Which symptom is characteristic of stem rust on wheat?

The presence of brick red pustules is a characteristic symptom of stem rust on wheat, caused by the fungus Puccinia graminis f.sp. tritici. These pustules, known as uredinia, appear on the stems and leaves of the wheat plants, and they are filled with spores that can be spread by wind and rain to other plants, facilitating the disease's spread.

This distinctive symptom arises during the infection process, where the fungus develops within the host tissue, leading to the development of these reddish-brown pustules that rupture as they mature, releasing spores into the environment. The color and form of these pustules are key diagnostic features that can be used to identify stem rust in the field.

Other symptoms, such as yellowing leaves, wilting, or leaf drop, can be attributed to a variety of other plant stress factors, including nutrient deficiencies, water stress, or different pathogens, but they are not specific to stem rust. Thus, the presence of brick red pustules is particularly important for recognizing and managing stem rust in wheat crops.