Analysis of Physiological Races of Wheat Stem Rust (<em>Puccinia graminis f. sp. tritici</em>) in Gurage Zone, Ethiopia

Research Article

Ann Agric Crop Sci. 2018; 3(2): 1037.

Analysis of Physiological Races of Wheat Stem Rust (Puccinia graminis f. sp. tritici) in Gurage Zone, Ethiopia

Abrahim A¹*, Wabela B¹, Zerga K¹ and Hailu E²

¹College of Agriculture and Natural Resource, Wolkite University, Ethiopia

²EIAR, Ethiopian Institute of Agricultural Research, Ambo Plant Protection Research Center, Ethiopia

*Corresponding author: Arif Abrahim, College of Agriculture and Natural Resource, Wolkite University, Wolkite Ethiopia

Received: October 29, 2018; Accepted: November 22, 2018; Published: November 29, 2018

Abstract

Wheat is one of the important cereal crops of Ethiopia. It is threatened by abiotic and biotic constraints. Stem rust caused by Puccinia graminis f. sp. tritici is amongst the biotic factors which can cause up to 100% yield loss if susceptible cultivar grown and epidemic occurs. The highland of Ethiopia is considered as a hot spot for the development of stem rust diversity. Guraghe highlands are one of the considerable areas for wheat production in the country. However, the production of wheat in this region is threatened by wheat stem rust disease. So, this study was conducted with the following objectives: to determine pathogenic variability of Puccinia graminis f.sp. tritici populations in Guraghe highlands. Field survey was carried out in four districts of Guraghe zone. Twenty five (25) peasant associations were assessed in each districts and diseased plant parts were collected and kept in labelled paper bags. All samples from the fields were inoculated on universally susceptible wheat cultivar for aggressiveness of isolates. Then virulence isolates were used for race analysis. It is inoculated on differential lines of wheat stem rust. Races TKTTF and TKTTC from the area were most virulent with frequency of 46.67% and 13.33%. Sr11 and Sr24 resistance genes were found to be effective 100% to all races detected. A variant of Ug99 virulent against the stem rust resistance gene Sr24 was not detected in this study. The survey revealed high occurrence and distribution of wheat stem rust in the study area. Race TKTTF was the most prevalent race following with race TKTTC in the study area. Differential hosts carrying Sr24 and Sr11 were an effective gene which confers resistance to all pathogen races. Therefore, it is imperative for the national agricultural research center to replace the susceptible cultivars with currently effective resistance genes. The high frequency of occurrence for specific pathogen race indicated that there is low level of variability in the Pgt population of the surveyed areas. Moreover, it is better to monitor pathogen populations over time to track further virulence evolution and to ensure that currently effective resistance genes are applied within a system of resistance-gene management.

Keywords: Stem rust; Race; Puccinia graminis f.sp. tritici; Differential lines; Resistance genes; Virulence/ avirulence genes; Isolates

Abbreviations

CSA: Central Statistical Authority; EWRTN: Ethiopian Wheat Rust Trap Nursery; SR: Stem Rust Resistance Gene; SNNP: Southern Nations Nationalities People

Introduction

Wheat is an important cereal crop in Ethiopia that is widely cultivated in a wide range of altitude [1]. It is the main staple food for about 36% of the Ethiopian population [2,3]. The area under wheat production is estimated to be about 1.4 million hectares, which makes the country the largest wheat producer in Sub-Saharan Africa [1,2]. Wheat ranks third in area coverage and total production after tef and maize. Bale region is one of the major wheat producing areas in Ethiopia and it is recognized as one of the wheat belts in eastern Africa. About 226,292 hectares of land are devoted to wheat production by subsistence farmers and profit oriented state farms in Bale [2]. In Ethiopia, wheat is produced in many regions of the country. Oromia, Amhara, SNNP and Tigray are the major wheat producing regions in the country with area coverage of 875641.45, 529609.63, 137294.72 and 108865.39 ha, respectively. Furthermore, 47,259 farmers were involved with unestimated area coverage in Gurage Zone in 2015 main production season [4]. The average productivity is lower than the average yield of other wheat producing areas in the country.

The national average yield of wheat in Ethiopia is about 1.4 t/ha2. This is by far below the world’s average, which is about 2.5t/ha [5]. Multifaceted biotic and abiotic factors are responsible for this low yield. Cultivation of unimproved low yielding varieties, insufficient and erratic rainfall, poor agronomic practices, diseases and insect pests are among the most important constraints to wheat production in Ethiopia [1,6].

Cereal rusts are the most destructive diseases of wheat worldwide [7,8]. Stem rust caused by the fungus Puccinia graminis f.sp. tritici Eriks. and E. Henn, has been the most devastating of all wheat diseases under favorable conditions. This disease is also known as black rust of wheat due to the abundant production of shiny black teliospores that form at the end of the season or with unfavorable conditions [9].

Under favorable environmental conditions, stem rust can cause yield losses of up to 100% in susceptible wheat varieties [10]. The yield loss due to this disease is usually greatest when the disease becomes severe before the grain is completely formed, but yield losses are generally influenced by the resistance level of the cultivar grown, the weather conditions and the onset of the disease [11,12]. In Ethiopia, yield losses due to stem rust have been reported to be in the range of 61-100% depending on the susceptibility of the variety and environmental conditions [13,14].

