Timothy D. Murray

Perennial wheat: The development of a sustainable cropping system for the U.S. Pacific Northwest

Perennial wheat offers a new solution to the long-standing problems of soil erosion and degradation associated with conventional annual small-grain cropping systems in the Pacific Northwest region. Using classical breeding methods, new types of wheat have been developed that maintain the key characteristics of annual wheat, but continue to grow after harvest. Following dormancy in the winter, growth is initiated from the roots or crowns in the spring, allowing a crop to be harvested every fall. By retaining constant soil cover over multiple years, wind and water erosion would be dramatically reduced. In addition, the costs associated with annual seeding and tillage would be minimized, and unlike many reduced tillage systems, it is expected that standard seeding equipment would be suitable for stand establishment. Other potential benefits of perennial wheat include improved wildlife habitat, more efficient use of available water, provision of a potent carbon sink, and the possibility of integrating straw retrieval into a small grains cropping system. Past attempts in the first half of the last century failed to develop perennial wheat as a viable crop, primarily because of low yields, and the research was ultimately abandoned. Perennial wheat production may now be viewed as acceptable for highly erodible land or for obtaining carbon sequestration credits. This paper presents an overview of solutions to the obstacles encountered by previous researchers, introduces some of the newly developed perennial wheat lines, and discusses considerations for management practices.

Pathogenicity, host-specificity, and population biology of Tapesia spp, causal agents of eyespot disease of cereals.

Abstract Eyespot is an important stem-base disease of cereal crops in temperate regions. Cultural, genetic and molecular criteria have been used to separate the fungi responsible into two species, Tapesia yallundae (previously W-type) and T. acuformis (previously R-type). Discovery of the apothecial sexual stage (teleomorph) of Tapesia yallundae on straw stubble was a key factor in this change for a pathogen previously believed to be asexual (anamorph: Pseudocercosporella herpotrichoides ). Sexual reproduction is controlled by a two-allele heterothallic system in both species, although mating appears to be rare in T. acuformis . Infection of cereal hosts is achieved by formation of multicellular plaques, and the colonization process is described. The host range includes wild grass species as well as small-grain cereals, and new genetic sources of resistance to the disease have been identified in Triticum species and wild relatives. Recent advances in the molecular genetics of the pathogens will aid analysis of pathogenic variation in eyespot.

Resistance to stripe rust and eyespot diseases of wheat in Triticum tauschii

A collection of 279 Triticum tauschii (syn. Aegilops squarrosa) accessions was evaluated for resistance to stripe rust (Puccinia striiformis) and eyespot (Pseudocercosporella herpotrichoides) diseases. Seedlings were inoculated with four different races of P. striiformis that represent all known virulences in the Pacific Northwest, and a genetically modified strain of P herpotrichoides expressing β-glucuronidase. Seventeen percent (44) of the T. tauschii accessions were resistant to all Pacific Northwest races of stripe rust, and 45% (115) were resistant to eyespot. Thirty-nine of the 279 accessions were resistant to the stripe rust races and the eyespot pathogen. Accessions resistant to stripe rust were mainly from the Caspian Sea region of Iran and Azerbaijan, with the majority belonging to T tauschii subsp. strangulata and T. t. subsp. meyeri. There was no clear association between resistance to eyespot and geographical origin or taxonomic subgroup.

Evaluation of Dasypyrum villosum Populations for Resistance to Cereal Eyespot and Stripe Rust Pathogens

Resistance to Pseudocercosporella herpotrichoides (cause of eyespot) and Puccinia striiformis(cause of stripe rust) was evaluated in a germ plasm collection of Dasypyrum villosum (syn. Haynaldia villosa) and a set of disomic addition lines, a substitution, and a translocation line of D. villosum chromosomes in a wheat background. Three races of P. striiformis and a β-glucuronidase-transformed strain of Pseudocercosporella herpotrichoides were used to inoculate plants and evaluate disease reactions. Of the 115 D. villosum accessions tested, 33 (28.6%) were resistant to one or more races of Puccinia striiformis and 8 accessions were resistant to all races. All 219 accessions of D. villosum tested were resistant to Pseudocercosporella herpotrichoides and 158 (72%) of the accessions had lower β-glucuronidase activity than the resistant wheat line VPM-1. Most of the accessions of D. villosum resistant to the stripe rust pathogen originated from Greece; however, there was no distinction among origins for resistance to the eyespot pathogen. Chromosome 4V was confirmed to carry the gene for resistance to P. herpotrichoides. At least one gene for resistance to Puccinia striiformis was located on the short arm of chromosome 6V of D. villosum in the 6VS/6AL-translocation line; this gene was named Yr26.

