Descriptions

Outline

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Puccinia graminis

Puccinia imagesPuccinia graminis, the causal agent of stem rust, has caused serious disease of small cereal grains (wheat, barley, oat, and rye) worldwide. P. graminis is the first sequenced representative of the rust fungi (Uredinales), which are obligate plant pathogens. The rust fungi comprise more than 7000 species and are one of the most destructive groups of plant pathogens. Stem rust of wheat has been a serious problem wherever wheat is grown and has caused major epidemics in North America (1). In 1999, a new highly virulent race TTKS (Ug99) of P. graminis was identified in Uganda, and since then has spread, causing a widening epidemic in Kenya and Ethiopia (2). Due to its devastating ability to cause epidemics, P. graminis was developed as a biological warfare agent during the Cold War (3) and is now considered one of the most important potential agricultural bio-terrorism threats to U. S. agriculture (4).

Puccinia graminis is a heteroecious rust fungus with five spore stages and two hosts (5). The asexual stage (uredinial) infects cereal and grass hosts and is the economically important form of the pathogen. The sexual stage begins in the resting spore (teliospore) on leaves and stalks of the gramineous host. The sexual stage is completed on the aecial (alternate) host, barberry. P. graminis, as a species, has a broad host range that includes more than 300 species of cereals and grasses. Wheat stem rust, P. graminis f.sp. tritici is known to naturally infect 28 species belonging to eight genera, including wheat, barley and rye.

References

  1. A. P. Roelfs, in The Cereal Rusts Vol II. Diseases, Distribution, Epidemiology, and Control A. P. Roelfs, W. R. Bushnell, Eds. (Academic Press, Orlando, 1985) pp. 3-37.
  2. www.globalrust.org
  3. R. F. Line, C. S. Griffith, in Stem Rust of Wheat, From Ancient Enemy to Modern Foe P. D. Peterson, Ed. (APS Press, St. Paul, 2001) pp. 83-118.
  4. Public Health Security and Bio-terrorism Preparedness and Response Act (2002).
  5. K. J. Leonard, L. J. Szabo. 2005. Pathogen profile: Stem rust of small grains and grasses caused by Puccinia graminis. Molecular Plant Pathology 6:99-111.

Puccinia triticina

Puccinia triticina, the causative agent of wheat leaf rust (also known as brown rust of wheat), is one of the most serious diseases of wheat in North-America and throughout the world. Severe epidemics caused by leaf rust and stem rust, caused by the related species P. graminis f. sp. tritici, plague North-American wheat production. Wheat resistance to cereal rusts is precarious at all times, as new races evolve regularly and threaten sustainable crop production. Genetic resistance remains the most economical and environmentally sound method of minimizing yield losses due to rust fungi, but development of wheat cultivars with long-lasting leaf rust resistance has been complicated by the highly variable nature of P. triticina.

P. triticina is a basidiomycete and belongs to the subphylum Pucciniomycotina, which contains approximately one-third of all described species of basidiomycetes species. Puccinia is the largest genus of rust fungi and currently contains approximately 4,000 species. P. graminis, P. striiformis, and P. triticina represent distinct lineages within the cereal and grass rusts.

P. triticina has a complex life cycle which includes five different spore types and two hosts, wheat and its alternate host meadow rue (Thalictrum speciosissimum) on which it completes its sexual stage. The asexual uredinial stage on wheat is the economically important part of the life cycle which can progress from initial infection to sporulation within ten days under warm and humid conditions, potentially leading to epidemics. During infection on wheat, the pathogen undergoes a high degree of morphological and physiological differentiation. After landing on a wheat epidermis, the dikaryotic urediniospore germinates within hours when sufficient moisture is available. Germination and formation of infection structures are affected by chemical, temperature and surface contact responses. The emerging germ tube extends until a stomatal pore is encountered. Thigmotropic responses to the topology of the leaf surface and negative phototropism help direct the germ tube to a stoma where an appressorium is produced over the stomatal aperture.

Puccinia striiformis

Puccinia striiformis f. sp. tritici (Pst) causes stripe rust, an important disease of wheat worldwide. It has become increasingly important in the U.S. where epidemics since 2000 have caused yield losses of over 246 million bushels in spite of millions of dollars spent on fungicides. Pst populations change their virulence amazingly rapidly by mechanisms that are largely unknown. Pst is recalcitrant to classical genetic manipulations because it is an obligate biotroph without a sexual stage; our best chance of understanding its genetic nature and interaction with wheat lie in comparing genomic and transcriptome sequences of different isolates and comparisons to related Puccinia species.