A rust fungus was observed on smooth brome grass (Bromus inermis Leyss.) leaves growing in the fields of the University of Wisconsin (UW) Agricultural Research Station at Arlington, WI. The population (WPc-95A) was classified as Puccinia coronata Corda. Reports of P. coronata on B. inermis are rare, so a study of the pathogen host range, alternate host, and morphology of urediniospores and teliospores was undertaken. Fourteen grass species representing 10 genera were inoculated with P. coronata WPc-95A, which was maintained with repeated inoculations on B. inermis cv. PL-BDR1. Seventy-two 30-day-old seedlings of B. inermis were inoculated with urediniospores of the fungus. Infection type, pustule density, and disease incidence were recorded 15 days after inoculation. The same grass cultivars were also inoculated with aecio-spores collected from Rhamnus cathartica L. located on the UW campus. To test for host specificity, urediniospores produced on aeciospore-susceptible grass species were used to reinoculate plants of B. inermis and the host species from which the urediniospores were derived. B. inermis, B. riparius Rehm., Festuca pratensis Huds., and Lolium perenne L. were susceptible to P. coronata WPc-95A. The two Bromus spp. had the highest disease incidence. R. cathartica was found to be an alternate host of P. coronata WPc-95A, as it is for P. coronata isolates found on F. pratensis. However, cross-inoculations with urediniospores from R. cathartica-derived aeciospore infections indicated that only urediniospores of B. inermis origin were capable of infecting B. inermis. Thus, P. coronata WPc-95A appears to belong to a forma speciales previously undescribed in North America.
Common smooth bromegrass (Bromus inermis Leyss.) is octoploid, 2n = 8x = 56, with a genome structure of AAAAB1B1B2B2. Tetrasomic inheritance patterns have been observed in smooth bromegrass, but disomic inheritance is also expected from cytologic observations. Smooth bromegrass is susceptible to the crown rust fungus (Puccinia coronata Corda.). The objective of this study was to determine the inheritance of smooth bromegrass resistance to P. coronata. Seven smooth bromegrass clones, three susceptible and four resistant, were selfed and crossed in a diallel with bulked reciprocals. Inoculations were made with a population of P. coronata from PL-BDR1 smooth bromegrass. Resistance of smooth bromegrass to this population of P. coronata is complex. At least three genes appear to be involved in this host-pathogen interaction, one tetrasomic dominant gene which determines susceptibility (S) and two dominant genes (R1 and R2) that may be complementary and could be inherited either tetrasomically or disomically. Other genes may be involved in the smooth bromegrass-P. coronata interaction, possibly accounting for the lack of fit to expected ratios of some progeny. Heterogeneity for avirulence phenotype in the pathogen population may also have contributed to lack of fit of some progeny. Multiple resistance genes were detected because a pathogen population, likely consisting of genotypes with different genes for virulence, was used to challenge the host.
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