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June 23, 2003
Source: Australasian Plant Pathology, New Disease Notes, Vol 32 [edited]

First detection of wheat stripe rust in Western Australia: evidence for a foreign incursion
CR Wellings <colinw@camden.usyd.edu.au >, The University of Sydney, Australia, DG Wright, Dept of Agriculture, Western Australia, F Keiper, The University of Sydney, R Loughman, Dept of Agriculture, Western Australia. Australasian Plant Pathology, 32 (2), 321-322

Wheat stripe rust (caused by _Puccinia striiformis_ f.sp. _tritici_) [Pst] was reported for the first time in Western Australia (WA) in August 2002. Although present in eastern Australia since 1979, Pst has failed to move westwards due presumably to quarantine precautions, the influence of west to east weather movements and the imposing geographic barrier afforded by the Nullarbor Plain and Great Victoria Desert.

The characteristics of the initial pathotype detected in WA indicate a foreign pathogen incursion, suggesting that the latter factors continue to restrict east to west movement of this pathogen in continental Australia.

The recent detection of Pst in WA represents the colonization by Pst of the last major wheat-producing region of the world that had remained free of the disease, despite rigorous quarantine measures that have contributed to disease exclusion since the establishment of the grains industry in the 1800s.

The initial infections were recorded in cv. Stiletto (carrying resistance gene Yr6) in the south eastern wheatbelt of WA. Within 2 weeks, there were 15 confirmed occurrences and within 4 weeks confirmed infected fields had increased to 95.

By the end of the wheat growing season in 2002, the disease was present over a wide area. Rapid spread of stripe rust in the early phase of the epidemic was consistent with experiences in eastern Australia in 1979 (5).

The initial disease sample from Newdegate was accessioned as part of the annual pathogenicity survey at the Cereal Rust Laboratory, Plant Breeding Institute Cobbitty (Sydney) in order to determine the Pst
pathotype. The pathotype was virulent for Yr6, Yr7, Yr8, Yr9, YrA and avirulent for Yr1,Yr2, Yr3, Yr4, Yr5 and several uncharacterized resistances in the differential cultivars (3) as modified by (6). Field responses on cultivars carrying Yr17 (eg Camm) suggested avirulence for this gene.

The unique combination of virulence/avirulence characteristics in this pathotype indicated that the origin of the initial isolate was not eastern Australia. The Newdegate isolate differed in at least 5
pathogenicity features compared with pathotype 110 E143 A+, which was common in eastern regions during the previous 2 years. The pathotype of the Newdegate sample was concluded to be 134 E16 A+, based on modified international pathotype nomenclature (6).

Further preliminary evidence was obtained from an examination of DNA extracted from urediniospores of the Newdegate isolate. Examination of only 5 AFLP primer combinations revealed considerable polymorphism between the Newdegate isolate and the 10 isolates of eastern Australia and New Zealand, with an average of 22 percent polymorphism among amplified loci. This finding further supported the hypothesis of a foreign incursion of Pst to WA.

The characteristics of the Newdegate isolate appear to contrast in 2 virulence features with those reported from South Africa (1). Features of the east African Pst population in the early 1990s (Kenya, Ethiopia), including pathotypes with various combinations of virulence, may resemble those of the Newdegate isolate (4).

A recent report from the USA suggests that Pst race 79, first detected in 2000 (2), has pathogenicity features very close to those of pathotype 134E16 A+. However further work will be required to explore possible origins of the Pst incursion into WA.

References

(1) Boshoff WHP, Pretorius ZA, van Nierkerk BD (2002) Establishment, distribution, and pathogenicity of Puccinia striiformis f.sp. tritici in South Africa. Plant Disease 86, 485-492.

(2) Chen X, Moore M, Milus EA, Long DL, Line FR, Marshall D, Jackson L (2002) Wheat stripe rust epidemics and races of Puccinia striiformis f.sp. tritici in the United States in 2000. Plant Disease 86, 39-46.

(3) Johnson R, Stubbs RW, Fuchs E, Chamberlain NH (1972) Nomenclature for physiologic races of Puccinia striiformis infecting wheat. Transactions of the British Mycological Society 58, 475-480.

(4) Louwers JM, van Silfhout CH, Stubbs, RW (1992) Race analysis of yellow rust in wheat in developing countries; Report 1990-1992. IPO-DLO Report 92-11, 23pp.

(5) O'Brien L, Brown JS, Young RM, Pascoe, T (1980) Occurrence and distribution of wheat stripe rust in Victoria and susceptibility of commercial wheat cultivars. Australasian Plant Pathology 9, 14.

(6) Wellings CR, McIntosh RA (1990) Puccinia striiformis f.sp. tritici in Australasia: pathogenic changes during the first 10 years. Plant Pathology 39, 316-325.

[In addition to wheat, Pst infects other cereals including barley (_Hordeum vulgare_), rye (_Secale cereale_) and triticale (an artificial genus hybrid of tetraploid or hexaploid wheat (_Triticum_ spp.) as the male plant, and diploid rye (_Secale cereale_) as the female in the cross) as well as several grass hosts. Stripe rust occurs worldwide in production areas, especially at higher elevations which tend to have higher relative humidity and cooler temperatures. The disease is prevalent this year in the Pacific Northwest of USA where it is the worst outbreak in many years. Disease management employs the use of resistant cultivars and applications of chemical fungicides containing propiconazole.

I cannot close without recognizing an event of some historical significance to plant pathology. As reported above, the last major wheat production area in the world has fallen victim to stripe rust infection, despite all attempts to prevent a foreign incursion of the fungus. Unfortunately, Nature has her ways. -Mod.DH]