Inheritance of gummy stem blight resistance in bottle gourd

Introduction

Gummy stem blight caused by Didymella bryoniae has emerged as a major limiting factor of bottle gourd cultivation in India because, the disease severity varies from season to season and limited success has been achieved in controlling GSB with chemical methods. Therefore, the best method for overcoming GSB in bottle gourd is to develop more resistant cultivars by molecular breeding. To date, there are no reports on molecular markers and mapping for use in breeding GSB resistance in bottle gourd. However, some works have been reported in melons and cucumber. GSB resistance reactions and mechanisms of resistance are truly based on location and environmental specific. Many scientists have reported that different types of gene action have been attributed to GSB resistance in cucurbits, including single dominance gene (Joseph et al., 2007) [6] , single recessive gene (Hassan et al., 2018) [4] and one study indicated that resistance to GSB was quantitative and mainly governed by three pairs of additive epistatic major genes (Zhang et. al., 2017) [11] . A study indicated that the resistance to GSB in cucumber seedlings was quantitative and mainly governed by two pairs of major QTLs and multiple minor QTLs (Liu et al. 2017) [8] and one report suggested that resistance to GSB may be under the control of a more complex genetic system (Gusmini et. al., 2017) [1] . Nevertheless, the inheritance of GSB resistance in bottle gourd is yet unclear. Hence, an attempt is made to study and to generate information on genetics of gummy stem blight disease resistance of bottle gourd.

Materials and Methods

The cross combination of resistant × susceptible i.e., IIHR-77-6-1 × IIHR-64 and IIHR-95×IIHR-18 were taken to study the generation mean analysis for gummy stem blight resistance. In season II, the susceptible and resistant parents were crossed to produce F 1 's in April-September, 2020. In season III, The F 1 's was selfed to get F 2 and B 1 & B 2 combinations were generated by back crossing of F1 to their respective parents in November to March, 2020-21. In Season IV, the six generations of two crosses were evaluated in the field during May-September, 2021. The materials used in this experiment to study the genetics of GSB resistance are six generations viz., P 1 , P 2 , F 1 , F 2 , B 1 and B 2 populations were raised in the field during May-September, 2021, to evaluate fruit yield and inheritance of GSB disease. Data were recorded from 10 plants of parents (P 1 and P 2 ) and F 1 each, 40 plants of each B 1 and B 2 each and 208 F 2 plants of cross IIHR-77-6-1 × IIHR-64 and 249 F 2 plants of cross IIHR-95 × IIHR-18 for GSB incidence. Susceptible line IIHR-24 and Pusa Naveen was raised along the border as well as after every five rows to maintain uniform spread of disease. Populations were screened under natural epiphytotic condition and disease incidence was recorded based on evaluation of the intensity of the disease symptoms on leaves and stem of the individual plant. The disease scoring was started a month after planting at weekly interval for 8 weeks until plant growth ended. Ratings were done on 0 to 9 scales as described by Gusmini et al., 2002 [3] . The individual plant ratings thus recorded were used to calculate the mean percentage disease index (PDI), Area under disease progression curve (AUDPC). Statistical analysis for the recorded data was performed using WINDOSTAT Version 9.3 package (Indostat services, Hyderabad).

Results and Discussion

For this study, IIHR-77-6-1 × IIHR-64 and IIHR-95 × IIHR-18 crosses were selected. Further, in each cross six generations, viz., P 1 , P 2 , F 1 , F 2 , B 1 and B 2 were produced. Both the segregating (F 2 , B 1 and B 2 ) and non-segregating (P 1 , P 2 and F 1 ) generations were screened for gummy stem blight disease reaction under field condition. The mean PDI data were subjected to Mather scale, joint scaling test and six generation mean analysis in order to study inheritance pattern of gummy stem blight resistant genes. The means for six generations in two crosses (IIHR-77-6-1 × IIHR-64 and IIHR-95 × IIHR-18) are given, primarily to compare these population's means with one another (Table 1). The results of mean PDI indicating that all the generations were recorded mean PDI within the parental range in both the crosses IIHR-77-6-1 × IIHR-64and IIHR-95 × IIHR-18 population (Fig. 1). The mean PDI data obtained from six generations studied were subjected to generation mean analysis to obtain information on additive, dominance and duplicate epistatic interactions (Table 2). The results of the scaling tests and generation mean analysis for the both combinations are presented in Table 2. The scaling tests conducted on each cross were found to be significant (Hayman, 1958) [5] . The significant values of scaling tests indicated the inadequacy of simple additivedominance model and the presence of epistasis. These crosses are called interacting crosses.

