Stem rust

Abstract: Rust fungi are some of the most devastating pathogens of crop plants. They are obligate biotrophs, which extract nutrients only from living plant tissues and cannot grow apart from their hosts. Their lifestyle has slowed the dissection of molecular mechanisms underlying host invasion and avoidance or suppression of plant innate immunity. We sequenced the 101-Mb genome of Melampsora larici-populina, the causal agent of poplar leaf rust, and the 89-Mb genome of Puccinia graminis f. sp. tritici, the causal agent of wheat and barley stem rust. We then compared the 16,399 predicted proteins of M. larici-populina with the 17,773 predicted proteins of P. graminis f. sp tritici. Genomic features related to their obligate biotrophic lifestyle include expanded lineage-specific gene families, a large repertoire of effector-like small secreted proteins, impaired nitrogen and sulfur assimilation pathways, and expanded families of amino acid and oligopeptide membrane transporters. The dramatic up-regulation of transcripts coding for small secreted proteins, secreted hydrolytic enzymes, and transporters in planta suggests that they play a role in host infection and nutrient acquisition. Some of these genomic hallmarks are mirrored in the genomes of other microbial eukaryotes that have independently evolved to infect plants, indicating convergent adaptation to a biotrophic existence inside plant cells.

Abstract: In much of the world, resistance to stem rust in wheat, caused by Puccinia graminis f. sp. tritici, is based at least in part on the gene Sr31. During February 1999, high levels of stem rust infection were observed on entries in wheat (Triticum aestivum) grown in a nursery at Kalengyere Research Station in Uganda. Because several of the rusted entries were known to carry the 1BL-1RS chromosome translocation containing the Sr31, Lr26, and Yr9 genes for rust resistance, virulence to Sr31 was suspected. Urediniospores, collected in bulk from rusted stems of seven entries containing Sr31, were suspended in light mineral oil and sprayed on primary leaves of 7-day-old seedlings of South African wheat cv. Gamtoos (=Veery #3, pedigree: Kvz/Buho‘S’//Kal/BB). Plants were kept overnight at 19 to 21°C in a dew chamber before placement in a greenhouse at 18 to 25°C. After ≈14 days, urediniospores were collected from large, susceptible-type stem rust pustules and subsequently increased on Gamtoos, which served as a sel...

