Ephiphytotics of southern rust occurred in the southern USA in 1972, 1973, and 1974 raising concern over the relative susceptibility of the maize germplasm in the United States to Puccinia polysora Underw. Slow rusting is a common form of resistance to many rust diseases, but slow rusting has not been evaluated in the maize/P. polysora interaction. For this reason, tests were designed to evaluate the slow rusting character of 23 single crosses and 33 inbreds in 1983 and 1984. The area under the disease progress curve (AUDPC) was calculated for each entry using weekly assessments of pustule density. Individual weekly assessments of pustule density were also analyzed to compare the relative effectiveness of the two methods for identification of slow rusting. Significant differences were observed among both inbreds and single crosses for AUDPC and for pustule density. The maize inbreds and single crosses evaluated displayed considerable variation for the slow rusting trait. Rank correlations between years were higher for AUDPC than pustule density, although rank correlations over years between pustule density and AUDPC were all high. The indication is that weekly assessments, if correctly timed, are as effective for identifying slow rusting as AUDPC, although they do not provide the details that can be gained by the multiple assessments used in calculating AUDPC.
B.A. Bailey, W Schuh, R.A. Fredericksen, AJ. Bockholt and J.D. Smith
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