Genotypic Variability for Alternaria alternata (Fr.) Keissler Infection in Bread Wheat
Muhammad Shahab Saqib,
S. Jawad A. Shah,
Naturally infected grains of nineteen candidate wheat lines along with local check were studied for black point symptoms at Nuclear Institute for Food and Agriculture (NIFA), Peshawar, during 2005-06. Data were recorded on the percent incidence, black point symptoms and thousand grain weights. Results indicated that all tested candidate varieties were found susceptible to A. alternata but they differed in symptoms, incidence and severity of infection. Variability in number of affected kernels among candidate varieties fluctuated between 1.2-20.1% while its mean was < 10%. Low level incidence (i.e., < 3%) was recorded in three candidate varieties i.e., 99B4012, NRDW-1 and Wafaq 2001. Percentage of the kernels with >50% disease coverage (i.e. infection severity) varied among the tested cultivars and it was highest (i.e., 30%) in V-99022. Symptoms which were observed in all cultivars included black point, dark smudge and black specks. Based on the disease incidence level of each candidate variety, an inference regarding disease situation under field conditions was derived in which up to 46 infected seeds capable of producing disease will be entering per m2 of the field. Field prediction along with incidence and symptoms severity may be utilized as selection criteria for commercial and upcoming breeding lines and cultivars.
Severe losses in wheat yields have been reported due to different wheat diseases, including rusts (Kisana et al., 2003), smuts, bunt, downy mildew, foot and root rot and black point; observed in various regions of Pakistan (Hafiz, 1986). Black point is one of these diseases caused by Alternaria alternata (Fr.) Keissler, characterized by dark brown to blackish discoloration towards the embryonic end of the seed (Rana and Gupta, 1982) and appears at maturity of the crop during wet spring season but remains inconspicuous initially (Khanum and Nigar, 1985).
Black point infection of reduces emergence (Yu Li et al., 2001; Ijaz et al., 2001), seedling vigor (Kaur et al., 2002) and crop return by downgrading the market value of grains (Desclaux et al., 2000). Malaker and Mian (2002) concluded that reduction in germination, seedling emergence, plant stand, root and shoot growth, vigor index and grain yield were directly related with the severity of black point. This reduced emergence and seedling vigor will result in lower yield and yield components (Ram and Joshi, 1979). It also affects the quality of end products such as pasta, noodles and semolina and reduces their marketability (Hoyle, 1999). Discolored grain is discounted in value because the milled flour contains dark specks (Wiese, 1987). Its incidence also causes downgrading of crop produce (Anonymous, 1991).
Host based resistance is the most effective and economical approach for its
control. Resistance to black point disease is under different genetic control
in wheat cultivars (Conner et al., 1990). Variability, however, does
exist for black point infection. Large differences in black point incidences
were detected among cultivars of soft white spring wheat grown under irrigation
in the field (Corner and Thomas, 1985). Durum wheat have been reported to be
more susceptible than hard red spring wheat to black point (Brentzel, 1944;
Greaney and Wallace, 1943; Machacek and Greaney, 1938). However, Southwell et
al. (1980) identified several durum wheats with black point resistance that
was either equal or superior to that of the bread wheat cultivar. Similarly,
Timgalen. Greaney and Wallace (1943) also noted differences in black point severity
among cultivars in both T. durum and T. aestivum. They concluded
that resistance provides the best means for controlling this disease. Wang et
al. (2002) concluded from his study that significant genotype difference
in susceptibility to black point infection indicated that selecting cultivars
resistant to kernel discoloration would be an efficient measure to control this
Keeping in view the sketchy information available on resistance of breeding material in national wheat improvement program, this research based study was designed primarily to acquire skills of identifying this disease on the bases of symptoms and subsequent screening of germplasm.
MATERIALS AND METHODS
Preparation of working samples: Seeds of the nineteen candidate lines and varieties (hereafter referred to as varieties) were grown at NIFA during sowing season 2005-2006. The trial was planted in three replicates. Seed lot of each cultivar was obtained from each replication as it was raised in the field after harvesting the crop from Crop Breeding Division. Names along with other details of varieties are shown in Table 1. Working seed sample with three replicates of each variety was prepared containing 1000 randomly selected kernels which were counted by using an automatic seed counter. Kernel weight of each replication was also recorded with the help of Sortorius balance.
