When stem rust is caused by Puccinia graminis, the first symptoms are small yellow flecks that develop into long, narrow, yellow blisters on the stem, leaves and leaf sheaths of young seedlings or plants at any stage of growth.
Puccinia graminis is a obligate parasite with many specialized forms, each of which is capable of infecting one or a few host species.
The blisters break open, revealing a powdery mass of brick red spores, the so called summer spores or repeating spores. The spores escape from the pustules and are carried by the wind, or other forces, to other plants where they germinate, invade the plants, and produce more pustules.
Stem rust pustules develop mostly on the underside of leaves, but may penetrate and sporulate on the upper side. In general, leaves of adult plants are not as receptive as stem tissue for stem rust infection. As infected plants mature, uredinia are replaced by telia, changing color from red to dark brown; the disease called black stem rust.
When the wheat plants approaches maturity, the red pustules turn black as the fungus forms black, winter spores. After the grain is harvested, these spores remain on the debris in the field; in the spring they germinate to form basidiospores of two different mating types.
These tiny colorless spores have as their sole host the native barberry. If they are blown to one of these plants, they may germinate and form yellow pustules containing pycniospores, which must fertilize receptive (opposite mating type) hyphae, after which they can form thousands of aeciospores.
These spores can infect only wheat starting the cycle again. The kernels of the plants affected by this rust become wrinkled and badly shriveled. Stems of the plants are weakened and lodge, or break off easily. Sometimes losses range up to 90%, and the crop is not worth harvesting.
In the early 1900s black stem rust was an especially damaging disease of wheat in the US. Steps were taken to eliminate the alternate barberry host, which continued until 1990.
Black stem rust of wheat
