Adult Plant Resistance of Wheat Entries to Black Rust Race 40-A

Genetic resistance is the most efficient, economical and environmental friendly approach against black rust of wheat. Although, evolution of new physiological races/pathotypes which can overcome the response of current resistant cultivar has engender to focus on evaluation of large number of wheat germplasms, breeding materials, cultivars for their reaction to black rust. Field based assessment for adult plant resistance in promising fifty entries against predominant pathotype 40-A of black rust revealed that forty eight entries showed resistance with lower AUDPC value in the range of 0 to 240 as compared to highly susceptible check varieties i.e. Lok 1 and Agra Local which showed AUDPC value of 1305 and 2000, respectively. But, amongst the forty eight resistant entries, forty entries showed durable resistance due to slower development of disease under high disease pressure which could be due to the presence of effective adult plant resistance Sr genes. Result of correlation coefficient between CI with both the disease parameters i.e. AUDPC (r=99**) and FRS (r=98**) also showed highly significant positive association.

Black/stem rust of wheat caused by Puccinia graminis f. sp. tritici Eriks. and E. Henn., belonging to family Pucciniaceae of order Uredinales and class Basidiomycetes of Basidiomycotina, has historically been one of the major constraints in realizing stabilized yields of worldwide wheat production. It has historically caused devastating yield losses on spring wheat in the United States (Kolmer 2001). According to Prasada (1960), nearly one million tonnes of wheat costing about ` 392 million were damaged during 1958-59 due to black rust. In India, the Central and Peninsular southern part where the warmer temperature prevails during wheat growing season the crop is prone to black rust, but has not in the wheat belt of Northern Hill Zone and Northern Pain Zone (Joshi and Palmer 1973). Besides, many times wheat cultivars often appear to lose their resistance due to the changes in virulence pattern of pathogens by mutation that ultimately can overcome the effects of existing available resistance genes. Emergence of a new virulent race of black rust i.e. Ug99 reported in Uganda in 1999 can infect most of the resistance genes like Sr 31, Sr 36, Sr 24 (Singh et al. 2008) and it has created a greatest threat to all of the wheat growing areas of world. But, under this situation relaying in genetic resistance is the most efficient, economical and environmental friendly approach to control wheat rust (Line and Chen 1995). Utilization of race-nonspecific resistance which is mainly polygenic has often been described as slow rusting or partial resistance (Parlevliet 1979) and is known to be long lasting and more durable Print ISSN : 1974ISSN : -1712 Online ISSN : 2230-732X than race specific resistance (Herrera-Fossel et al. 2007). Mainly, presence of non-race specific genes shows susceptibility at seedling stage but exhibits moderately to highly resistant responses at adult plant stages; like the presence of Sr2 gene which is a slow rusting gene or adult plant resistance (APR) gene in wheat genotypes shows a characteristics phenotypic appearance of pseudo black chaff (PBC) (Nzuve et al. 2012). When the non-race specific genes are combined with 4 to 5 genes of minor or additive effect, a near immunity could be achieved (Singh et al. 2008). The emphasis on use of non-race specific genes formed the basis of durable resistance in wheat resistance breeding programs (Nzuve et al. 2012). Screening of genotypes by field based assessment is employed for partial resistance and can be assess through different measures, viz. final rust severity (FRS), area under disease progress curve (AUDPC) and coefficient of infection (CI) (Pathan and Park 2006). Slow rusting parameters can be used for grouping of different cultivars based on their resistance reaction (Ali et al. 2007;Sandoval-Islas et al. 2007;Shah et al. 2010). Thus, in present context broadening of resistance base by the utilization of genetically diverse source of resistance are essential for enhancing the durability of resistance to black rust due to continue evolution of pathogen especially in the perspective of the changing climatic scenario. A combination of several minor genes may confer durability of field resistance leading to contribute towards slow development of rust due to increase latent period, development of smaller pustule size and reduced number of uredia per unit area etc. (Tomar et al. 2014). Therefore, there is a need for the identification of new sources for durable resistance which exhibit a better field durability than those possessing the vertical resistance genes only. The present study was conducted to evaluate promising wheat entries at adult plant stage for exploiting new sources of durable resistance to predominant pathotype 40-A of black rust.

