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Puccinia polysora Epidemics on Maize Associated with Cropping Practice and Genetic Homogeneity. M. C. Futrell, Research Plant Pathologist, Plant Science Laboratory, Agricultural Research Service, U.S. Department of Agriculture, and the Mississippi Agricultural and Forestry Experiment Station, Mississippi State 39762; Phytopathology 65:1040-1042. DOI: 10.1094/Phyto-65-1040.

Southern corn rust (which is incited by Puccinia polysora) caused losses to late-planted corn in the lower Mississippi River valley in 1972, 1973, and 1974. The northern limit of the epiphytotics was Natchez, Mississippi, in 1972; Cairo, Illinois, in 1973; and southern Illinois and Indiana in 1974. The 1974 epiphytotic extended eastward from the Mississippi River into the Tennessee, Cumberland, and Ohio River valleys. This epiphytotic reached western Kentucky by late June and probably would have spread into the main part of the Corn Belt except for dry weather in June and July. Yields of heavily rusted late-planted corn were 5,320 kg/hectare (ha), compared with 9,604 kg/ha for nonrusted early planted corn. Severe stalk lodging was associated with heavy rust infection. Two major factors are believed to contribute to increased rust incidence on corn in the lower Mississippi River valley: (i) late-season corn production has increased with attempts to grow two corn crops annually, and (ii) limited germplasm in currently grown hybrids has reduced genetic diversity leaving corn genetically vulnerable to P. polysora.

Additional keywords: yield-reduction, stalk lodging, Zea mays, genetic vulnerability, epidemiology.