Evaluation location: Colorado, United States
Differences among lines in the 2000 leaf spot nursery were highly significant in all tests at each of three evaluation dates. There were three replications in each test, which were arranged in randomized complete block designs. Two-row plots were 12 feet long, with 22-inch row spacing and an 8 - to 10-inch within-row plant spacing. The trial was planted on April 27th in Windsor, CO. Inoculation was performed on July 6th and July 13th. Evaluations were made on August 31th and September 7th and 14th, with the peak of the epidemic occurring on or about the last date. The field was sprayed twice with Betamix Progress, Upbeet, and Stinger (June 2nd and June 12th) to control weeds. The field was thinned by hand and irrigated as necessary.
The high temperatures in the summer of 2000, combined with very low moisture, made it difficult to keep the humidity in the nursery high, and contributed to a mild leaf spot epidemic. The Cercospora epidemic was slow to develop and had not become severe enough to rate until the end of August. Disease severity had started to increase by mid September, and our next rating was expected to be more severe. However, heavy rain shortly before our fourth rating prevented entry into the field, and this was followed by snow and a frost that damaged leaves so that consistent ratings could not be made after September 24. At our third evaluation, means of the resistant and susceptible internal control were 2.5 and 3.75 (scale of 0-10), respectively, across the nursery. In 1999 (September 14), these means were 3.1 and 6.4, respectively. Means of contributor lines ranged from 1.7 to 6.0.