Assessment of Wheat Breeding Lines for Slow Yellow Rusting (Puccinia striiformis West. Tritici)
S. Jawad A. Shah
A preliminary infection experiment was carried out during 2005-06 to identify slow yellow rusting sources of resistance in 20 NIA-wheat breeding lines along with Morocco as susceptible check at Nuclear Institute for Food and Agriculture, Peshawar. It was revealed that the current pathotypes of Puccinia striiformis West. tritici (Pst) were found virulent on all tested genotypes except NIA-6 and NIA-14. Host-pathogen compatibility in remaining lines facilitated the assessment of slow rusting trait and it was assessed through the epidemiological parameters viz., Final Rust Severity (FRS), Area Under Rust Progress Curve (AURPC) and Infection Rate (IR). Maximum FRS of 70% was rated in five lines while in the remaining 13 lines it ranged between 10 to 60%. Relative AURPC values up to 30% of the susceptible check were recorded for 10 lines while up to 70% for eight lines, showing desirable and good slow rusting behavior, respectively. Relative infection rate values ranged from 0 for NIA-7 to 1100-1300, recorded for eight lines. NIA-7 has a constant severity of 10% over assessment dates with lower AURPC value and was considered as better slow rusting line. Among various epidemiological parameters used for assessment of slow rusting trait, AURC was found to be a reliable parameter. Infection rate, on the other hand did not reveal the actual slow rusting behavior of the tested lines. Based on slow rusting trait, the tested lines may probably have genes for varying degrees of slow yellow rusting and can be used for further manipulation in wheat improvement program after confirmatory studies.
Food security and self-sufficiency are not achievable without enhancing wheat production in Pakistan. Non-availability of improved varieties with durable disease resistance is one of the major reasons for various epidemics and low wheat production in the country. Severe losses due to different wheat diseases, including stripe rust have been reported both in past and recently (Saari et al., 1995; Kisana et al., 2003). Stripe rust or yellow rust of wheat, caused by Puccinia striiformis Westend f. sp. Tritici, is among the most important foliar diseases of wheat and its regular outbreaks in the country have caused serious losses to national economy (Roelf and Bushnell, 1985; Saari et al., 1995; Kisana et al., 2003). The only economic and practical control of rust diseases can be achieved through genetic resistance (Singh et al., 2004).
Rust resistance in improved wheat cultivars in the past was solely based on major genes that often were overcome by virulent pathogenic mutant strains within five to seven years (CIMMYT, 1992). This was the case for Yr9 based cultivars, Pirsabak-85 and Pak-81, when these became susceptible during 1994-95 and resulted in yield losses up to 40% (Saari et al., 1995). Similarly, the resistance of Inqilab-Inqilab-91, based on Yr27, was broken down and was severely infected by yellow rust (Kisana et al., 2003) causing severe losses. Such major gene(s)-based resistance are lost very rapidly due to evolution of virulence by the pathogen to these genes. The alternative is to search for partial resistance based on minor genes, which impart durability. The ability of certain genotypes to retard rust development is a form of partial resistance and is referred to as slow rusting or durable resistance (Singh et al., 2004; Clifford, 1972).
Several researchers have reported rust resistance in different wheat breeding
lines of Pakistan (Mirza et al., 2000). However, their studies were based
solely on vertical resistance. Little research has been reported on screening
of wheat germplasm for slow rusting. The present study was thus designed to
assess wheat breeding lines for slow rusting. The study reports the preliminary
findings of a study which was carried out to evaluate the slow yellow rusting
performance of 20 wheat breeding lines.
MATERIALS AND METHODS
Seed source and field sowing: The study was conducted at Nuclear Institute for Food and Agriculture (NIFA), Peshawar. Seeds of 20 breeding lines were received by NIFA under a coordinated program from Nuclear Institute for Agriculture (NIA) Tandojam, Sindh, Pakistan. Each entry was separately planted in strips of small adjacent plots with 2 rows plot-1 of 2.5 m length separated by 0.3 m with a distance of 0.6 m between entries. Morocco, a super susceptible wheat cultivar, was sown around entries as spreader and also to serve as an adult plant susceptible check.
Inoculum source and inoculation: Rust inoculum was obtained from Crop Diseases Research Program (CDRP) of National Agricultural Research Center (NARC), Islamabad which carried virulence for yellow rust resistance genes Yr1, Yr6, Yr7, Yr8, Yr9, Yr17, Yr18 and Yr27. Artificial inoculation was carried out with this Pakistani Yr inoculum by spraying all test entries and spreader rows with urediniospores suspended in double distilled (having 2-3 drops of non-phytotoxic wetting agent) with 300 mg of spores per liter of water, 2-3 times after the sun set as described by (Roelfs et al., 1992).
Estimation of disease severity and of slow rusting parameters: Data
were scored four times at seven days interval, starting when Morocco reached
50% severity according to the Modified Cobb Scale (Paterson et al., 1948).
