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Frequency of Single-Gall Isolates of Cronartium quercuum f. sp. fusiforme with Virulence Toward Three Resistant Loblolly Pine Families. E. G. Kuhlman, Principal research plant pathologist, U.S. Department of Agriculture Forest Service, Southeastern Forest Experiment Station, Forestry Sciences Laboratory, Athens, GA 30602; Phytopathology 80:614-617. Accepted for publication 12 January 1990. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1990. DOI: 10.1094/Phyto-80-614.

Basidiospores from 11 of 19 aeciospore isolates of Cronartium quercuum f. sp. fusiforme derived from single galls were highly virulent on half-sib progeny of resistant loblolly pine selection 10-5 (family 10-5). Virulence was indicated by a high frequency of seedlings with galls (68?83%) 9 mo after inoculation compared to a normal frequency of 34?50% for the other isolates of this family. The high proportion of isolates with increased virulence toward progeny of 10-5 was unexpected because seedlings of this resistant selection have been widely planted with no evidence of breakdown in resistance, and the sources of the rust isolates were geographically distant from the original source of the 10-5 ortet. These isolates with virulence toward family 10-5 were common in North Carolina and Maryland, whereas the parent, 10-5, originated 400?775 km away in South Carolina. In contrast, only one of 77 isolates had virulence toward resistant family 11-20 in a second study. The one virulent isolate was from Colleton County, SC, which was the source of another isolate highly virulent toward family 11-20 in a previous test. In a third study, increased virulence toward resistant family 29R was confirmed in three of six isolates. One of these isolates came from a gall on a half-sib offspring of 29R. Isolates with virulence toward families 10-5 and 11-20 were collected in pine stands that had been established with progeny not selected for rust resistance.

Additional keywords: fusiform rust, Pinus taeda.