Effectiveness of adult and seedling resistance in management of stem rust (Puccinia graminis f.sp. tritici) of wheat (Triticum aestivum l.) in CIMMYT lines

Abstract

Stem rust of wheat (Triticum aestivum L.) caused by Puccinia graminis f.sp. tritici had been controlled globally through the use of resistant varieties. However, the emergence of a new and more virulent race Ug99 (designated TTKSK) and its variants reversed these gains. The objectives of this study therefore were: to assess the progress in using adult plant resistance (APR) in controlling stem rust in CIMMYT wheat lines and to evaluate CIMMYT wheat lines for seedling and adult plant resistance to stem rust races TTKSK, TTKST, TTTSK and TTKTK. Both field and greenhouse experiments were conducted at Kenya Agricultural and Livestock Research Organization (KALRO), Njoro (0o 20' S, 35o 56' E). The field experiment involved testing 744 wheat lines originating from 1st, 3rd, 5th, 7th and 9th CIMMYT stem rust resistance screening nurseries and four checks. The experiment was laid in an Alpha lattice design. Seedling and adult resistance to the stem rust races TTKST, TTKSK, TTKTK and TTTSK were determined in the greenhouse, where lines were inoculated artificially. Progress in using APR was assessed using disease severity data from the nurseries between years 2005 and 2016. The proportion of lines which showed severities of ≤30% were higher across all the nurseries in all years of evaluation compared to those that had severities of ≥35%. The number of lines which exhibited low severities (≤30%) increased in the first, third and fifth nurseries in the first three years of evaluation but later reduced in 2015. In 2016, these proportions increased again. In the seventh and ninth nurseries, the proportions declined in 2015 but later increased in 2016 from the initial proportion. The number of lines which revealed high disease severities (≥35%) kept on reducing and increasing with time from the first to the ninth nurseries but these proportions were still lower than those for the lines which showed severities of ≤30%. Out of the 39 lines evaluated for seedling resistance, only three of them showed high infection type (IT) of 3 with race TTKST, one showed IT 3 with race TTKTK, four showed IT 3 with race TTKSK and two showed IT 3 with race TTTSK, the rest showed infection types of between 0 and 2 which is considered low. For APR, only 0.13% of the lines showed a severity of ≤5% to race TTKST while 99.87% showed a severity of ≥10%. To race TTKTK, 43.59% of the lines revealed a severity of ≤5% while 56.41% showed a severity of ≥10%. To race TTKSK, 43.59% of the lines revealed a severity of ≤5% while 56.41% showed a severity of ≥10% and to race TTTSK, 46.15% of the lines exhibited a severity of ≤5% while 53.85% showed a severity of ≥10%. APR is a good breeding strategy as 75.08% of the lines showed severity of ≤30%. Lines SRG7, SRG13, SRG24 and SRG35 showed the lowest final disease severities, infection types and AUDPC in all the four races, therefore can be used as sources of new resistant genes and also be released as new varieties for farmers to adopt.