Abstract

Stem rust is one of the three rusts that are important constraints to wheat cultivation throughout the world, India being no exception. Not all the three rusts are equally important in all the seven wheat agroclimatic zones of the country. Stem rust is considered to be important only in CZ, PZ and SHZ. The system of varietal development in India ensures that a variety is tested for at least three consecutive years in the targeted zone and thereby its resistance to the locally important diseases is confirmed. We report the prevalence of stem rust pathotypes in different agroclimatic zones of India and the profile of various stem rust resistance genes deployed to counter them.

Introduction

Rusts are the most important group of diseases which threaten successful wheat cultivation the world over. India has the distinction of not having had any rust epidemic since the 1972-1973 crop season (Joshi et al. 1975). India is divided into seven agroclimatic zones on the basis of climatic requirements of wheat production. These are Northern Hills Zone (NHZ), North Western Plains Zone (NWPZ) , North Eastern Plain Zone (NEPZ) , Central Zone (CZ), Peninsular Zone (PZ) and Southern Hill Zone (SHZ). The wheat varieties suitable for cultivation in these zones have different requirements pertaining to growth duration, water and temperature relations as well as resistance to diseases and insect pests. Before commercial release wheat varieties are tested in the specific zone for at least three consecutive years (DWR., 2002). The first year of testing is referred to as National Initial Varietal Trial (NIVT) whereas second and third year of testing are called as Advance Varietal Trials (AVT) I and II respectively. After the pre-trial Initial Plant Pathological Screening Nursery (IPPSN) the resistant varieties are entered in NIVT. The number of varieties is drastically reduced on account of poor performance with respect to agronomic traits, resistance to diseases etc., before they reach the AVT II stage. However, the very presence of a genotype in AVT II demonstrates that the variety has given promising performance in the zone under question and has acceptable level of tolerance / resistance to the locally prevalent pathotypes / races of different diseases and insect pests endemic to the zone.

Not all the three rusts viz., leaf rust (Puccinia recondita), stem rust (Puccinia graminis tritici) and stripe rust (Puccinia striiformis) are equally important in different agro-ecological zones. Though leaf rust is found all over the country, the other two rusts only have regional importance. Stem rust , for example, is threat only in CZ , PZ and SHZ. We studied the stem rust reaction of AVT II genotypes of different zones during 1997-98 to 2001-02 under Delhi conditions and present the results in the light of the stem rust pathotypes prevalent in their respective zones, and the resistant genes possessed by the genotypes studied. Analysis of the relative importance of different stem rust resistance genes across different agro-ecological zones is presented.

Materials and Methods

The test material comprised of AVT II entries of T. aestivum and T. durum that were in final year of yield trial Under All India Coordinated Wheat Improvement Programme (AICWIP) in different zones of the country. The material was planted in late December each year during 1995-96 to 2001-2002. The nursery was bordered from all sides with one row of local oat (to check for external contaminating inoculum) followed by two rows of infector genotype 'Agra local' for creating stem rust epiphytotic conditions. Fresh viable uredospores inoculum of stem rust pathotypes was used to create the infection. The infection was established on 'Agra local' in the glasshouse from November onwards. The inoculum was further multiplied as described by Joshi et al. (1988). The inoculum suspension of the pathotypes 40A (62 G 29), 40-1 (62 G 29-1), 1176 (37 G 19) and 122 (7 G 11) containing 0.1% spoor suspension (Kapoor and Joshi, 1941) was used to inoculate the border rows and every 20th row of the infector genotype at GS37 (Zadoks et al., 1974) using a hypodermic syringe in the nursery to create epidemic conditions. Severity and response were recorded twice in the crop season using Loegering's assessment scale (Loegering, 1959). The same set of AVT II entries were screened under glass house conditions in the crop season against all the known pathotypes of stem rust and observations on the infection types were recorded using the assessment scale reported by Stakman et al. (1962) at DWR regional Station Shimla. The seedling reaction and the reaction produced on the near iso-genic lines in set A (Sr13, Sr9b, Sr11, Sr28, Sr8b, Sr9e, Sr30, Sr37) and B (Sr7b, Sr21, Sr28, Sr5, Sr11, Sr7a_Sr13, Sr14) were matched for the postulation of genes as proposed by Nayar et al. (1997).