Epidemics of stem rust of wheat often occur in different parts of the world. Resistant cultivars are continuously being developed to prevent such epidemics, although these become susceptible to new pathogenic races sooner or later. A number of physiologic races are known to occur in P. graminis f.sp. tritici [15]. An epidemic of stem rust of wheat occurred in 1972, in Ethiopia, due to the loss of resistance in cultivar Lakech, which was grown on large area. Similarly, the cultivar Enkoy went out of production in the country after the epidemics in 1992. Currently cultivar Kubsa has become highly susceptible to stem rust. Thus monitoring of the races and their virulence is important part of rust management strategy to avoid crop losses [16-18]. There are limited reports on the distribution of stem rust races in some parts of Ethiopia [19,20]. Even though Gurage zone is one of the wheat production areas in the country; there is no any information on the virulence and race composition of wheat stem rust disease. Therefore, this study was conducted with the following objectives:

• To analyze the pathogenic variability of Puccinia graminis f.sp. tritici populations.

• To determine the occurrence and distribution of pathogen races in the study area.

Materials and Methods

Site description

The study was conducted during 2017, in major wheat producing areas of Gurage zone and all the laboratory activities were conducted at Ambo plant protection research center. Geographically, Guraghe Zone is located between 7.8°-8.5° North latitude and 37.5°C-38.7° East longitude of the equator. Wolkite, the capital of the zone, is 155km away from Addis Ababa to south west direction. Gurage zone has a total area of 5932km2. It has 13 woredas’ with a total population estimated about 13432464. The zone comprises altitudes ranging from 1,001 to 3,500 meters above sea level (m.a.s.l). It is classified into three agro-climatic zones: Dega (high altitude) covers 28.3% of the area and ranges between 2,500-3662 m.a.s.l, Woindega (midaltitude) at 1,500-2,500 m.a.s.l, encompasses about 64.9% of the area, and Kolla (lowland) at 1,000-1,500 m.a.s.l covers 6.8% of the area. The mean annual temperature of the zone ranges between 13- 30°c and the mean annual rain fall ranges 600-1600mm. The rainfall pattern in the Gurage Zone is bimodal in which 80% of rain falls in the Kremt period of June to August whereas 20% in the Belg period of February to May. According to land utilization data of the region, 298,369 ha cultivated land, 67,678ha forest, bushes and shrub covered land, 70,249.31ha grazing land, and 14,234ha of land, were covered by others [21].

The pathogen sample collection

Samples were collected from farmer fields and the EWRTN. Forty one (41) samples were collected during the main cropping season of 2017. Sterile scissor with alcohol was used in each field to cut samples. Three to four infected wheat stem tissues with length of 3 to 4 cm were collected and kept in labeled paper bags and transported to Ambo plant protection research center and stored in the refrigerator at 4-5°C until used for variability study. The collected samples were bulked together based on their area of collection and type of cultivars.

Growing test plant in the greenhouse

Five to six seeds of susceptible wheat cultivar (Morocco) were planted in to 7cm diameter plastic pots containing soil, sand and compost mixtures in a 2:1:1 (v/v/v) ratio. The seedlings were allowed to grow until the first leaves were fully emerged. During inoculation, leaves were firstly rubbed gently between moistened fingers to remove the waxy layer from the surface which hinders the penetration of the germ tube of the pathogen spores. Bulk spores from each sample were suspended in distilled water with a drop of Tween20 and then sprayed until runoff using an atomizer. Hands and all materials used during inoculation were disinfested with 70% ethyl alcohol after each inoculation. The seedlings were incubated at relative humidity of about 100% and temperature of about 22°C in a plastic chamber for 24 hours. Then, it was transferred to a greenhouse bench having temperature of 18-27°C. Seedlings were kept for 11-14 days for development of symptom. Samples with viable spores were selected to multiply inoculums for generation of monopustule isolates on the susceptible cultivar, Morocco.

Generation of monopustule isolates

According to the procedures mentioned above, five to six seeds of the universally susceptible wheat variety (Morocco) was planted in plastic pots and then allowed growing for 6-7 days until they produced the first leaves. Seedlings were inoculated very lightly with a population of urediospores of each sample to obtain monopustule isolates.

Urediospores were scrapped from sample leaves/stems in to sterile water in watch glasses using sterile scalpel and were inoculated on seedlings by spatula method of inoculation. The inoculated seedlings were immediately transferred to incubation chamber having saturated atmosphere (95-100%RH) and was kept for 24 hours there at 10-15°C. Thereafter, the seedlings were transferred to greenhouse benches having temperature 18-27°C which is maintained with air conditioners. Seedlings were kept there until urediospores developed. In case where the pustules were aggregated and no isolated pustule occurred, inoculation was repeated on susceptible variety until separate pustules developed and monopustule isolates were generated. A clear plastic cage with a mesh cloth on the top was used to isolate and maintain each isolate.

Multiplication of monopustule isolates

The mono-pustule isolates (Urediospores) were multiplied on the variety “Morocco”. Seedlings were raised following the procedures mentioned in above to multiply mono-pustule isolates. Inoculation of seedlings were done in isolation hood. For the multiplication of spores, each isolate was inoculated until sufficient urediospores were collected for differential host test.

Determination of variability

Seedlings of 20 North American stem rust standard differentials were raised with universal susceptible variety “Morocco”. Greenhouse inoculations were done using the methods and procedures developed by Stakman et al. (Table 1) [22].