Alert: Lr24 virulence looks like it's here to stay

THE VIRULENCE for leaf rust resistance gene Lr24, detected for the first time in October 2000, was again detected throughout eastern Australia during the 2001 season.

This virulence has had important implications for the Australian wheat industry because some 13 recommended cultivars carry the resistance gene Lr24 and are therefore potentially under threat.

Detailed field testing by the National Cereal Rust Control Program (NCRCP) at the Plant Breeding Institute Cobbitty during the 200 I season using the new Lr24 pathotype has determined the adult plant response of current wheat cultivars and breeders lines. Such information often cannot be obtained reliably in greenhouse tests.

Of particular importance, the tests confirmed that Dennis, Gilesm, Petriem and Sunsoft 9Srt> were very resistant to Lr24 virulent rust as adult plants. Other cultivars were moderately susceptible (Anlacem, Babble, Cunningham, Datatinem, Janz, Langm, Mira Mitrert, Mulgaram, Perouse, Tasmanrt, Sunpict) or susceptible (Krichauff, Worrakatta) in the field.

Unlike greenhouse tests, where environmental parameters such as temperature are carefully controlled, some variability is experienced in field testing and it is likely that, under certain conditions, the relative rating of moderately susceptible to susceptible cultivars will change.

The early detection and confirmation of virulence for Lr24 by the NCRCP last year provided some lead time during which information such as varietal response to the new rust pathotype could be assessed. The virulence was detected during Australia-wide surveys of the cereal rust pathogens conducted by staff of the NCRCP.

These surveys place particular emphasis on monitoring the effectiveness of currently deployed resistance genes, and provide an early warning system should virulence develop on a given resistance.

By preserving new pathotypes as they are detected in surveys, the NCRCP has established an invaluable collection of cereal rust isolates. The collection is maintained in liquid nitrogen at _1960 Celsius. The rust collection was vital in gaining a better understanding of the new Lr24 virulent rust.

Greenhouse tests using older pathotypes maintained in the collection showed clearly that the new pathotype had developed by mutation in an existing one. This information gave an immediate indication of which cultivars were under threat, and which were not. At the same time, adequate quantities of the Lr24 virulent pathotype were produced and stored in liquid nitrogen, for greenhouse and field testing of advanced lines from all breeding programs.