WHEAT STEM RUST (Puccinia graminis f. sp. tritici) RACES AND INTEGRATED MANAGEMENT OF THE DISEASE IN NORTHWEST ETHIOPIA

Abstract

Wheat rusts are among the major diseases that cause high yield loss on wheat in Ethiopia. Wheat is mainly affected by stem rust, Puccinia graminis f. sp. tritici (Pgt) in warmer highlands which is usually managed by using host resistance and fungicides. Pgt is capable of producing new physiological races that can overcome resistant varieties resulting in epidemic under favorable environmental conditions, which results in serious yield losses and the development of fungicide resistant pathogen races. Northwest Ethiopia is one of the major wheat producing regions, where stem rust is a major production constraint. Experiments were carried out to determine the virulence spectrum of Pgt in northwest Ethiopia. Eightysix stem rust samples were collected during the 2017 and 2018 cropping seasons and analyzed on 20 standard stem rust differential lines. In-vitro and in-vivo experiments were conducted to identify inorganic salts that have antifungal activities against Pgt under controlled environment conditions. Eight inorganic salts were tested in a completely randomized design at concentrations ranging from 5 to 20 g l-1. Field experiments were conducted at Adet and Aneded, Ethiopia, in the 2017 and 2018 main cropping seasons to study the effectiveness of old and new wheat varieties against stem rust. Fourty-nine bread wheat varieties that are under production as well as old varieties that are out of production due to stem rust susceptibility were evaluated in a simple lattice experimental design. Finally, experiments that studied integrated management of stem rust using host resistance and inorganic salt foliar sprays were conducted at two locations in a randomized complete block design with three replications. Race analysis experiments identified four races of Pgt (TTTTF, TKTTF, TKPTF, and TTRTF) from 86 isolates. Of these, 60.4 and 60% of the isolates were race TTTTF followed by TKTTF (Digalu race) with a frequency of 38.7 and 37.3% in 2017 and 2018, respectively. These two races accounted for almost 99% of the Pgt population. The least abundant races were TKPTF and TTRTF that accounted for 1% in both seasons and were detected only at single location (Farta in South Gondar in 2017 and Ambagiorgis in North Gondar in 2018, respectively). The differential hosts carrying resistance genes Sr5, Sr21, Sr9e, Sr7b, Sr6, Sr8a, Sr9g, Sr36, Sr17, Sr9a, Sr9d, Sr10, SrTmp, Sr38 and SrMcN were ineffective to the four races identified in northwest Ethiopia. Two differential hosts carrying the resistance genes Sr24 and Sr31 were effective against all the four races identified in this study. Therefore, since non of the identified races virulent on xiii genes Sr31 and Sr24, can be used as sources of resistance in combination with other stem rust management systems in the study areas. The controlled in-vitro experiment results demonstrated that there was a significant (P < 0.05) difference among inorganic salts and their concentrations on the germination of Pgt uredospores in-vitro. Potassium carbonate completely inhibited the germination of Pgt uredospores at three concentrations (i.e. 10, 15, and 20 g l-1) followed by potassium bicarbonate, sodium bicarbonate, and ammonium bicarbonate with an inhibition rate of 99.7, 97.33 and 97%, respectively. The lowest inhibition of spore germination was recorded from potassium dihydrogen phosphate (6.7%) and potassium chloride (9.3%) at 5 g l-1. The in-vivo study showed that six inorganic salts significantly (P < 0.05) reduced the number of pustules and pustule length on the susceptible wheat cultivar ‘McNair’invivo. In this study, potassium carbonate and potassium bicarbonate were identified as the best inorganic salts to manage wheat stem rust. The results provide an important basis for further study on the alternative uses of potassium carbonate and potassium bicarbonate salts in the control of wheat stem rust. Results of the field studies indicated that the wheat varieties evaluated varied significantly (P < 0.05) in severity, area under disease progress curve, disease progress rate and grain yield. Varieties Hoggana, Shorima and Sanete at Adet and Hoggana, Shorima and Abola at Aneded were resistant to stem rust in both seasons. Varieties Sofumer, Enkoy, ET13A2, Tuse¸ Senkegna, Dinknesh, Alidoro, Tay, Honqollo, Tsehay, Mandoyu, Mitike and Medawalabu at both locations and variety KBG-01 and Dashen at Adet and G’ambo, Galema, Danda’a, Katar, Pavon76, Guna, Densa, K6290A, Sulla, Dereselign, and Dure varieties at Aneded were moderately resistant in both seasons. Variety Enkoy which is currently out of production due to stem rust susceptibility was moderately resistant at both locations. Hence, Enkoy can potentially be reintroduced in to production if it is competitive in yield with the recently released varieties. Results from the integrated management study showed that the lowest rust severity, area under disease progress curve and slower disease progress rate occurred on the resistant and moderately resistant varieties sprayed with Tilt, followed by potassium carbonate. Integrated use of potassium carbonate and the resistant variety ‘Hoggana’ had comparable effect on wheat stem rust to the commercially recommended fungicide ‘Tilt’ at Aneded but not at Adet. The partial budget analysis showed using the recommended fungicide Tilt 250 EC the highest NB and Rate of return on both varieties followed by potassium carbonate. Hence, it needs further investigation as to its viability as an alternative option to manage the wheat stem rust disease. More extensive studies are suggested to evaluate new inorganic salts and additional wheat varieties to work out more effective management strategies against wheat stem rust.