The Area under Disease Progress Curve (AURPC) was estimated by using the formula
adopted by Pandey et al. (1989). Relative AURPC was worked out by comparing
AURPC value of each entry with Morocco. Infection rate r, the unit leaf area
damaged by disease per day was estimated for the whole epidemic period after
the time when Morocco showed >50% severity, following Vanderplank (1968).
Relative infection rate was worked out for each entry by comparing r value with
Grain yield and 1000-kernel weight: Grain yield was calculated for each entry by harvesting entire two rows and weighing their grains through electronic balance. Kernel weight was taken for 500 kernels, which was converted to 1000-kernel weight.
RESULTS AND DISCUSSION
Parameters used as criteria to identify slow yellow rusting genotypes under field conditions included pattern of disease increase, relative final disease severity, relative AURPC and relative infection rate. Results regarding these parameters are described as under:
Pattern of virulence and disease increase: Results regarding Yr virulence
pattern and disease increase over time are presented in Fig. 1,
which clearly shows that disease pressure was considerably high as indicated
by the severity of susceptible check. Yr pathotypes were considered virulent
on all tested lines except NIA-6 and NIA-14. These two lines may probably carry
major gene or combination of major genes based resistance, effective against
all virulences used. These lines need further inheritance studies or marker
assisted identification for exploring basis of their resistance displayed in
the current study. Susceptible check Morocco showed almost linear pattern of
increase in disease severity over time, between initial and final disease scoring.
NIA-2 and NIA-5 exhibited severity after second scoring. Most of the lines,
including NIA-2, NIA-17 and NIA-18 were having more increase in severity during
first and second scoring. NIA-1, NIA-3, NIA-8, NIA-12 and NIA-17 attained 70%
severity after 144 days. However, NIA-1, NIA-3 and NIA-12 reached to only 30%
severity at 134 days after sowing, while NIA-8 and NIA-17 showed 50% severity
at 134 days after scoring. Increase in disease severity was slower in NIA-4
and NIA-5 which reached to only 20% severity. Similarly NIA-7, being highly
resistant, showed a constant severity of 10%. Previously, other researchers
have also reported variation among different advanced and candidate breeding
lines of Pakistan for stripe rust resistance (Shah et al., 2003; Mirza
et al., 2000).
|| Pattern of virulence and disease increase over time for 20
NIA-breeding lines at NIFA, during 2005-06
Final disease severity: Maximum relative final disease severity recorded
among the tested lines were 70% of the susceptible check for five entries (i.e.,
NIA-1, NIA-3, NIA-8, NIA-12 and NIA-17), while the remaining 13 were between
10 to 60% of Morocco. NIA-4, NIA-5, NIA-15 and NIA-20 exhibited disease severity
up to 30% of the susceptible check and were regarded as better slow rusting.
NIA-1, NIA-2, NIA-3, NIA-8, NIA-9, NIA-10, NIA-11, NIA-12, NIA-13, NIA-16, NIA-17,
NIA-18 and NIA-19, displayed relative final disease severity up to 70% of susceptible
check and were marked as moderately slow rusting (Fig. 2).
This final disease severity is assumed to represent the cumulative result of
all resistance factors during the progress of epidemic (Parlevliet and van Omeren,
1975). Previously, Broers et al. (1996) also carried out field assessment
of quantitative resistance to yellow rust for ranking of wheat breeding lines.
According to the resistance level based on Disease Severity (DS) along with
other slow rusting parameters, they found that resistance levels ranged from
very low (in Taichung 23) to very high (in Parula) among the tested wheat breeding
||Relative yellow rust severity for NIA-wheat breeding lines
along with susceptible check, at NIFA, during 2005-06
||Relative AURPC estimation of Yr for NIA-wheat breeding lines
along with susceptible check, at NIFA, during 2005-06
AURPC value: Based on the AURPC values, breeding lines were categorized
into two distinct groups. The first group comprised genotypes exhibiting relative
AURPC values up to 30% of check, while lines showing AURPC values up to 70%
of check were placed in second group. In these lines rust initiated and sporulated
but with final chlorotic and necrotic strips (MR and/or MS-infection types).
Subsequently, the progress of rust development remained slower and restricted.
Lines of group 1 were marked as better slow rusting and that of group 2 were
marked as moderately slow rusting since they also developed epiphytotic of very
low potential as indicated by their AURPC values despite the ultimate expression
of high infection type (Fig. 3). Breeding lines with such
traits are expected to possess genes that confer partial resistance (Parlevliet,
1988). NIA-2, NIA-4, NIA-5, NIA-7, NIA-9, NIA-10, NIA-11, NIA-12, NIA-13 and
NIA-20 exhibited relative AURPC values less than 30% of Morocco and were marked
as better slow rusting. Lines having relative AURPC values up to 70% of susceptible
check were grouped as moderately slow rusting in group 2. These were NIA-1,
NIA-3, NIA-8, NIA-15, NIA-16, NIA-17, NIA-18 and NIA-19. Both group 1 and 2
included lines with varying degrees of partial resistance which has been advocated
to be more durable (Singh et al., 2004). Furthermore, lines with acceptable
levels of slow rusting restrict the evolution of new virulent races of the pathogen
because multiple point mutations are extremely rare in nature (Schafer and Roelfs,
1985). None of the tested line was marked as susceptible or highly susceptible
and may be advanced further.
