Field Screening of Mungbean × Mashbean InterSpecific Recombinant Genotypes Against Yellow Mosaic Disease ( YMD )

M (Vigna radiata L. Wilczek) is an excellent source of protein and an important short duration grain legume crop in humid and sub-humid climate of the world (Akhtar et al., 2011). Different metabolic processes occurring in a plant culminate in the final product, i.e., yield. Any disruption/disturbance in one or more of such processes caused due to biotic and abiotic stresses faced by the plant may reduce the actual yield, and due to these stresses, the average yield of mungbean is low. Among these stresses, diseases are the major causes of low yield (Malik and Bashir, 1992). The severity of various stresses is largely due to varying weather conditions that prevail throughout the year and may extend to next coming years, thus, lowering yield of pulses at farmer’s field and keeping it below the potential yield/economic level. The low yielding cultivars and susceptibility to diseases particularly to YMD transmitted by whitefly (Bemisia tabaci Genn.) are the major constraints causing low seed yield. YMD, caused by mungbean yellow mosaic India virus (MYMIV) is very devastating in Pakistan especially in the summer season Abstract | Yellow mosaic disease (YMD) caused by mungbean yellow mosaic India virus (MYMIV) is an important constraint of mungbean during the summer season in Pakistan. The use of resistant varieties is the only way to reduce the losses caused by YMD. Resistant sources had already been reported in mungbean, but no such information seems to be available for mung × mash interspecific recombinants. In this study, 72 mung × mash interspecific recombinant genotypes and a susceptible mungbean variety Mung Kabuli were screened against Yellow Mosaic Disease (YMD) using disease severity ratings by visual scoring of symptoms to calculate percent disease index (PDI). Tested recombinants responded differently to the disease. None of the tested genotypes was found to be disease free (field immune). However, combined data showed that 43 genotypes were highly resistant with 1.18 to 10 PDI while 29 were resistant with 10.24 to 24.85 PDI. Mung Kabuli (positive control) showed susceptible response with PDI value of 60 %. Thus 43 highly resistant genotypes proved to be a good source of resistance to YMD despite high disease pressure and can, therefore, be used directly as varieties to manage the disease after evaluation for acceptable agronomic characteristics, adaptation, and stability in various regions or can be used as a resistant source in further breeding programs. Ghulam Abbas1, Khalid Pervez Akhtar1, Muhammad Ahsan2, Muhammad Jawad Asghar1, Fiaz Ahmad1 and Muhammad Rizwan3*


Introduction
M ungbean (Vigna radiata L. Wilczek) is an excel- lent source of protein and an important short duration grain legume crop in humid and sub-humid climate of the world (Akhtar et al., 2011).Different metabolic processes occurring in a plant culminate in the final product, i.e., yield.Any disruption/disturbance in one or more of such processes caused due to biotic and abiotic stresses faced by the plant may reduce the actual yield, and due to these stresses, the average yield of mungbean is low.Among these stress-es, diseases are the major causes of low yield (Malik and Bashir, 1992).The severity of various stresses is largely due to varying weather conditions that prevail throughout the year and may extend to next coming years, thus, lowering yield of pulses at farmer's field and keeping it below the potential yield/economic level.The low yielding cultivars and susceptibility to diseases particularly to YMD transmitted by whitefly (Bemisia tabaci Genn.) are the major constraints causing low seed yield.YMD, caused by mungbean yellow mosaic India virus (MYMIV) is very devastating in Pakistan especially in the summer season Abstract | Yellow mosaic disease (YMD) caused by mungbean yellow mosaic India virus (MYMIV) is an important constraint of mungbean during the summer season in Pakistan.The use of resistant varieties is the only way to reduce the losses caused by YMD.Resistant sources had already been reported in mungbean, but no such information seems to be available for mung × mash interspecific recombinants.In this study, 72 mung × mash interspecific recombinant genotypes and a susceptible mungbean variety Mung Kabuli were screened against Yellow Mosaic Disease (YMD) using disease severity ratings by visual scoring of symptoms to calculate percent disease index (PDI).Tested recombinants responded differently to the disease.None of the tested genotypes was found to be disease free (field immune).However, combined data showed that 43 genotypes were highly resistant with 1.18 to 10 PDI while 29 were resistant with 10.24 to 24.85 PDI.Mung Kabuli (positive control) showed susceptible response with PDI value of 60 %.Thus 43 highly resistant genotypes proved to be a good source of resistance to YMD despite high disease pressure and can, therefore, be used directly as varieties to manage the disease after evaluation for acceptable agronomic characteristics, adaptation, and stability in various regions or can be used as a resistant source in further breeding programs.
Ghulam Abbas 1 , Khalid Pervez Akhtar 1 , Muhammad Ahsan 2 , Muhammad Jawad Asghar 1 , Fiaz Ahmad 1 and Muhammad Rizwan 3 * (Malik, 1991).The disease is characterized by the appearance of yellow specks or spots on young leaves, and the emerging trifoliate leaves manifest irregular yellow and green patches causing a reduction in leaf size.In severe cases, there is complete yellowing of leaves followed by stunted growth, few flowers, and pods maturing late with shriveled seeds.YMD is also the major threat to mungbean production in India, Sri Lanka, Bangladesh, Papu New Guinea, Philippines and Thailand (Malik and Bashir, 1992;Honda et al., 1983;Chenulu and Verma, 1988;Varma et al., 1992;Jones, 2003) and inflict on heavy yields losses annually.According to an estimate yellow mosaic of mungbean, urdbean, cowpea and soybean induced an annual yield loss of US$300 million in India (Varma et al., 1992).The disease incidence of MYMV ranged from 4-40% in Pakistan (Malik and Bashir, 1992) depending upon crop variety and location, leading to 100% yield losses (Ilyas et al., 2010).However, in naturally infected susceptible cultivars it varies with the time of infection and yield losses may reach up to 100% (complete crop failure).
Therefore, the use of resistant varieties is the only way to reduce the losses caused by YMD.Resistant sources had already been reported in mungbean but no such information seems to be available for interspecific recombinants.Most of the available resistance sources reported in the literature do not provide complete resistance against YMD.Furthermore, there are reports of resistance breaking strains appearance of MYMV that overcome already available resistance sources.Therefore, to broaden the genetic base of MYMV resistance in mungbean and to mitigate its resistance breaking strain there is dire need to develop/identify new resistant sources.So, keeping this fact in view, we planned the present study to check the response of mungbean × mashbean interspecific recombinants against YMD and to examine whether the resistance present in parents is transferred in recombinant genotypes or not.

