Field assessment of quantitative resistance to yellow rust in ten spring bread wheat cultivars

Ten spring bread wheat cultivars with a susceptible seedling reaction to race 14E14 of yellow rust were tested at three locations to assess the level and stability of quantitative resistance. Quantitative resistance was expressed in terms of disease severity (DS), area under the disease progress curve (AUDPC), apparent infection rate (r), infection type (IT) and infection density. Large genotypic differences were observed for all variables measured. Morocco was the most susceptible cultivar. Based on its high infection type (IT=9) throughout the epidemics, it most likely does not possess any resistance. All other cultivars carry quantitative resistance. The levels ranged from very low (Taichung 23) to very high (Parula). Resistance levels were lower in Quito, Ecuador than at the other locations. Most likely, the lower temperatures in Quito resulted in a reduced expression of quantitative resistance to yellow rust and to obtain the same protection as at the other two locations, more resistance genes are needed. Therefore, to accumulate genes for quantitative resistance, Quito is considered to be the better location.

Though significant cultivar-location interactions were detected, they were small compared to the cultivar and location effect. Therefore, they are considered of little importance and it is concluded that quantitative resistance is a stable trait, in the sense that cultivar rankings are hardly affected by environment.

The contribution of infection growth to the development of yellow rust was demonstrated. Between 29 and 66% of the increase in disease severity could be contributed to growth of infections. These figures are probably an underestimation of the real contribution as new infections are very small, thus reducing the average size of infections and their contribution to the increase of disease severity.