EFFECT OF STEM RUST INFECTION ON GRAIN YIELD AND YIELD COMPONENTS OF

Stem rust, caused by Puccinia graminis f. sp. tritici is an important disease of wheat worldwide. To estimate grain yield losses due to stem rust, replicated experiments including twelve wheat cultivars i.e.  Gemmeiza 7, Gemmeiza 9, Gemmeiza 10, Gemmeiza 11, Sakha 61, Sakha 93, Sakha 94, Sids 12, Sids 13, Giza 168, Misr 1 and Misr 2 were evaluated for adult plant resistance at Sakha Research Station, Kafr El-Sheikh, Egypt during 2011/12 and 2012/13 growing seasons. The field experiment was surrounded by spreader area of highly susceptible varieties i.e. Morroco and Max inoculated with a mixture of stem rust races as a source of inoculum. Disease severity was recorded each 10 days and area under disease progress curve (AUDPC) was estimated and ranged from 100 to 475 in 2011/12 and 100 to 750 in 2012/13. It was found that, yield losses ranged between 2.47 % in the wheat cultivar Misr 2 to 6.29 % in the wheat cultivar Sids 12 during 2011/12, while during 2012/13 ranged from 1.96 % in the wheat cultivar Gemmeiza 7 to      8.21 % in the wheat cultivar Misr 1. High correlation was found between yield losses with disease severity and AUDPC.


INTRODUCTION
Stem rust caused by Puccinia graminis f. sp. tritici, is the most destructive disease to wheat. Under favorable conditions, stem rust may cause yield losses up to 100 % to the susceptible varieties (Roelfs, 1985 &. The new stem rust race which was designated as Ug99 in Uganda in 1999 has threatened wheat production globally (Pretorius et al., 2000). Wanyera et al. (2006) designated this strain as race TTKS using the letter code stem rust nomenclature system (Roelfs and Martens 1988). A fifth set of differential lines was added, thus Pgt-Ug99 is race TTKSK and variants with added virulence to Sr 24 and Sr 36 are TTKST and TTTSK, respectively (Jin et al. 2008(Jin et al. , 2009. Four other variants of the Ug99 race lineage (TTKSF, TTKSP, PTKSK, and PTKST) are present in different parts of Africa (Park et al. 2011). Race TTKSK and its variants are virulent to about 90 % of the world's wheat cultivars (Singh et al. 2008). Race Ug99 is virulent to a number of stem rust resistance genes, most notably Sr 31for which Ug99 was the first reported virulent race. Also, Ug99 is highly damaging which was reported to cause yield losses of more than 71 % in experimental fields (CIMMYT, 2005). Host resistance is the effective control method for stem rust and has been used worldwide for over 50 years, but TTKSK is virulent to most Sr genes (Jin et al. 2007). Among 56 designated and a few undesignated stem rust resistance genes in wheat, only eight designated genes in the primary gene pool (Sr 13, Sr 14, Sr 22, Sr 28, Sr 33, Sr 35, Sr 42, and Sr 45) confer resistance to TTKSK (Pretorius et al. 2000;Jin et al. 2007;Hiebert et al. 2011). Although host resistance to stem rust has generally provided adequate protection without the need for chemicals (Loughman et al., 2005 andSingh et al., 2008), it is important to evaluate popular commercial cultivars and other improved wheat varieties that may have the potential to replace current susceptible varieties . The objectives of the present investigation were:-1-To examine the reaction of 12 commercial wheat cultivars to stem rust under field conditions. 2-To assess yield losses due to stem rust infection in 12 wheat cultivars when exposed to high stem rust pressure.