Species and mating-type distribution of Tapesia yallundae and T. acuformis and occurrence of apothecia in the U.S. Pacific Northwest.

ABSTRACT Eyespot of wheat is caused by the discomycete fungi Tapesia yallundae and T. acuformis. T. yallundae is considered the most important causal agent of the disease in this region but no apothecia of either species have been found in the U.S. Pacific Northwest (PNW). Two compatible isolates of T. yallundae from the PNW were used to inoculate a field plot in the fall of 1998 and apothecia developed in the spring and fall of 2000 on standing wheat stubble. In the spring of 2000, wheat stubble from eight naturally infected fields was examined for the presence of apothecia of T. yallundae and T. acuformis. Apothecia of T. acuformis were found in two fields but no apothecia of T. yallundae were found. This is the first report of apothecia of the eyespot pathogens occurring in the PNW. Species and mating-type distribution of T. yallundae and T. acuformis in the PNW were determined from 817 isolates collected from diseased wheat over 3 years at spatial scales ranging from within fields to across states. In all, 460 isolates were identified as T. yallundae and 357 isolates were identified as T. acuformis with MAT1-1/MAT1-2 ratios not significantly different from 1:1 based on chi(2) tests at most scales tested. The apparent increase in frequency of T. acuformis from previous surveys may indicate a shift in the predominant species causing eyespot. The occurrence of apothecia under field conditions, along with the widespread distribution of mating types of both species, suggests that sexual reproduction may be occurring in both species.

A New Source of Resistance to Tapesia yallundae Associated with a Homoeologous Group 4 Chromosome in Thinopyrum ponticum

ABSTRACT Wheat (Thinopyrum ponticum line SS767; PI 611939) with 42 chromosomes previously was identified as a new source of eyespot resistance. Individual plants of SS767 were tested for reaction to Tapesia yallundae, the major pathogen of eyespot in the Pacific Northwest region of the United States. Resistance of this line was similar to the resistant winter wheat cv. Madsen (carrying gene Pch1 for eyespot resistance). Polymerase chain reaction analysis with primers specific for the J or E genomes revealed that SS767 contains Thinopyrum chromatin. Cytological and Cbanding analyses demonstrated that SS767 is a chromosome substitution line in which wheat chromosome 4D is replaced by a homoeologous group 4 chromosome of Thinopyrum ponticum. Genomic in situ hybridization using St genomic DNA from Pseudoroegneria strigosa as a probe, which can differentiate chromosomes from different genomes of Thinopyrum, indicated that this chromosome belongs to the J genome. Molecular analysis of an F(2) population segregating for chromosome 4J and resistance to eyespot confirmed that eyespot resistance in line SS767 is associated with chromosome 4J of Thinopyrum ponticum. This is the first report of genetic control of resistance to eyespot derived from Thinopyrum ponticum. This source of resistance provides a new opportunity to improve wheat resistance to eyespot by adding to the diversity of resistance sources available.

Characterization of tetraploid wheat germplasm for resistance to Pseudocercosporella herpotrichoides, cause of eyespot disease

Eyespot disease, caused by Pseudocercosporella herpotrichoides, can be devastating to winter wheat grown in northern Europe and the northwest USA. Accessions from eight different tetraploid wheat species randomly extracted from core collections were scored for resistance to eyespot disease using a β-glucuronidase (GUS)-transformed strain of P. herpotrichoides. The GUS values for the combined population followed a quasi-Gaussian distribution. Three species, Triticum dicoccoides, T. durum and T. turanicum, showed significant variation (P < 0.001) in disease response with T. dicoccoides having the lowest disease scores, i.e. highest levels of resistance. All tetraploid accessions were less resistant than resistant diploid T. tauschii accessions. Thirteen percent of tetraploid accessions had disease scores that ranged between the average of the resistant accessions of T. tauschii and the moderately resistant hexaploid germplasm line Cerco. Eight accessions (three accessions of T. dicoccoides, two of T. turgidum and three of T. durum) with low disease scores (resistance) to infection were selected for further genetic analysis.

