Frequency of Single-Gall Isolates ofCronartium quercuumf. sp.fusiformewith Virulence Toward Three Resistant Loblolly Pine Families

Basidiospores from 11 of 19 aeciospore isolates of C. q. f. fusiforme derived from single galls were highly virulent on half-sib progeny of resistant loblolly pine selection 10-5. Virulence was indicated by a high frequency of seedlings with galls 9 mo after inoculation compared to a normal frequency 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 (...)