Infection rate: Relative infection rate (r) ranged from 0 for NIA-7
to 1350 for NIA-1, NIA-3, NIA-10, NIA-11 and NIA-12 (Fig. 4).
NIA-7 showed a constant disease severity, thus showing no increase per unit
time with r value of 0. The infection rate assumed for susceptible check Morocco
as 100. The apparent infection rate of certain lines was more than Morocco due
to the fact that disease scoring was started when disease severity was >50%
on Morocco. Therefore the actual infection rate for Morocco may even be more.
Slow rusting behavior was assessed through comparison of relative infection
rate. Those having r value up to 100 were marked as better slow rusting.
||Slow rusting parameters along with 1000-kernel weight and
grain yield of NIA-wheat breeding lines at NIFA, Peshawar, during 2005-06
||Relative infection rate r of Yr for NIA-wheat breeding lines
along with susceptible check, at NIFA, during 2005-06
Those having r value ranging from 100 to 400 were marked as moderately slow
rusting. The remaining lines were marked as poor slow rusting. NIA-19 and NIA-20
showed RIR value up to 100 and were marked as better slow rusting. NIA-8, NIA-13,
NIA-15, NIA-16, NIA-17 and NIA-18 exhibited r value up to 350, marked as moderately
slow rusting. NIA-1, NIA-3, NIA-10, NIA-11 and NIA-12 were having maximum r
values (1350). NIA-2, NIA-4, NIA-5 and NIA-9 also showed higher value of r (1100
to 1300). These were found to be poor slow rusting lines based on their relative
Present study demonstrated that the apparent infection rate r seemed to produce unreliable estimates of slow yellow rusting resistance when compared with disease severity and AURPC because it did not mark Morocco as the fast ruster. Moreover, more variation in r among the tested lines than the disease severity and AURPC, is partly because apparent infection rate is a regression coefficient with larger error variance. Similar results were found for stem rust and leaf rust of wheat (Broers, 1989; Rees et al., 1979). When the epidemic is studied from the very beginning, only then r might give accurate estimate of the level of resistance for cultivars/lines with low level of resistance assuming that the disease progress curve follows a logistic pattern which is not necessary for the case of yellow rust.
Relation of slow rusting parameters with yield parameters: Maximum 1000-kernel weight was produced by NIA-12 (50 g) followed by NIA-11 (49 g) and NIA-10 (47 g) which were having relative AURPC value of 30%, 25 and 31% of susceptible check, respectively (Table 1). However, all of the three were having maximum infection rate and higher final disease severity. Susceptible check having maximum relative disease severity and relative AURPC value produced kernels with least kernel weight (36 g) after NIA-7 (35 g). The minimum kernel weight of NIA-7 despite of its highly better slow rusting behavior may be due to its lower genetic potential and not due to rust infection. Slight negative correlation was observed between 1000-kernel weight and relative AURPC value.
Maximum grain yield per plot was produced by NIA-1 (1.2 kg) followed by NIA-11 (1.0 kg), having relative AURPC values of 33 and 25%, respectively. However, both of them were having higher relative disease severity and r values. Susceptible check i.e., Morocco having maximum relative disease severity and relative AURPC values, produced minimum grain yield (0.56 kg). NIA-1, NIA-5, NIA-10, NIA-13, NIA-14 and NIA-15, having relative AURPC values up to 40%, produced grain yield of 0.8 g. Several other researchers have also reported significant correlation between rust infection and grain yield losses (Allan et al., 1963). Because of negative correlation of relative AURPC of varying degree with 1000-kernel weight and grain yield, it is suggested that relative AURPC may preferably be used to monitor the influence of disease on grain yield as compared to other epidemiological parameters.
CONCLUSION AND RECOMMENDATIONS
The present study revealed that most of the tested lines displayed better performance under high disease pressure when compared with susceptible check. None of the lines was marked as susceptible or highly susceptible while showing resistance of all categories including immune to partial resistance to yellow rust. Among various slow rusting parameters, AURPC was found to be a reliable parameter during the present study. Based on slow rusting behavior, the tested lines were supposed to be having genes for varying degrees of slow yellow rusting and may be used for further genetic manipulations. The tested lines may be advanced further, if having better performance, otherwise. However, these lines should be tested over years and locations for yellow rust along with other desirable characters, before approval.
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