Effect of barberry eradication on changes in populations of Puccinia graminis in Minnesota

Proportions of formae speciales of Puccinia graminis in collections of aeciospores from barberry were determined from cereal rust survey records from 1912 to 2002 in Minnesota. The frequency of P. graminis f. sp. avenae in aeciospore collections fluctuated between O and 10% from 1920 to 2002, even though oat was the dominant small grain crop in Minnesota until 1970. In early years, P. graminis f. sp. tritici was common, but the frequency of P. graminis f. sp. tritici in aeciospore collections declined to a low of 4% in the 1980s, whereas P. graminis f. sp. secalis increased to 96%. After 1990, the frequency of P. graminis f. sp. tritici increased and P. graminis f. sp. secalis declined in aecial collections, possibly indicating a changing proportion of P. graminis f. sp. secalis and tritici on wild grasses near barberry bushes. Diversity of races among uredinial collections of P. graminis f. sp. tritici from wheat in Minnesota declined sharply from 1912 to 1930 and remained low to 2002. Although the races of P. graminis f. sp. tritici most common in uredinial collections also were most common in the aecial collections in five of nine decades from 1912 to 1999 as well in the years 2000 to 2002, the diversity of races was greater among aecial than uredinial collections. Diversity in aecial collections remained nearly constant for 90 years, indicating a continuing contribution of the sexual stage to diversity of P. graminis f. sp. tritici.