Cross-I. (IIHR-77-6-1 × IIHR-64)

Based on mean PDI P 2 , F 2 and B 2 recorded susceptibility to GSB disease. B 1 and F 1 showed moderately resistant reaction and resistant reaction were observed in P 1 . The maximum mean PDI was recorded in P 2 (77.77) followed by F 2 (51.81) and B 2 (46.94). The P 1 recorded least mean PDI (23.33) among the generations and P 2 recorded high variances of 1700.95. In the cross IIHR-77-6-1 × IIHR-64, scaling tests (A, C and D) were significant, indicating that inadequacy of simple additive-dominance model and presence of epistasis. All the main gene components contributed towards susceptibility rather resistance with high dominance effect (-71.15). In interacting crosses, additive × dominance (57.77) was significant and possessed a positive value suggesting their contribution towards resistance rather than susceptibility to disease. Additive × additive (36.14) and dominance × dominance (37.26) components were showed non-significant. Among all the gene effects, additive × dominance component was of much higher in magnitude for resistance due to its negative value. Similar results were also reported in by Woluka et al. (2007) [10] . Opposite signs of ĥ and l ̂ components revealed duplicate epistasis and number of effective gene controlling resistance was 2.1 (Castle Wright's estimation).

Cross II (IIHR-95 × IIHR-18)

Based on mean PDI P 2 recorded highly susceptible to GSB disease, whereas F 2 and B 2 showed susceptible reaction, F 1 and B 1 exhibited moderate resistance and P 1 exhibits resistance reaction. The maximum mean PDI was recorded in P 2 (90.00) followed by F 2 (50.02) and B 2 (Fig. 2). The P 1 recorded least mean PDI (18.88) among the generations. Highest variance was observed in F 2 segregating population (382.44). In the cross IIHR-95 × IIHR-18, all scaling tests (A, B, C and D) were significant, indicating that inadequacy of simple additive-dominance model and presence of epistasis. Similar results were observed by Gusmini et al., 2005 [2] . Among the main gene components all are towards susceptibility rather resistance with high dominance (ĥ) (-52.86) effect. In interacting crosses, dominance × dominance (25.64) component possessed a negative value suggesting their contribution towards resistance rather than susceptibility to disease and additive × additive (ĵ) (23.05) component contributed towards susceptibility. Among all the gene effects, dominance × dominance (l ̂) component was of much higher in magnitude for resistance due to its negative value. Opposite signs of (ĥ) and (l ̂) components revealed duplicate epistasis and number of effective gene controlling resistance was 2.4 (Castle Wright's estimation). In this study, the crosses IIHR-77-6-1 × IIHR-64 and IIHR-95 × IIHR-18 were made to produce six generations: P 1 , P 2 , F 1 , F 2 , B 1 and B 2 . Both segregating (F 2 , B 1 and B 2 ) and nonsegregating (P 1 , P 2 and F 1 ) generations were assessed for disease resistance, and data on percent disease incidence (PDI) along with various other traits were collected. The evidence suggests that resistance or susceptibility is controlled by nuclear genes, with no cytoplasmic influence. While it is challenging to completely rule out the presence of multiple allelism (Kempthrone, 1957), it has not been deemed significantly impactful. Since F1 generations were included in this study, the estimates derived from nonsegregating progenies are unlikely to be affected by linkage. In the IIHR-77-6-1 × IIHR-64 and IIHR-95 × IIHR-18 crosses, the mean values of F 1 and F 2 exhibited significant fluctuations compared to each other, with their means F 2 exhibited Significant fluctuations compared to each other, with their means skewing towards the susceptible parent, indicating dominance for susceptibility in these crosses. The significance of scaling tests for both crosses suggested that resistance was not governed solely by simple additive or dominant gene effects. Instead, gene interactions appeared to play a role in its inheritance (Malek and Shamsuddin, 1998) [9] . This indicates that the phenotypic expression of genes at one locus may be influenced by genes at other loci, a phenomenon known as non-allelic interaction. In six parameter model, all main and interacting crosses showed the significant except additive × additive and additive × dominance. Different signs of dominance (ĥ) and additive × dominance (l ̂) components revealed that GSB resistance governed by duplicate epistasis decreasing effect genes between dominance. The study shows that the IIHR-77-6-1 and IIHR-95 genotypes can serve as sources of gummy stem blight resistance for future crop improvement programs in bottle gourd.

Acknowledgement

AcknowledgementThis research was a part of doctoral programme of first author and financial support from ICAR Indian Institute of Horticulture Research (IIHR).

References

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