Polymerase chain reaction detection of Ustilago hordei in leaves of susceptible and resistant barley varieties

Although Ustilago hordei infects barley seedlings, symptoms of the disease covered smut are not visible until heading. Natural or artificial inoculati on usually results in inconsistent infection, even in highly susceptible li nes. Thus, breeding for resistance to covered smut is time consuming and di fficult. The ribosomal DNA internal transcribed spacer (ITS) regions of U. hordei were sequenced and a primer pair was developed for polymerase chain reaction (PCR). These primers amplified a 574-bp fragment from DNA of Ustil ago spp., but did not amplify DNA from barley or other common barley pathog ens. DNA extracted from as few as four U. hordei sporidia was detected by t his method. U. hordei DNA was amplified from leaf tissue of inoculated susc eptible and resistant plants at different stages of plant development in di fferential varieties. Growth of the fungus in different leaves of an indivi dual plant was variable. Several highly resistant varieties were shown to c ontain U. hordei DNA in the first three to four leaves, but not in later le aves. Thus, although the fungus can infect many resistant plants, the plant s remain symptomless. Detection of U. hordei prior to heading should assist efforts far breeding for resistance and provide clues concerning the mecha nisms of resistance employed by different resistance genes.