Results and Discussion

Since the material used in the study under report comprised of AVT II entries of AICWIP yield evaluation trials, they possessed resistance at least to the locally prevalent pathotypes of stem rust. Seedling and APR reaction of the checks and test entries of different zones to various stem rust pathotypes at Delhi is presented in Table 1. The results indicate that the newly bred material tested in trials is clearly more resistant to the stem rust pathotypes than the checks. A change in the choice of the pathotypes for this screening, however, is warranted as is evident from a perusal of the Table 2 that presents the distribution of different pathotypes across various agro-climatic zones. A total of 527 diseased plant samples, received from different zones from farmers' fields, were analysed for stem rust pathotypes, the highest number of samples (431) being from SHZ. This zone also had maximum (six) pathotype, however only two pathotypes viz., 62G29 (40A) and 62G29-1(40-1) accounted for 96% of samples. Pathotype 40A (Sinha et al., 1978) is virulent on many Sr genes but avirulent on Sr 7a, 13, 21, 24, 30, 37 and pathotype 40-1 (Bharadwaj et al., 1990) is additionally virulent on Sr 24. In the rest of India, pathotype 75G5 (21A-2), virulent on Sr 7b, 9b , 13 ,17, 28, 30 is more frequent. A total of 35 samples from PZ were analysed and they contained five pathotypes among them. The two pathotypes 62G29 and 7G 43 accounted for 14 samples each. Though 62G29 was abundant (43%) in SHZ samples also, the pathotype 7G43 was unique to PZ. The pathotype 62G29 was also more or less confined to PZ and SHZ, as out of 245 samples that contained this pathotype (40A), 244 were from SHZ, whereas PZ and CZ contributed only one sample each. Central zone, had four different pathotypes, but three were found only once, the remaining 11 being pathotype 75G5. Another interesting observation was that the predominant pathotype of CZ was also the predominant pathotype of NWPZ, NEPZ and NHZ . Over all, the pathotype 75G5 accounted for 78% , 90% and 85% of all the samples analysed from CZ, NW/EPZ and NHZ respectively. This analysis clearly shows that we need to screen the newly developed wheat lines against the pathotypes that are more prevalent in a target environment, i.e. 75G5 in NWPZ, NEPZ and CZ, 62 G 29 and 7G43 in PZ, and 62 G 29 and 62 G 29-1 in SHZ.

A study of the profile of stem rust resistance genes postulated in AVT II entries from SHZ and PZ (Table 3) yielded interesting results. In SHZ which has the maximum pathotype diversity there are just four resistance genes and their combinations, whereas in PZ whose predominant pathotype is also one of the two predominant pathotype of SHZ seven different genes or their combinations are found. In CZ, a region that has severe stem rust, elite AVT II genotypes made had eight stem rust resistance gene or their combinations. Sr24 was found only in varieties in CZ and PZ.

Even though the northern regions of the country do not have any serious stem rust problem, the varieties found in these three zones had the maximum diversity of stem rust resistance genes. NHZ varieties were postulated to have seven genes or their combinations and NWP/NEP had all available genes and their combinations with the sole exception of Sr24. This could be due to use of predominantly local germplasm with minimal introductions from elsewhere, and whatever gets incorporated remains there because there is no selection pressure for stem rust resistance genes. In spite of such selection free breeding, it was interesting to note that Sr12 was found in varieties of NW/EPZ, Sr8b only in northern regions and Sr5 in northern and central regions. Meaningful interpretation having bearing for future breeding programmes could be derived. SHZ, the hot spot for stem rust was the only zone having more than one predominant pathotype but managing to combat them with minimum number of genes i.e. Sr2, Sr11, Sr31 and Sr9b. All other zones had only one predominant pathotype each . The northern and central regions had the same predominant pathotype viz., 75G5, where as the predominant pathotype of PZ was 62G29.

Unlike pathotype distribution, a clear cut picture does not emerge with regards to the utilization of resistance genes. The situation seems to have been confounded more by the presence of linked rust resistance genes (Table 4). Sr2, Sr 11 and Sr31 have been the most extensively used genes. Though there were few genes exclusive to one or the other region indicating their role in conferring resistance to the pathotypes specific to the zone in question, the lack of a very clear cut utilization pattern perhaps exist because there is no selection operating against ineffective genes. The genes required for resistance in a zone are intentionally selected for, but those not required are not selected against. The information generated provides crucial information about breeding for stem rust resistance to wheat breeders and pathologists.

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