Seed inspection and disease assessment: Working sample of each cultivar
was carefully examined with the help of magnifying lens having built in light
source. Kernels with all visual symptoms (including black point, dark smudge
and speck) as described in compendium of wheat diseases (Wiese, 1987) and pictorial
guide for identification of field wheat diseases and pests (Prescott et al.,
1986) were manually separated. Samples were than rated for percent incidence
(number of infected kernels) and severity (severely affected, >50% of area
of kernel) of kernel discoloration including all symptoms (hereafter referred
to as total kernel discoloration) and also for individual symptoms of black
point, dark smudge and speck. Disease causing organism was confirmed in a parallel
separate study (data not shown), which was found to be Alternaria alternata.
Forecast of disease foci under field conditions: Based on percent disease
incidence in each variety and 100 kg ha-1 seed rate were used as
parameters in the methodology described by Neergaard (1962) to forecast the
number of infected seeds entering per hectare which are capable of production
disease. Details of varieties/lines are given in Table 1.
The experiment was harvested and threshed plot wise. Grain of each plot was
P = I/10 qs
||No. of inoculated loci per ha (inoculums density)
Quantity of seeds ha-1
No. of seeds g-1
RESULTS AND DISCUSSION
Disease incidence and severity: Analyses of variance showed highly significant
differences (p<0.01) in tested varieties for disease incidence and kernels
area discolored which are shown in Table 2. Highest incidence
(i.e., 21.1%) was observed in 91BT010-84, followed by 93T347 (17.3%), V-00183
(14.9%), V99022 (14%), 99B2237 (10.7%) and 99B2278 (10.5%). Incidence values
grouped under <10%, <5% and <3% were recorded in ten, one and three
|| Candidate varieties along with their parentages belonging
to various institutes
||Incidence and severity of kernel discoloration of candidate
wheat varieties grown at NIFA during 2005-2006
|* = Discoloration of infected kernels is >50%; values with
different letter(s) are significantly different in columns
Absence of resistance combined with conducive environment in the field could
have resulted in higher disease intensity. Iiyas et al. (1999) concluded
that variability in kernel infection dependants on the resistance/susceptibility
of wheat lines. Moreover, varieties differed in maturity time and days to heading.
Hussain et al. (1995) reported that Faisalabad-83, Faisalabad-85 and
Zardana took 95-100, 100-105 and 110-115 days to heading respectively. Similarly,
in the current study this may also be one of the contributing factors in generating
variability in seed infection levels.
Variability was observed in percent severity of kernels discoloration (i.e.
severely affected, >50% of area of kernel) among the varieties which were
grouped under six levels i.e., <5% (V-01180), <10% (CT-00231 and 7-03),
<15% (SN-122, 91BT010-84, 99B2237, RWM-9313 and Wafaq-2001), <20% (V-00125,
SARC-5, Diamond, 99B4012, 99B2278, SD-66 and NRDW-1), <25% (V-00183, 93T347,
Inqilab-91 and Local White) and <35% (V-99022). Varieties with <10% kernels
having discoloration >50% were rated as relatively resistant while remaining
levels were considered as susceptible. As this study was carried out on naturally
infected material so various arguments could be placed for explaining possible
causes of variability in number of seeds which were having discoloration >50%.
It was reported by Hoyle (1999) that cultivar and the stage of plant growth
appear to be related to the severity of disease. Machacek and Greaney (1938)
speculated that infection in the field occurs either during anthesis, when the
florets are open, or at the late stages of seed development, when the developing
kernel forces the lemma and palea apart and becomes exposed to airborne spores.
Several studies (Fokkenna, 1971, Fokkema, 1973; Fokkema and Lorbeer, 1974; Warren,
1972) have shown that the presence of exogenous nutrients from pollen upsets
the balance between pathogens and the phyllosphere mycoflora, allowing the pathogen
to germinate and grow more rapidly than it would in the absence of pollen. A
similar enhancing effect of pollen on black point development in susceptible
wheat has also been observed by Conner et al. (1990). Varieties tested
under this study were either introduction from CIMMYT or CIMMYT derived germplasm
which are under trials for adaptation in Pakistan. Un-adopted germplasm and
wheat breeding lines were reported by Fernandez et al. (1994) to be more
susceptible to black point than the conventional local genotypes.
Symptom categories of kernel discoloration: Kernel discoloration symptoms observed on wheat varieties included black point, dark smudge and dark speck. Black point darkening confined to the germ end, while dark smudge is more progressed form of the infection which has spread along the crease and sides of kernels. Dark speck was recorded on the seed surface. Details of each symptom class in the tested varieties are as under.
