather point to the possibility that a main recessive gene and the interaction of smaller background genes are responsible for regulating the downy mildew resistance in cucumbers in both cross combinations.
3.3. Nature and magnitude of gene effects for downy mildew resistance
The purpose of providing the means for six generations in two crosses (Swarna Agethi × IIHR-438 and IIHR-431 × IIHR-433) is mainly to compare the means of these populations with one another, indicating that there is enough diversity between the two crosses under study. Table 4 presents the findings from the scaling tests. The results of the scaling test demonstrated that the simple additive-dominance model was inadequate and that there was epistasis in both crosses.
| Cross | Swarna Agethi × IIHR-438 | IIHR-431 × IIHR-433 |
|---|---|---|
| A | 5.56*±2.53 | -3.07 ± 2.96 |
| B | -12.79 **± 3.26 | -3.35 ± 3.36 |
| C | -4.01 ± 3.81 | 19.69**±3.78 |
| D | -1.60 ± 2.46 | -12.78**±2.48 |
| m̂ | 45.14** ± 0.81 | 46.69**±0.72 |
| d̂ | 14.97**±1.86 | 18.12**±2.03 |
| ĥ | 23.96**±4.98 | 45.08**±5.12 |
| î | 3.20 ± 4.88 | 25.49**±4.97 |
| ĵ | -18.35** ±3.96 | -0.29 ± 4.22 |
| l̂ | -10.43 ± 8.23 | -31.92**±8.94 |
| Type ofepistasis | Duplicate | Duplicate |
| **p = 0.01, *p = 0.05 | ||
For both crosses, the nonallelic interaction was estimated using the Heyman (1958) approach. The estimates of the various genetic components, namely mean (m), additive (d), dominance (h), additive × additive (i), additive × dominance (j), and dominance × dominance (l), are displayed in Table 4. Among all the gene effects in the cross Swarna Agethi × IIHR-438, the additive x dominance component had a much greater amplitude for resistance and a negative direction. The duplicate form of epistasis was revealed by the opposite signs of the (h) and (l) components, and according to Castle Wright's assessment, there were 2.1 effective genes influencing resistance. The inheritance pattern of the resistance gene appears to be caused by additive and dominant effects, digenic nonallelic interactions, and epistatic interaction (additive × dominance). Due to its negative value, the dominance × dominance (l) component in the cross IIHR-431 × IIHR-433 had a substantially larger amplitude for resistance among all the gene effects. Duplicate epistasis was shown by the opposite signs of the (h) and (l) components, and there were 2.4 effective genes regulating resistance. The findings showed that the inheritance of resistance genes was caused by both additive and nonadditive factors.
Downy mildew is the most common foliar disease of cucumbers in the globe. Whenever there is a severe infection, the plant dies within 6 to 12 days after the onset of symptoms, depending on the host genotype, inoculum concentration, and weather (Lebeda, 1990). Fungicide-based disease control lost some of its effectiveness on particularly vulnerable types or in situations where the disease pressure was too great. Therefore, in order to create varieties resistant to the downy mildew disease, breeders are urged to use resistant breeding. Diverse pathotypes and races of the pathogen, environmental factors, sources of resistance, and distinct resistance mechanisms may have different inheritance patterns, all contribute to the contradictory findings regarding the inheritance of downy mildew resistance in cucumbers (Criswell et. al., 2010), that sheds light on the kind and scope of the relevant gene effects is generation mean analysis (Hayman, 1958).
There is evidence to imply that downy mildew is a hereditary trait based on the progeny of different parental lines exhibiting resistance or vulnerability to it. While the mean PDI of all generations in the Swarna Agethi × IIHR-438 cross fell within the parental range and in the IIHR-431 × IIHR-433 cross, both F1 and BC1 are marginally higher PDI than the susceptible parent. In BC2 population of both cross, the mean illness ratings of the backcrosses were lesser than those of the susceptible parent but closer than those of the resistant parent. In comparison to P1, F1, BC1, and checks, the generations P2, F2, and BC2 showed slower illness progression, lower AUDPC, and lower PDI (%) (Fig. 2 and Fig. 3). These generations have a remarkably slow rate of disease development, which is practically significant since it allows them to withstand an epidemic without become infected for an extended period of time and gives other disease control techniques more time to be implemented. They may also help to significantly reduce chemical inputs and can withstand a delayed chemical spray interval. Previous research on downy mildew in cucumbers showed that the illness progressed more slowly (Call, 2012, Bhutia et al., 2015).
