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Results and discussion
The initial disease rating (IDR) on 40th day of inoculation, the
final disease rating (FDR) on 68th day of inoculation and AUDPC
values recorded on the test genotypes are given in Table
1. Based on the AUDPC values, the genotypes were categorized into
three distinct groups. The group 1 included those genotypes which
either remained complete free of disease (immune) or exhibited
TR-infection type at IDR and 5-10R at FDR. The AUDPC values on these
genotypes were measured below 1% of the susceptible check variety
Agra Local (AUDPC :1738). The genotypes of this group represented a
strong vertical resistance (major genes), which may not impart a
durable protection.
The genotypes exhibiting AUDPC values in the range of 1-20% of the
check were placed in group 2, and between 21-50% of the check in
group 3. Genotypes belonging to groups 2 and 3 did develop yellow
rust but terminal severity was always low.
The genotypes of group 2 and 3 are characterized to be partially
resistant since they developed epiphytotic of very low potential as
indicated by their AUDPC values despite the ultimate expression of
high infection type. Genotypes with, these traits are designated as
possessing genes that accord partial resistance (Parlevliet 1975,
1979, 1988). These genotypes exhibited AUDPC values ranging between
153-872, which is less than 50% of value of Agra Local, the cheek
(AUDPC: 1780). Group 2 and 3 genotypes are further distinguished as
the first cluster with better slow rusting (AUDPC below 20% of the
cheek), while those in the latter group represented lower level of
slow rusting (AUDPC between 20-50% of the cheek).
Parental lineages of group 2 and 3 genotypes were examined through a
computer package named GRIPI developed by CIMMYT, Mexico (Skovmond et
al. 1995). The pedigree analysis revealed that slow rusting lines of
group 2 essentially had Yaktana 54 in their parental lineage
(Fig. 1). Yaktana 54 could not be traced in
the pedigrees of genotypes of group 3 which had less apparent slow
rusting and higher terminal disease severity. The adult plant
resistance expressed as slow rusting of higher level in genotypes of
group 2 may be attributed to the presence of adult plant resistance
gene Yr18. This APR gene for yellow rust resistance might have
descended into Yaktana 54 from one of its parent cultivar Frontana,
which is the designated source of Lr34 (an APR gene for brown
rust resistance) having tight linkage with Yr18 (Singh 1992).
The gene Yr18 has been found to interact and produce enhanced
level of resistance (Singh 1992; Johnson 1988; Milus and Line 1986
a,b). Therefore adding partially effective additive genes to
genotypes already carrying Yr18 can lead to development of
wheat lines with higher levels of resistance.
The partial resistance has been advocated to be more durable (Singh
et al. 1991). Lines with acceptable levels of slow rusting invariably
carry a combination of Lr/Yr genes and this restricts the
evolution of pathogen since the changes of multiple point mutations
are extremely rare (Schafer and Roelfs 1985). Stability of the
pathogen population, and the avirulence 1 virulence structure would
result in greater durability of cultivar resistance. Gene
Yr18, if incorporated in varietal background has the potential
to impart durable resistance to yellow rust as evident from its
performance in several bread wheat cultivars (Singh 1992). As shown
in Fig. 2, genotypes of group 2 possessing
Yr18 were able to combine both good yield and wide adaptation.
Therefore, these genotypes may be considered as important genetic
stocks having dual advantage of resistance to yellow rust as well as
potential of higher yield.
Acknowledgments
This research paper is outcome of the project No. DWR RP-1 of the
Directorate of Wheat Research, ICAR, and the technical assistance of
Mr. Mangal Singh is acknowledged.
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