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Causal organism - Puccinia striiformis
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There are distinct forms of the fungus which are specific to crops.
P. striiformis f.sp.tritici can attack wheat and triticale.
P. striiformis f.sp.hordei can only attack barley.
Within the forms of P. striiformis there are races which can only attack
particular varieties.
The characteristic symptom is of parallel rows of yellowish orange coloured
pustules
on the leaves of adult plants. epidemics
of yellow rust often start as individual plants showing symptoms, usually
in the autumn. These usually develop slowly over winter and are often missed
until the early spring when small patches or foci
of infected plants can be seen in fields 
The yellow to orange coloured pustules on leaves in the very early stages
are very difficult to distinguish from brown rust
However, yellow rust lesions tend
to spread as a band across young leaves, often with a yellow band on the leaf
moving ahead of the sporulating lesion
On older leaves pustules tend to occur in obvious stripes
Severe attacks quickly give rise to chlorosis and later necrosis of leaves
Infected leaves can rapidly desiccate in May/June if weather conditions are
warm and dry. In severe attacks yellow rust infection of the ears can occur,
resulting in the formation of masses of spores between the grain and the glumes.
At the end of the season, secondary black spores (teliospores)
are sometimes produced amongst the stripes of pustules