The apparent infection rate (AIR) values in both crosses fluctuated and occasionally did not hold steady over several generations. The AIR increased as the plant grew, reach its maximum in the P1, BC1, and F1 populations. Determining the resistance level in genotypes with plant age will be made easier with the use of this information. The susceptibility of the population and favorable environmental conditions that may favor the development of disease can be ascribed to the severity of the disease (Patil 1997).
The test cross with the resistant parent segregating according to the ratio 1:1 further examined the digenic recessive model (inhibitory recessive epistasis), as the F2 populations in both crosses followed the 3:13 ratio (3 resistant: 13 susceptible). Resistance to downy mildew disease was a recessive trait in both crosses, whereas susceptibility to the illness was entirely dominant. The inheritance of the resistant gene was controlled by two sets of genes interacting in a dominant and recessive manner, according to the segregation pattern. This hypothesis was validated by backcrosses with the resistant parent, yielding a ratio of 1 susceptible to 1 resistant. It appears that two gene pairs may interact, maybe in an epistastic manner, where resistance is conferred by two non-allelic resistance genes; also, one dominant gene may be dominant over the other and one recessive gene may be dominant over the other. Additional findings from the chi-square analysis were validated by the Castle Wright estimate of the minimal number of functional genes. Previous research on cucumber plants ((Bhutia 2015, Kozik et al. 2013), Call (2012), Criswell et al. (2010), and Petrov et al. (2000)) revealed a pattern of inheritance akin to that of downy mildew disease.
Understanding the kind and extent of gene effects present in breeding material is required before deciding on the type of breeding procedure to be employed. The inheritance of disease resistance was found to be caused by epistatic interaction (additive x dominance) and additive and dominance effects, which are types of digenic nonallelic interactions. These findings were obtained from the Swarna Agethi × IIHR-438 cross. All other components were positive and contributed to susceptibility rather than resistance, with the exception of the additive x dominance component. Given that the epistatic interaction was more significant than the primary effects, the best breeding strategy in this case seems to be a reciprocal recurrent selection strategy followed by the pedigree technique of selection.
The inheritance of resistance in the IIHR-431 × IIHR-433 cross additive was caused by dominance, additive x additive, and dominance x dominance effects. Only dominance x dominance effects, however, made a significant contribution to resistance. Selection must be postponed since selection in the previous generations would not be successful in enhancing resistance because the quantity of dominance x dominance epistasis was in the direction of resistance and duplication epistasis was predominating in this cross. Therefore, recurrent selection breeding would be a suitable approach. According to Badr and Mohamed (1998), two pairs of dominant and recessive interaction genes (13 susceptible: 3 resistant) in the cucumber resistance line of TX302 regulate resistance to downy mildew. Downy mildew resistance in cucumbers Palmetto and Yomaki types is regulated by either dominant suppression epistasis or inhibitory recessive epistasis (El-Hafaz et al. 1990). Compared to IIHR-438 and IIHR-433, IIHR-438 appears to be more resistant to epistasis inheritance, making it a potential candidate for breeders. At now, endeavors are underway to map this resistance in order to facilitate marker-assisted selection.
Author Contribution
J.C.B.- Overall planing and conducted the whole research workD.C.M.- collected the cucumber germplasm, collection of observations and manuscript preparation M.P.- Supervision of whole research work and layout for research experiments, laboratory facility provided to carryout the workK.V.R- overall monitored the breeding work and selection of design for genetic studies. D.H.M- data compilation and Statistical analysis of generation mean analysisS.S.- he has monitored the disease incidence in field experiment and also in plant pathology lab
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No competing interests reported.