Fungal disease of barley
Leaf rust is a fungal disease of barley caused by "Puccinia hordei". It is also known as brown rust and it is the most important rust disease on barley.
Symptoms.
Pustules of leaf rust are small and circular, producing a mass of orange-brown powdery spores. They appear on the leaf sheaths and predominantly on the upper leaf surfaces. Heavily infected leaves die prematurely.
Crop losses.
Leaf rust of barley is considered a relatively minor disease in the United States. However, sporadic outbreaks have occurred in the southeastern and Midwestern regions of the country.
Pathotypes and host resistance.
Most of the barley cultivars grown in the United States are susceptible to "Puccinia hordei". 
Nineteen seedling resistance genes (i.e. "Rph"1 to "Rph"19) have been identified, but only three ("Rph"3, 7 and 9) have been deployed in commercial cutlivars worldwide.
In the United States, the "Rph"7 gene effectively controlled the disease for over twenty years. However, in 1993, pathotypes with virulence to the "Rph"7 resistance gene were identified in Virginia, California, and Pennsylvania.
Recently, the first simply inherited gene conferring adult plant resistance to leaf rust in barley was designated "Rph20".
"Rph20" originated from the two-rowed barley landrace "H. laevigatum" (i.e., "Hordeum vulgare" subsp. "vulgare"); parent of the Dutch cultivar 'Vada' (released in the 1950s).
To date there have been no reports of an "Rph20"-virulent pathotype.
References.
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