When stem rust us 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.
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.
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.
The principal means of managing or controlling of stem rust is the use of resistant cultivars. Unfortunately, the resistance of these cultivars is only temporary, as an infective race of the rust develops, or appears, usually in about five years.
This means that new cultivars must be in constant development. Fungicides applied by spraying, or systemic fungicides applied to the soil just before sowing are often effective in controlling this disease.
Black stem rust in wheat