Infection of Winter Wheat by a β-Glucuronidase-Transformed Isolate of Cephalosporium gramineum

ABSTRACT Field-grown winter wheat was inoculated with a beta-glucuronidase-transformed isolate of Cephalosporium gramineum in two field seasons to elucidate the mode of infection in resistant and susceptible cultivars. Colonization of viable root epidermis and cortical cells occurred as soon as 15 days postinoculation and the pathogen was found in the vascular tissues by 20 days postinoculation, well before freezing soil temperatures occurred. Penetration occurred directly through the root epidermis and through wounds adjacent to emerging secondary roots. The pathogen also penetrated through root cap cells and colonized meristematic tissues near root tips to gain access to the vascular system. Lower stem base colonization was observed where the pathogen penetrated directly through the epidermis, wounds, or senescent tissues. Appressorium-like structures, which appeared to aid penetration of cell walls, were often found within cells of both roots and stems after initial colonization. The mechanisms of resistance were not apparent, but less colonization occurred in resistant than in susceptible cultivars.

Genetic Variation of Wheat streak mosaic virus in the United States Pacific Northwest

Wheat streak mosaic virus (WSMV), the cause of wheat streak mosaic, is a widespread and damaging pathogen of wheat. WSMV is not a chronic problem of annual wheat in the United States Pacific Northwest but could negatively affect the establishment of perennial wheat, which is being developed as an alternative to annual wheat to prevent soil erosion. Fifty local isolates of WSMV were collected from 2008 to 2010 near Lewiston, ID, Pullman, WA, and the United States Department of Agriculture Central Ferry Research Station, near Pomeroy, WA to determine the amount of genetic variation present in the region. The coat protein gene from each isolate was sequenced and the data subjected to four different methods of phylogenetic analyses. Two well-supported clades of WSMV were identified. Isolates in clade I share sequence similarity with isolates from Central Europe; this is the first report of isolates from Central Europe being reported in the United States. Isolates in clade II are similar to isolates originating from Australia, Argentina, and the American Pacific Northwest. Nine isolates showed evidence of recombination and the same two well-supported clades were observed when recombinant isolates were omitted from the analysis. More polymorphic sites, parsimony informative sites, and increased diversity were observed in clade II than clade I, suggesting more recent establishment of the virus in the latter. The observed diversity within both clades could make breeding for durable disease resistance in perennial wheat difficult if there is a differential response of WSMV resistance genes to isolates from different clades.

Population genetic structure of Tapesia acuformis in Washington State

ABSTRACT Eyespot of wheat is caused by Tapesia yallundae and T. acuformis. Historically, T. yallundae has been considered the more important causal agent of the disease in Washington state and consists of a large homogeneous population with a genetic structure consistent with both sexual and asexual reproduction. T. acuformis has increased significantly in Washington in the past 10 years and apothecia were found recently under natural field conditions, indicating that T. acuformis may have a more important role in eyespot of wheat than previously was thought. To determine the genetic structure of T. acuformis in Washington, 141 single conidial isolates were sampled from four subpopulations in the eastern wheat-growing region of the state. Isolates were scored for mating type and six amplified fragment length polymorphism markers. All markers segregated in a 1:1 ratio and were determined to be unlinked based on genetic analysis of 24 progeny from an in vitro cross. No significant differences in allele frequencies (0.127 < P < 0.809) were found among individual loci across the four subpopulations and over all loci based on contingency table analysis of the log-likelihood ratio statistic G(2). Likewise, no overall differences between subpopulations were detected using the population differentiation statistic theta (theta = -0.004, P = 0.537). Random mating could not be rejected within each subpopulation or for the combined data using clone-corrected data sets based on (i) 1:1 ratio of mating-type, (ii) multilocus gametic disequilibrium analyses (index of association), (iii) phylogenetic analyses (parsimony tree length permutation test), and (iv) genotypic diversity analyses. T. acuformis has a genetic structure similar to that of sympatric populations of T. yallundae in Washington, with both sexual and asexual reproduction contributing to the structuring of this species.