Materials and Methods
Seventy-two true breeding recombinant genotypes developed through inter-specific hybridization between mungbean and mashbean at NIAB, Faisalabad were evaluated in the field for their response against YMD in a Randomized Complete Block Design (RCBD) with three replications during the summer season in 2015.Two rows of each entry were sown in a row plot of 2.1 m 2 keeping row to row and plant to plant distances of 30 cm and 10 cm, respectively.Two to three seeds per hill were dibbled and after germination thinning was done to maintain a single healthy plant per hill.One row of susceptible check Mung Kabuli was planted after every test entry.As the disease spread through whitefly, the crop was sown late about 15 days after the sowing of another surrounding field with mungbean to exert maximum inoculums pressure before the disease screening nursery establish.Fertilizer was applied at the rate of one bag DAP per acre.The weedicide (Dual Gold) was also sprayed on the soil before sowing to avoid excessive weeds.Irrigation was applied to the experiment at flowering and pod filling stages.No plant protection measures were applied against whitefly to ensured high inoculum pressure throughout the growing season.All other agronomic practices were kept uniform.The experiment was observed weekly, and data for disease symptom severity was recorded according to the rating system described in Table 1 to calculate percent disease index (PDI) and the level of resistance/ susceptibility of the genotypes (Akhtar et al., 2011).

Results and Discussion
A low level of vector whiteflies Bemisia tabaci started to appear immediately after germination and it continued its buildup during the whole growth period of the crop.The first disease symptoms were started as scattered few small yellow spots on few young leaves of susceptible positive control Mung Kabuli after 20-23 days of germination.The number of infected plants and disease severity values increased with the passage of time depending upon the genetic makeup of genotypes.Plants of the susceptible control Mung Kabuli infected at an early stage of growth expressed severe disease symptoms like complete yellowing or chlorosis followed by necrosis within 10-12 days of infection.
In Pakistan YMD remains a serious problem throughout.Many research organizations especially NIAB, Faisalabad developed YMD resistant varieties but the causal pathogen is a geminivirus that can change its strain very quickly against a resistant source and a resistant variety became susceptible after few years of release with the emergence of a new strain as previously reported in case of cotton in Pakistan (2010).Thus, a continuous effort is needed for the identification of new sources of resistance with wider genetic base.

Conclusion
The use of resistant varieties is the only way to reduce the losses caused by YMD.In present study, the interspecific recombinant genotypes i.e.MMH 15521, MMH 1125, MMH 3132, MMH 3563, MMH 3615, MMH 4615, MMH 53105 and MMH 5615 were found to be highly resistant with minimum PDI value.These genotypes, therefore, can be either released as new varieties or may be used as resistant source in further breeding programs.

Table 1 :
Disease Scale for rating of Mungbean Yellow Mosaic Disease (MYMD).