MATERIALS AND METHODS
This experiment was carried out at Sakha Agricultural Research Station in two successive growing seasons i.e. 2011/12 and 2012/13, using 12 wheat cultivars i.e. Gemmeiza 7, Gemmeiza 9, Gemmeiza 10, Gemmeiza 11, Sakha 61, Sakha 93, Sakha 94, Sids 12, Sids 13, Giza 168, Misr 1 and Misr 2. The wheat cultivars were grown in a randomized complete block design with three replicates. The plot size was 6 × 7 m = 42 m 2 , each plot contained 20 rows with 7 m long and 30 cm between rows. The experiment was planted 15 days after the regular sowing date (the first half of December) to expose the plants to suitable environment of rust incidence and development. Plots were surrounded by spreader area planted with a mixture of highly susceptible wheat genotypes to stem rust i.e. Morocco and Max to spread inoculum. To provide and maintain the rust inoculum pressure the experiment was also inoculated by injection method twice in a week during the growing season. To maintain crop stand/vigor normal agronomic practices including recommended fertilization dose and irrigation schedule were followed. To keep protected plots almost free from stem rust, the fungicide Sumi-eight 5 EC (1H-1,2,4-Triazole-1-ethanol,.beta.-[(2,4-dichlorophenyl)methylene] -.alpha.-(1,1-dimethylethyl)-,(.beta.E) (35 cm /100 litter water) was applied at 10, 26 February and 8 March. Stem rust severity and reaction were evaluated for each plot every 10 days intervals from rust appearance along with the stages of plant growth using the modified Cobb , s scale (Peterson et al., 1948) and the host response scale described in Roelfs et al. (1992). The area under disease progress curve (AUDPC) was calculated for each variety according to the equation adopted by Pandy et al. (1989). AUDPC = D [1/2 (Y1 + Yk) + (Y2 + Y3 + -----+ Yk-1)] Where: D = days between two consecutive records (time intervals) Y1 + Yk = Sum of the first and last disease records. Y2 + Y3 + -----+ Yk-1 = Sum of all in between disease scores.  At maturity the crop was harvested and yield of each cultivar of 42 m was weighted by conventional balance. The influence of stem rust severities on yield was determined by comparing the yield of infected and protected cultivars. Yield loss was estimated using the simple equation as follows:-Loss % = 1-yd/yh X 100 (Colpauzos et al., 1976). Where: Yd = yield of diseased plants.

Yh = yield of healthy plants.
Data of 1000-kernel weight (g) and grain yield (kg) was calculated following Hassan (2004). Randomly selected thousand kernel from each entry were counted with a seed counter and were weighed with an electronic balance to calculate 1000-kernel weight. The grain weight from the threshed spikes was measured entire harvested plots was weighed with an electronic balance to calculate grain yield per plot for each cultivar. Least significant differences (L.S.D. at 5%) was used to compare yield components according to (Snedecor, 1957). Correlation coefficient was also used to detect the relationship between yield loss and AUDPC.

RESULTS AND DISCUSSION
The present study clearly showed that the wheat cultivars showed high stem rust disease severity exhibited maximum values of AUDPC and yield losses. While the wheat cultivars showed low disease severity displayed minimum values of AUDPC and yield losses. It was also evident that the susceptible wheat cultivars suffered more yield losses than those of moderately resistant to resistant to stem rust.