Abstract: Fusarium head blight, also known as scab, can be a devastating disease on all classes of wheat and barley. Among the species of Fusarium that cause scab, F. graminearum is the predominant pathogen. Wheat and barley are highly susceptible to infection when the crop is in the flowering to soft dough stages and when weather includes frequent precipitation, high humidity, or heavy dews. Multiple symptoms occur: reduced yields, discolored and shriveled kernels (tombstones), depressed seed weights, and reduced seed quality and vigor. The mycotoxin deoxynivalenol (DON, commonly known as vomitoxin) also can be produced by F. graminearum on infected grain. Grain contaminated with DON often is unsuitable for flour, cereals, or malt and is too toxic as feed for nonruminant animals (19). Distribution of Fusarium head blight. In recent years, Fusarium head blight has reemerged worldwide as a disease of economic importance (19). In the United States and Canada, the reemergence of Fusarium head blight in the 1990s has caused epidemics of varying severity on barley and on all classes of wheat. It is documented as causing epidemics in 26 states (19) and the Canadian provinces of Quebec (A. Comeau, Agriculture and Agri-Food Canada, Ste. Foy, Quebec, Canada, personal communication), Ontario (A. W. Schaafsma, University of Guelph, Ridgetown College, Guelph, Ontario, personal communication), and Manitoba and Saskatchewan (R. Clear, Canadian Grain Commission, Winnipeg, Manitoba, personal communication). The United States Department of Agriculture (USDA) ranks Fusarium head blight as the worst plant disease to hit the nation since the stem rust epidemics of the 1950s (35). Wheat and barley losses caused by scab epidemics in the United States during the 1990s are estimated at close to $3 billion. American wheat farmers lost over 500 million bushels of wheat valued at about $2.5 billion (T. Sayler, Prairie Ag Communications, Fargo, ND, personal communication) and Midwestern barley producers lost $400 million on barley (J. Mittleider, North Dakota Barley Council, Fargo, personal communication). Economic losses to wheat producers in Canada in the 1990s are estimated in U.S. dollars at $220 million in Quebec and Ontario (A. Comeau, Agriculture and Agri-Food Canada, Ste. Foy, Quebec, personal communication) and, from 1993 to 1998, at $300 million in Manitoba (A. Tekauz, Agriculture and Agri-Food Canada, Winnipeg, Manitoba, personal communication). Why Fusarium head blight has been so severe. The disease has been favored by a combination of factors including unseasonably wet weather, particularly when plants are most susceptible. The increased practice of conservation tillage has favored inoculum of the pathogen to survive in residues of host crops such as wheat, barley, and corn. A high percentage of land has been planted to small grains, with short intervals between crops. Also, plant resistance to scab has been limited or absent in wheat and barley cultivars. These factors have resulted in the buildup and abundance of inoculum of F. graminearum. “Breadbasket of the World” under siege. The Northern Great Plains has a short growing season, so much of the land is tied to cereal production that includes hard red spring wheat, durum wheat, winter wheat, and barley. Beginning in 1993, northwestern Minnesota, eastern North Dakota, northeastern South Dakota, and southern Manitoba in Canada were particularly hard hit by Fusarium head blight and disease severities reached 20 to 80% (19). Epidemics of Fusarium head blight, however, have continued to reoccur every year from 1993 to 1998 at varying intensities, particularly in northeastern North Dakota, northwestern Minnesota, and southern Manitoba. In the last couple of summers, F. graminearum has spread further westward across North Dakota (M. McMullen, North Dakota State University, Fargo, personal communication) and from western Manitoba into Saskatchewan (R. Clear, Canadian Grain Commission, Winnipeg, Manitoba, personal communication). In the 1999 season, Fusarium head blight was a minor disease problem in Minnesota and North Dakota (although infections occurred as far west as the central portion of the state) and less of a problem in Manitoba compared with previous years. The Red River Valley, which I refer to occasionally in this presentation, is a 40to 60-mile region on each side the Red River that separates North Dakota and Minnesota and flows north into Manitoba. The flatlands of the Red River Valley are the lake bed of the prehistoric Lake Agassiz. It’s black loamy soil is among the richest farmland in the world. This region is the “Breadbasket of the World” and farmers here typically are prosperous. I live and work in northeastern North Dakota and northwestern Minnesota and have witnessed the impact of Fusarium head blight on the region. It has severely crippled the wheat and barley industries, driven producers to financial ruin and human hardship, and resulted in rural communities dwindling away. The epidemic has generated nearly $10 million in new public and private funding to support Fusarium head blight research and education in the United States (T. Sayler, Prairie Ag Communications, Fargo, ND, personal communication) and also has resulted in lobbying to change agricultural policies. In this address, I will focus on the development and impact of this crisis, with an emphasis on northeastern North Dakota and northwestern Minnesota. The summer of 1993 held great promise. Fields of wheat and barley were lush and green, and yield potentials promised to meet or exceed the bumper crops harvested in 1992. At harvest, growers were stunned and distressed when they found severe scab infecContribution No. 9910102 of the Minnesota Agricultural Experiment Station.

Abstract: Leaf rust caused by Puccinia triticina is the most common and widely distributed of the three wheat rusts. Losses from leaf rust are usually less damaging than those from stem rust and stripe rust, but leaf rust causes greater annual losses due to its more frequent and widespread occurrence. Yield losses from leaf rust are mostly due to reductions in kernel weight. Many laboratories worldwide conduct leaf rust surveys and virulence analyses. Most currently important races (pathotypes) have either evolved through mutations in existing populations or migrated from other, often unknown, areas. Several leaf rust resistance genes are cataloged, and high levels of slow rusting adult plant resistance are available in high yielding CIMMYT wheats. This paper summarizes the importance of leaf rust in the main wheat production areas as reflected by yield losses, the complexity of virulence variation in pathogen populations, the role cultivars with race-specific resistance play in pathogen evolution, and the control measures currently practiced in various regions of the world.