MATERIALS AND METHODS
During 2017 BDW4, BDW8, GW1,  GW11, GW173, GW322, GW451, GW474, GW475,  GW477, GW478, GW496, GW505, GW506, GW509 Table 1). The severity was determined by visual observations, below 5 per cent severity, the intervals were Trace and usually 5 per cent interval was used from 5 to 20 per cent and 10 per cent interval between 20-100 per cent severity. The coefficient of infection (CI) was calculated by multiplying severity score with constant values of response type viz. 0.2, 0.4, 0.6, 0.8, 1 for R, MR, X, MS and S respectively as shown in Table 1. Afterwards by using following formula, area under disease progress curve (AUDPC) was calculated for all the test entry under study.
where, X i and X i+1 are severities on date i and date i+1, respectively t i is the number of days in between date i and date i+1 n is the number of observation recorded

RESULTS AND DISCUSSION
Twenty two entries with trace severity and three entries with 5 to 10 per cent severity of different infection types, remaining twenty five entries with no infections were recorded in first day of data observation (Table 2). With the progress of disease, twenty one entries viz. BDW 4, BDW8, GW1, GW478, GW505, GW509, GW1320, GW1343, J-2013-26, J-2013-28, J-2013-46, VA2013-46, VA2013-48, VA2013-57, VA2017-02, VA2017-04, VA2017-05, VA 2017-06, VA2017-07, VA2017-08, and VA2017-09 were observed with resistance reaction ranging from trace to 10 per cent severity, while, ten entries viz. GW173, GW496, GW451, GW477, GW506, GDW1255, J-2013-09, VA2013-49, VA2017-03, and VA2017-10 were observed with moderately resistance type of infection ranging from trace to 20 per cent severity according to final rust severity score (Table 2). Moreover, five entries viz. GW11, GW322, GW1319, GW1339, and GW1349 were observed as moderately susceptible type of reaction which ranged from 5 to 10 per cent severity according to final rust severity score (Table  2). Four entries viz. GW1318, GW1321, GW1348, and J-2013-30 were recorded with susceptible type of infection but their per cent of severity i.e. 5 to 10 per cent, were under considerable level for resistance as compared to check varieties i.e. Lok 1 and Agra local which showed susceptible reaction of 100 per cent severity in final rust severity score ( Moreover, linear relationship between three parameters viz. final rust severity and AUDPC with CI were explained by coefficient of determination with 96 and 99 per cent, respectively as depicted in Fig. 1 and 2.    Besides, significantly positive correlation coefficients (r) were observed between CI with final rust severity (r = 98**) and AUDPC with CI (r = 99**) for all the test promising entries (Table 4).   Since, all these three disease parameters i.e. final rust severity, coefficient of infection and area under disease progress curve were strongly and positively correlated in the present study, so, it can be concluded that these are the most appropriate parameters for field based assessment for durable resistance to black rust. resistant to moderately resistant responses were of great importance while selecting for durable resistance to stem rust for enhancing more effective breeding program (Nzuve et al. 2012). Durum lines generally showed resistance to bread wheat virulent pathotypes 40-A and 40-1 among the Indian Pgt population (Mishra et al. 2009). Moreover, lines with acceptable level of partial or durable resistance restrict the evolution of new virulent races due to extremely rare chance of occurring multiple point mutations in nature inside the pathogen (Ali et al. 2007). Earlier, report also suggest that germplasm accessions exhibiting moderate resistance to resistance reaction may confer durable resistance which may be presence of some effective adult plant resistance (APR) genes against rust (Bhardwaj et al, 2006;Elangbam et al. 2015;2016). Many resistance genes for black rust have been designated (McIntosh et al. 2009) and among them genes Sr 2 (Rajaram et al. 1988), Sr 55 (Sybil et al. 2014), Sr 57 (Singh et al. 2012) contribute to adult plant resistance to black rust of wheat.

CONCLUSION
In order to combat the threat of evolution of new virulent races of black rust, exploitation of genetic resistance in wheat breeding programme became a critical tool. Hence, from the present findings it can be concluded that all the tested forty entries acquired partial or durable resistance by exhibiting good response of resistance with low AUDPC values under high disease pressure against predominant pathotype 40-A of black rust. Thus, it is important to assess critically the utility of such great diversity of entries for sources of durable resistance at adult plant stage for enhancing work of resistance breeding programme against black rust of wheat.