Reaction of commercial wheat cultivars to stem rust:
The reaction of the commercial wheat cultivars to stem rust at adult plant stage under field conditions is shown in Tables 2 and 3. The fungicide-protected plots remained almost free from stem rust during the two growing seasons of this study (2011/12 and 2012/13).
In 2011/12 growing season, all of the tested wheat cultivars showed different disease severity ranged from 5 to 30 % (Tables 2 and 3). The cvs. Gemmeiza 9, Gemmeiza 10 and Sakha 94 showed the least disease severity (10 R), followed by Misr 1 and Misr 2 (10 MR and 10 MS). While, the rest of the tested cvs. exhibited rust severity ranged from 5 % to 30 %. The rust severity of the cvs. Misr 1 and Misr 2 was 20 MR and 10 MS, respectively in season 2011/12, while in season2012/13 the rust severity of the same genotypes were 40 % and 30 %. This high severity occurred in these cultivars in season 2012/13 may be due to the appearance of new stem rust race (s). Emebet et al., (2006) found that the wheat varieties Hawi and Simba were highly susceptible at both seedling and adult growth stages despite their resistance in previous studies for stem rust isolates from Debre Zeit and Ambo. Also, the wheat cultivars Simba and Wetera were susceptible at both growth stages (Denbel et. al., 2013) and this may be due to the widest virulence race in this study. However, they were resistant to Debre Zeit and Ambo isolates of stem rust of wheat both in the seedling and adult plant growth stages (Emebet et al., 2006). Area under disease progress curve (AUDPC): Data in Tables 2, 3 and Figure 1 indicates that AUDPC run in a parallel line with disease severity.  Macharia and Wanyera (2012) found that the wheat line 102091 showed low level of AUDPC, while the wheat cultivars Chozi and Duma showed high levels of AUDPC to stem rust. Grain yield and yield losses: The 1000 kernel weight and grain yield per plot differences between protected and infected wheat genotypes due to the differences in the level of disease severity of stem rust as shown in Tables 2 and 3. In 2011/12, the loss % of the 1000 kernel weight ranged from 2.03 % to 6.71 %. The cvs. Sids 12 and Sids 13 gave the highest values of loss % of 1000 kernel weight (6.71 and 5.07, respectively) compared to the other genotypes. In 2012/13, the loss % in the 1000 kernel weight ranged from 2.20 % to 6.17 %. The cvs. Misr 1, Misr 2, Sids 12 and Sids 13 gave the highest values of loss % of the 1000 kernel weight (6.17 %, 5.69 %, 5.83 % and 5.44 %, respectively) followed by cvs. Gemmeiza 10, Gemmeiza 9, Giza 168, Gemmeiza 11, Gemmeiza 7, Sakha 61, Sakha 94 and Sakha 93.  the highest values of loss % of yield per plot (8.21 %, 6.81 %, 6.17 % and 5.34 %, respectively). While, the cvs. Gemmeiza 7, Gemmeiza 11, Gemmeiza 10, Sakha 93, Gemmeiza 9, Sakha 61, Sakha 94 and Giza 168 showed the lowest values of loss % of yield per plot. This trend is in a harmony with losses reported in previous studies obtained by Loughman et al., 2005). Ochoa and Parlevliet (2007) reported that yield loss was correlated strongly with area under disease progress curve, which means that high levels of partial resistance are needed to prevent significant yield loss. This study showed that stem rust reduced yield irrespective of the type and level of resistance possessed by the cultivars (Singh et al., 2008 andPretorius et al., 2007). The effect of rust on grain yield may be due to the energy expenditure in plant defense mechanisms rather than for growth and grain information (Smedegaard-Petersen and Tolstrup, 1985). Moreover, the tissue damage caused by hypersensitive reactions also contributes to yield reduction (Khanna et al., 2005). The present study showed that infection with stem rust can severely reduce grain yield on susceptible cultivars. Therefore, growing slow rusting cultivars will reduce the loss in grain yield. Association between AUDPC with loss in the 1000 kernel weight and loss in plot weight: The association of the 1000 kernel weight and loss in plot weight with AUDPC was determined through regression analysis during 2011/12 and 2012/13 growing seasons. Positive relation between AUDPC and loss in 1000 kernel weight during the two growing seasons (R 2 = 0.492 and 0.466) (Fig. 2). Also, regression analysis revealed a significant linear relationship (R 2 = 0.783 and 0.674) between loss in plot weight and AUDPC. On overall basis cultivars with maximum disease severity had lower mean grain yield and vice versa (Shaner et al., 1978). Ochoa and Parlevliet (2007) reported that yield loss was correlated strongly with AUDPC. El-Shamy et al. (2011) found that a significant correlation between mean disease severity and percentage loss for 1000-kernel and grain yield/plant.