Distribution, Epidemics dynamics and physiological races of wheat stem rust (Puccinia graminis f.sp. tritici Eriks and E. Henn) on irrigated wheat in the Awash River basin of Ethiopia

Wheat is one of the high value important major crops of the globe. However, wheat stem rust is considered one of the determinant threats to wheat production in Ethiopia and the globe. So this study was conducted with the objective to assess disease intensity, seasonal distribution dynamics, and genetic variability and to determine virulence spectrum of stem rust in the irrigated wheat areas of Ethiopia. A total of 137 wheat farms were evaluated from 2014/15 - 2019/20 in six districts of Awash River basin. Farm plots were assessed every 5 - 10 km interval with ‘X’ fashion, and data on disease incidence, severity, and healthy plants were scored with diseased wheat plant samples collection for stem rust race analysis. The seasonal trend of wheat stem rust disease was also compared to see the future importance of the diseases. The result revealed that the prevalence, incidence, and severity of stem rust were significantly varied among the different districts and seasons in the two regions. The survey results also indicated that about 71.7% of the wheat fields were affected by stem rust during the 2018/19 growing season. The overall incidence and mean severity of the disease during the same season were 49.02% and 29.27%, respectively. During 2019/20 about 63.7% of the wheat fields were affected by stem rust, which, however, the incidence (30.97%) and severity (17.22%) were lower than the previous season. Although the seasonal disease distribution was decreased, its spatial distribution was expanding into Lower Awash. The physiological and the genetic race analysis identified four dominant races (TTTTF, TKTTF TKKTF, and TTKTF) during 2018/19 and additional race (TKPTF) during 2019/20. Thus races are highly virulent and affect most of the Sr genes except Sr – 31 and Sr – 24. TTTTF and TKKTF are the widest virulence spectrum which affects 90% of the Sr genes. Thus, it can be concluded that the spatial and seasonal distribution of the disease was expanding. Moreover, most of the races were similar with rain-fed production, and thus care must be given for effective management of the diseases to both agro-ecologies. Therefore, these findings provide inputs or insight for breeders to think about the breeding programs in their crossing lines and wheat producers to reduce the damage of the disease in the irrigated ecologies. ETHICAL STATEMENT Thus, surveys were conducted with the lateral aim of rust epidemics early warning and monitoring support program in the Awash River basin. Samples for this study were collected from farmers’ fields of the irrigated production areas in the Awash River basin. The disease was an air-borne disease that is difficult to contain. Still, we give maximum care during surveying through spore-free through self-sanitation after Pgt infested field observation to minimize induced disease dissemination to the communities in the production areas that no specific permissions were required for these locations. Field sites are on public access, and P. graminis f. sp. tritici is already an air-born pathogen that doesn’t need special protection kinds. This work was our study experience in the endeavor in irrigated wheat technology dissemination.


Survey of Wheat Stem Rust Distribution in Awash River Basin Fields
146 The survey and surveillance assessment were done among the farmers' fields at every 5-10 km intervals 147 following the main road routes in wheat farms. Stem rust assessment was carried out along the double 148 diagonal 'pattern and samples were taken using 1m 2 quadrate. In each field wheat plants within the 149 quadrate were counted and recorded as infected and healthy, and the intensity of stem rust was 150 calculated. The prevalence, incidence, and severity of the diseases were calculated with the formula 151 given below.
152 153 154 155 The disease severity under field condition was recorded followed by the modified Cobb's scale as 156 developed by Peterson et al. (1948

Data Analysis
247 Survey data (prevalence, incidence, and severity) were analyzed by using descriptive statistical analysis 248 (means) over districts, varieties, and altitude range, and crop growth stages. Similarly, race analysis 249 was analyzed using those descriptive statistics (Gomez and Gomez, 1984  In 2018/19 the diseases escalated in terms of prevalence (71.7%), incidence (47.45%), and severity 283 (29.27%) in Awash River basin. The highest incidence of stem rust was noted in Gellalo with 284 (83.33%), the second-largest incidence was in Fentale (65%), and the third incidence was in Amibara 285 (60%). In Sire and Jeju districts of the Oromia region the incidence of Sr was 30% and 50% 286 respectively, whereas the lowest incidence was in Dubti (;) during physiological maturity. The disease 287 severity showed a similar trend as the incidence. The highest severity in Gellalo with range mean 288 values 60-80%, (70%) respectively. This was followed by Fentale and Amibara districts with a range of 289 0-80% for both districts and a mean value of (40%) and (35.63%) respectively. In Sire and Jeju 290 districts, the severity was (16.67%) and (28.5%) respectively. The rust-free district was Gewane due to 291 poor management performance of the wheat was also bad, due to management after booting they 292 stopped irrigation water completely. Even though the late planting date increases the outbreak of stem 293 rust diseases. Stem rust is polycyclic by its nature in those types of air-borne diseases the spore 294 pressure in the air was critical to the nature of disease virulence. Most of the varieties released in Werer 295 ARC including Fentale two were susceptible to stem rust. Fentale two ranges with 100% incidence and 296 80% severity in Werer Seed multiplication leased farm. The losses detected with this disease was also 297 affected the seed multiplication unit of the center and caused high yield loss. In 2018/19 the disease 298 invades most wheat production districts in the basin and incurs economic losses in terms of agricultural 299 input cost for fungicide purchases, application cost, and yield losses. The disease pressure was severing 300 in farms near to Awash River with high morning dew pressure and low on farms which are far from 301 riverbanks.
302 The pressure of the disease was more severe in the farms near Awash River with high morning dew 303 pressure and low on farms that are far from riverbanks.
304 Table 3   365 The data showed that the number of stem rust-infected fields decreased as the altitude decreased. The 366 incidence also decreased from 41.14% at upper awash to 17.5% at middle awash and 8.33% at lower 367 awash. The same result was recorded regarding the disease severity 24.11% at upper awash 9.38% at 368 middle awash and 3.89% which is a trace in lower awash fields. In rain-fed agroecology Ayele et al., 369 (2008) reported that the highest level of stem rust infection was recorded in the altitude range of 1600 -370 2500. High humidity along with high temperatures during the growing season in the low altitude areas 371 favors the growth of the pathogen (Roelfs et al., 1992). 390 Surveys in 2019/20 a total of 46 stem rust samples were collected and they were taken AARC for 391 physiological race analysis. During this cropping season, a total of 28 stem rust samples in Oromia 392 (Sire, Jeju, and Fentale) districts and 19 samples were collected from Afar regional state (Afambo, 393 Dubti, and Amibara) districts.
401 Table 9.  427 This study showed that the stem race races in the irrigated wheat production areas of the Awash River 428 basin were highly virulent to most of Sr genes. The only effective genes able to provide a good harvest 429 in the existing races are Sr 24 and Sr 31. On the other hand, more than 80% of the Sr genes were 430 defeated by 100% of the isolates. The resistance gene McNair (Sr McN) was ineffective to all isolates 431 tested (Table 4). During 2018/19 a total of 33 isolates were collected in cool seasons. Out of this only 4 432 were viable and analyzed on to the 20 stem rust differential lines in 2018/19. From the viable samples,