Determination of Stem Rust Ug99 Disease Severity in Selected Wheat Backcrosses in Kenya

Stem rust (Pucciniagraministritici) is a disease of economic importance in wheat (Triticumaestivum L.) worldwide. The objective of this study was to determine the disease severity of the BC1F1 plants in the nursery at Kenya Agricultural and Livestock Research Organization, Njoro. Kingbird (resistant) was crossed with Njoro Bw2 (susceptible) to obtain F1 plants. The F1plantswere backcrossed with Njoro Bw2 to obtain a BC1F1 progeny. Two hundred and forty seeds of the BC1F1 plants, twenty eight resistant and susceptible parents were planted in two replicates and the Modified Cobb’s Scale was used to record their disease severity scores. An excel work sheet programmed with the AUDPC formula was used to compute the Area Under Disease Progress Curve (AUDPC) values for BC1F1 plants. Variation of the disease severity scores and mean AUDPC among the wheat plants was analysed using Analysis of variance (ANOVA). Results showed 47.9% ofresistant plants that portrayed the pseudo black chaff (PBC) trait had low disease severity mean scores ranging from 0 to 1.0414 with mean AUDPC values below 99.8. The resistant plants without PBC were 43.3 % and had disease severity mean score ranging from 0.9098 to 1.3686 with mean AUDPC values ranging from 99.8 to 260.The BC1F1 plants that showed susceptibility to stem rust disease were 21 (8.8 %) and had the highest disease severity mean score ranging from 1.3223 to 1.4914 with the highest mean AUDPC values ranging from 282.5 to 431.3. The ANOVA computation showed that, variation in the disease severity and in the mean AUDPC values among the individual plants (P < 0.001) was significant, but variation in mean AUDPC values from one replicate to another (P > 0.001) was not significant. The BC1F1 wheat plants showed a low disease severity score in the field indicating that these plants received the APR genes for stem rust and this could be useful for wheat improvement through breeding.