Black point: Black point is the predominant symptom observed in all tested varieties with higher frequencies. Its prevalence was greater than 80% in thirteen varieties while in six varieties it ranged between >60 to <80% (Table 3). Only one variety has its prevalence <60%. Mean prevalence of black point in infected kernels of tested varieties was 83% (Table 4). About 98% black point symptom was prevalent in three varieties (i.e., V-00183, V-99022 and 99B4012) which was maximum and followed by V-00125 (95.2%), Diamond (92.7%), 99B2237 (90.1%) and 7_03(91.3%). Minimum prevalence of black point was recorded in V-01180 which reached 59%.
Dark smudge: Dark smudge symptoms were observed in all tested varieties (Table 3) but with low frequencies. Less than 4% of the dark smudge symptom was observed in 17 varieties. Only two varieties were in the range of >4 to <10% while a single variety has little less than 20% dark smudge symptom. Overall estimated mean for dark smudge across infected kernels was 3.1% (Table 4). Maximum prevalence of dark smudge was 18.3% which was recorded in NRDW-1 while it was not observed in V-99022.
||Frequencies of kernel discoloration symptoms in wheat varieties
grown at NIFA during 2005-2006
||Prevalence of black point symptoms in seeds of candidate wheat
varieties grown at NIFA, during 2005-2006
|Values with different letter(s) are significantly different
Dark speck: Dark speck symptoms were observed in infected kernels of all varieties (Table 3). Prevalence of dark speck in four varieties was recorded which fall between less than 1 to less than 4% while enhanced occurrence with little less than 10% was observed in seven varieties. Five varieties each were having occurrence of dark speck with little less than 20 and 40%. Overall mean across infected kernels in all varieties for dark speck was around 13% (Table 4). Maximum occurrence of 39.7% was recorded in V-01180 which was followed by CT-00231(33.3%), 91BT010-84 (25.4%), Inqilab-91(24.5%), Wafaq-2001(20.2%), SD-66(15.2%), 99B2278 (14.7%), NRDW-1(13.5%), Local white (11.4%) and SARC-5(11.3%). Less than 1% occurrence of dark speck was registered in 7_03 and V-00183 varieties lines.
It has been reported by Wang et al. (2002) that low temperature and
high rainfall during the period of grain filling results in sever kernel discoloration
in susceptible varieties.
||Relationship of 1000-kernel weight with incidence and black
point Symptom category of
|*: Major prevailed symptom; values with different letter(s)
are significantly different in columns
||Prediction in candidate wheat varieties based on black point
Current investigation was based on naturally infected material so a local
detailed focused exploratory study with quantitative inoculation of A. alternata
under controlled conditions on the current and upcoming wheat germplasm is required
to establish the causes of variability in displaying the above mentioned symptom
Thousand kernel weight: kernel weight varied between 36.9-55.6 g among
varieties (Table 5). Most of the varieties were having 1000-kernel
weight above 40 g. Mean 1000-kernel weight was around 44 g. Correlations were
worked out for thousand kernel weight with % incidence (including all symptoms)
and black point symptom. Correlation in both cases was found negative (Table
5) which probably shows a trend that with the incidence and black point
symptoms enhancement, 1000-kernel weight reduces.
Disease forecasting: Black point foci available in wheat field at the current level of incidence was estimated/predicted because sowing of black point infected kernels results in failure of germination and reduced seedling emergence (Zhang et al., 1990; McMullen and McKay, 2000). Furthermore, post germination infection will result in weakened stands or restricted grain fill (Riesselman, 1988). On the average across all varieties, 21 kernels capable of producing disease will be entering per m2 of wheat field if current material is used for sowing purpose (Table 6), however, this forecast varied between 3-46 kernels m-1 based on 100 kg seed ha-1.
Variability in incidence and kernel discoloration including all symptoms were established in candidate Pakistani wheat varieties. Incidence, >50% kernel coverage by the symptoms and number of infected seeds entering per m2 are the proposed collective criteria for selecting a relatively resistant line in a breeding program. Based upon the above criteria, V-01180, 99B4012, NRDW-1 and Wafaq-2001 are comparatively resistant to disease may perform better under field condition. Inheritance of kernel discoloration caused by A. alternata require for further study. Further inheritance and gene postulation study is needed to understand the underlying genetics of this variability and genetic resistance of wheat lines. For genetic improvement of wheat, the association of black point with other traits needs to be ascertained.
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