Abstract
Stem rust Puccinia graminis Pers. f. sp. tritici of wheat is the most important disease in Kenya. Emergence of race Ug99 and other variants virulent to host resistance genes including Sr31 has rendered 95% of Kenyan cultivars susceptible. This study aimed to identify new sources of resistance to stem rust in a collection of exotic genotypes. Three hundred and sixteen wheat genotypes were screened at the Kenya Agricultural and Livestock Research Organization (KALRO) in Njoro for two seasons in 2015. The host reaction to disease was evaluated based on the modified Cobb scale. The relative Final Rust Severity (rFRS), Average Coefficient of Infection (ACI) and relative Area Under Disease Progress Curve (rAUDPC) were used to characterize the genotypes for stem rust resistance. Agronomic traits were also recorded. Six genotypes namely ALBW-100, ALBW- 204, EPCBW-261, EPCBW-295, PCHP-309 and PCHPBW-310 with significantly low ACI, rAUDPC and rFRS were identified. Thirty five genotypes showed Pseudo-Black Chaff (PBC) phenotype associated with resistant gene Sr2, a source of partial resistance in wheat. The genotypes also showed low disease severity (20-25%) and Moderately Susceptible (MS) – Susceptible (S) infection types in both seasons. Genotypes had significant differences (p ≤ 0.05) on plant height, 1000-kernel weight and number of tillers indicating genetic variation which could be exploited in breeding for resistance to stem rust. The negative relationship between agronomic variables involving plant height, spikelet length and 1000-kernel weight showed harmful effects of stem rust on plant characteristics including yield. The stem rust resistant genotypes with good agronomic traits could be introgressed into adapted Kenyan backgrounds while the genotypes showing presence of PBC could be utilized to develop durable stem rust resistant wheat. Inheritance studies to elucidate the exact genes conferring resistance to stem rust could be conducted for breeders to exploit their genetic variability.
Highlights
Wheat (Triticum aestivum spp. aestivum L.) is an important food grain source worldwide
The six exotic wheat genotypes with high resistance to stem rust could be used as donors for introgression of resistance to the adapted Kenyan wheat backgrounds
The 35 genotypes with low moderately susceptible-susceptible (MSS) response and which showed presence of Pseudo-Black Chaff (PBC) could be integrated in the Durable Rust Resistance Wheat (DRRW) pipeline to develop durable sources of resistance to stem rust
Summary
Wheat (Triticum aestivum spp. aestivum L.) is an important food grain source worldwide. Ever since wheat was introduced in Kenya-nearly a century ago, stem rust has remained a key production challenge. Strategic introgression and deployment of resistance genes in commercial varieties throughout the 1950s greatly circumvented major stem rust epidemics. The threats due to stem rust disease have re-emerged owing to a new set of races, called the “Ug99 family”. The first recognizable variant in this family is the initially characterized race Ug99, designated as TTKSK, based on its effects on select host resistance genes differentials (Jin et al, 2008). First reported in Uganda in 1998, the race TTKSK was found in Kenya in 2001 This race has unique virulence to Sr31 and Sr38 resistance genes widely utilized in wheat worldwide and for which virulence had not been reported previously in the world (Pretorius et al, 2000)
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