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Inheritence; Interspecific; MYMV; Resistance; Susceptible; Segregation

Introduction

Mungbean is one of the important minor pulse crops with an excellent source of easily digestible protein (24%) with low flatulence which complements the staple rice diet in Asia. Greengram is also one of the pulses being imported in India to meet domestic requirement of pulses. Just like other minor pulses mungbean is cultivated mostly in marginal land under rainfed situation and also because of biotic and abiotic stress factors productivity of green gram in India is very less. Amongst these factors Mungbean Yellow Mosaic Virus (MYMV) which is the most devastating one and accounts for yield losses up to 85% [1]. This MYMV which is transmitted in a circulative tenacious way by whitefly Bemisia tabaci has no stable resistance source among mungbean cultivars whereas ricebean (Vigna umbellata Thunb.) has been reported with high resistance to MYMV disease and is also cross compatible. Hence, transfer of resistant genes for MYMV from rice bean to mungbean may be a possible means to develop MYMV resistant mungbean varieties. In this context study was conducted with the objective to understand the inheritance of MYMV resistance in the interspecific crosses of mungbean and ricebean which may aid the breeders in effective selection of target genotype which ultimately leads to the development of MYMV resistant cultivars.

Experimental Section

Interspecific hybridization was carried out between two Vigna species viz., Vigna radiata (mungbean) and Vigna umbellata (rice bean) during summer-2017 at field unit of AICRN on Potential Crops Scheme, Main Research Station, University of Agricultural Sciences, Hebbal, Bengaluru. Green gram variety KKM-3 which is agronomically superior and well adopted variety but susceptible to MYMV was used as a female parent. Two rice bean lines KBR-1 and RBL-6 which are highly resistant to MYMV were used as male parents. The F1 hybrids produced were raised during Kharif 2017 along with parents under natural field condition. The mungbean variety, yellowmung which is highly susceptible to MYMV was used as disease spreader and two rows of this variety were planted all around the experimental plot in order to attract whitefly and enhance spread of MYMV. On individual plant basis, the per cent infection by MYMV in parents and F1 hybrids were recorded at 50 days after sowing, when 100 per cent disease was visible on the susceptible check. F2 seeds harvested from individual F1 plants of respective inter specific crosses were collected separately and evaluated during summer (February-April) 2018 under natural field condition and infector row technique was used. Each F2 was raised with minimum of 250-300 plant population. Yellowmung, a highly susceptible mungbean cultivar for MYMV, was used as spreader and was sown along with all the F2 populations of inter specific hybrids of mungbean and rice bean to create disease epidemic in the test plants. F2 populations (KKM-3 × KBR-1 and KKM-3 × RBL-6) were screened for yellow mosaic reaction using the disease incidence scale given by AVRDC (1-6). The susceptible check had 100 per cent disease incident at the time of scoring and Per cent Disease Index was calculated by,

Plants with MYMV score ≤ 3 were classified as resistant and those with MYMV score > 3 were classified as susceptible. The recorded observations were subjected to Chi-square test based on expected ratios. The goodness of fit between expected and observed Mendelian segregation ratios were worked out in order to infer on the inheritance pattern of the resistant genes. The Chi-square was calculated by using the following formula.

Where,

O = Observed frequency

E = Expected frequency

The significance of Chi-square value was tested by comparing the calculated Chi-square value with table value at 5 % and 1 % level of significance at appropriate degrees of freedom (n-1), where, n = number of classes of trait under consideration.

Discussion

Screening of interspecific F1 hybrids for MYMV resistance

The two male parents used in interspecific hybridization recorded highly resistant (HR) disease reaction taking a disease score of 1. Female parent KKM-3 used in the study were found to be susceptible with the disease score of 5. F1 interspecific hybrids of the cross KKM-3 × KBR- 1were found to be resistant taking a disease score of 2 where as F1’s of the cross KKM-3 × RBL-6 were highly resistant with the AVRDC score of 1 suggesting that there may be a successful introgression of MYMV resistance from ricebean to mungbean (Table 1).