June 2017 Additions
Emerson M. Del Ponte, Barbara Valent, Gary C. Bergstrom
Fusarium head blight (FHB, or scab) of wheat and barley and wheat blast (or brusone) are two major head diseases with devastating impact on yield and grain quality. These fungal diseases emerged as a major threat to global food security and human health over three decades ago. Since the early 1990s, FHB, caused mainly by Fusarium graminearum sensu lato, resurged in wheat and barley crops worldwide, likely due to increased adoption of conservation tillage practices, expansion of maize production, and climate variability. The disease causes serious economic losses especially during wet seasons that favor epidemics and lead to mycotoxin accumulation at unacceptable levels in grain (McMullen et al. 2012). On the other hand, wheat blast caused by Magnaporthe oryzae Triticum pathotype (MoT, synonym Pyricularia oryzae), first discovered in northern Paraná State, Brazil, in 1985, had been restricted to South America, where sporadic outbreaks in the tropical, warmer, growing regions of Brazil and Bolivia have been reported (Maciel 2011). However, as anticipated by experts after devastating blast outbreaks in the 2000’s, in March of 2016, the pathogen was found for the first time in Asia, in Bangladesh, where it suddenly devastated fields in 10 districts (Islam et al. 2016; Malaker et al. 2016). Infected fields were burned in an attempt to prevent inoculum survival, but blast reoccurred in Bangladesh in 2017. The potential for blast to spread and attack vulnerable wheat acreage in Asia is enormous. However, that was not the only important event for wheat pathologists in that year. Almost concomitantly to the discovery of the wheat blast in Bangladesh, members of the international research community had already planned to meet in Brazil to learn more about these two diseases.
Genetic diversity within and among aecia of the wheat rust fungus Puccinia striiformis on the alternate host Berberis vulgaris
Julian Rodriguez-Algaba, Chris K. Sørensen, Rodrigo Labouriau, Annemarie F. Justesen, Mogens S. Hovmøller
An isolate of the fungus Puccinia striiformis, causing yellow (stripe) rust on cereals and grasses, was selfed on the alternate (sexual) host, Berberis vulgaris. This enabled us to investigate genetic variability of progeny isolates within and among aecia. Nine aecial clusters each consisting of an aecium (single aecial cup) and nine clusters containing multiple aecial cups were selected from 18 B. vulgaris leaves. Aeciospores from each cluster were inoculated on susceptible wheat seedlings and 64 progeny isolates were recovered. Molecular genotyping using 37 simple sequence repeat markers confirmed the parental origin of all progeny isolates. Thirteen molecular markers, which were heterozygous in the parental isolate, were used to analyse genetic diversity within and among aecial cups. The 64 progeny isolates resulted in 22 unique recombinant multilocus genotypes and none of them were resampled in different aecial clusters. Isolates derived from a single cup were always of the same genotype whereas isolates originating from clusters containing up to nine aecial cups revealed one to three genotypes per cluster. These results implied that each aecium was the result of a successful fertilization in a corresponding pycnium and that an aecium consisted of genetically identical aeciospores probably multiplied via repetitive mitotic divisions. Furthermore, the results suggested that aecia within a cluster were the result of independent fertilization events often involving genetically different pycniospores. The application of molecular markers represented a major advance in comparison to previous studies depending on phenotypic responses on host plants. The study allowed significant conclusions about fundamental aspects of the biology and genetics of an important cereal rust fungus.
Sajid Ali, Julian Rodriguez-Algaba, Tine Thach, Chris K. Sørensen, Jens G. Hansen, Poul Lassen, Kumarse Nazari, David P. Hodson, Annemarie F. Justesen and Mogens S. Hovmøller
We investigated whether the recent worldwide epidemics of wheat yellow rust were driven by races of few clonal lineage(s) or populations of divergent races. Race phenotyping of 887 genetically diverse Puccinia striiformis isolates sampled in 35 countries during 2009–2015 revealed that these epidemics were often driven by races from few but highly divergent genetic lineages. PstS1 was predominant in North America; PstS2 in West Asia and North Africa; and both PstS1 and PstS2 in East Africa. PstS4 was prevalent in Northern Europe on triticale; PstS5 and PstS9 were prevalent in Central Asia; whereas PstS6 was prevalent in epidemics in East Africa. PstS7, PstS8 and PstS10 represented three genetic lineages prevalent in Europe. Races from other lineages were in low frequencies. Virulence to Yr9 and Yr27 was common in epidemics in Africa and Asia, while virulence to Yr17 and Yr32 were prevalent in Europe, corresponding to widely deployed resistance genes. The highest diversity was observed in South Asian populations, where frequent recombination has been reported, and no particular race was predominant in this area. The results are discussed in light of the role of invasions in shaping pathogen population across geographical regions. The results emphasized the lack of predictability of emergence of new races with high epidemic potential, which stresses the need for additional investments in population biology and surveillance activities of pathogens on global food crops, and assessments of disease vulnerability of host varieties prior to their deployment at larger scales.