A major gene for resistance to white pine blister rust in Western white pine from the Western cascade range

ABSTRACT A dominant gene for resistance to white pine blister was indicated by Mendelian segregation in full-sib families of western white pine parent trees selected for phenotypic resistance in six heavily infected stands in the Western Cascades of Oregon and Washington. Seedlings were artificially inoculated three times between 1959 and 1964 and observed for development of stem infection. Segregation at this locus (Cr2) occurred in only two of the six parent populations sampled: one a natural stand, Champion Mine (CM), and the other a plantation of unknown seed origin. At CM, reduced penetrance of this gene was expressed by altered Mendelian ratios (mostly less-than-expected resistant phenotypes) in families of specific combinations of certain parents, indicating the presence of modifier genes with effects that ranged from mild to almost complete suppression of Cr2. Between 1968 and 1994, an apparent shift in virulence at CM caused all of the resistant selections to become infected and die. Recent inoculations of many of the same or related families from these parents, made from grafted ramets in a seed orchard, showed that Cr2 conditions a classical hypersensitive reaction (HR) in needle tissues, the primary infection courts. In the latter tests, seedlings were challenged with wild-type and four other sources of inoculum at and near CM that were also suspected of having wider virulence than wild type. No seedlings segregating for HR that were inoculated with wild type subsequently developed stem symptoms, but the other inocula induced both susceptible and HR needle spots on Cr2- genotypes, and many of these seedlings did develop stem infections. This implied that spore genotypes with specific virulence to Cr2 are carried in these inocula.     

Evidence of Cytoplasmic Inheritance of Virulence in Cronartium ribicola to Major Gene Resistance in Sugar Pine

ABSTRACT Tests for Mendelian segregation of virulence and avirulence in Cronartium ribicola, causal agent of white pine blister rust, to a major gene (R) for resistance in sugar pine were made using haploid basidiospore progenies from single diploid telia as inoculum on resistant genotypes. The telia were sampled from a small deme in the Siskyou Mountains of northern California, where a few mature sugar pines known to be Rr genotypes had become infected after withstanding the chronic blister rust epidemic for several decades and where intermediate frequencies of virulence in the ambient basidiospore population were subsequently measured. Infection type on inoculated seedlings with R was qualitative: all progenies of 81 single telia tested over 3 different years were either virulent (compatible) or avirulent (inducing hypersensitive necrosis), never a mixture of both reactions. The complete absence of heterozygotes in the telia population is strong evidence that virulence is not controlled by a nuclear gene. The data are consistent with earlier tests showing that basidiospore inoculum derived from aeciospores isolated from infected Rr trees produced mostly (>90%) virulent reactions on R- seedlings. The evidence indicates that transmission of virulence is uniparental via the cytoplasm of aeciospores. Exchange of spermatia between haploid thalli does not appear to be involved.     

Influence of host resistance on the genetic structure of the white pine blister rust fungus in the western United States

Cronartium ribicola, the causal agent of white pine blister rust, has been devastating to five-needled white pines in North America since its introduction nearly a century ago. However, dynamic and complex interactions occur among C. ribicola, five-needled white pines, and the environment. To examine potential evolutionary influences on genetic structure and diversity of C. ribicola in western United States, population genetic analyses of C. ribicola were conducted using amplified fragment length polymorphism (AFLP) molecular markers. The fungus was sampled at six sites. Collections for two of the six sites were from separate plantings of resistant-selected western white pine and sugar pine. Heterozygosity based on polymorphic loci among populations ranged from 0.28 to 0.40, with resistant-selected plantations at the extremes. Genetic differentiation was also highest between these two populations. Principal coordinates analysis and Bayesian assignment placed most isolates that are putative carriers of virulence to major-gene resistance into a discernable cluster, while other isolates showed no clustering by site or host species. These results indicate that C. ribicola in western North America is not genetically uniform, despite its presumed single site of introduction and relatively brief residence. Moreover, major-gene resistance appears to have imposed strong selection on the rust, resulting in reduced genetic diversity. In contrast, no evidence of selection was observed in C. ribicola from hosts that exhibit only multigenic resistance.     

Origin and distribution of cr2, a gene for resistance to white pine blister rust in natural populations of Western white pine

ABSTRACT The distribution and frequency of the Cr2 gene for resistance to white pine blister rust (Cronartium ribicola) in western white pine (Pinus monticola) was surveyed in natural populations of the host by inoculation of open-pollinated seedlings from 687 individual seed parents from throughout most of the species' range. Because Cr2 is dominant and results in a conspicuous hypersensitive reaction (HR) in pine needles, the phenotype can readily be detected in offspring of susceptible seed parents fertilized by unknown Cr2 donors in the ambient pollen cloud. Gametic frequencies of Cr2 were thus determined as the proportion of total challenged seedlings that were pollen receptors exhibiting the Cr2 phenotype. Zygotic frequencies, the proportion of seed parents with progeny that segregated in Mendelian ratios for the Cr2 phenotype to the total number of parents, were a complementary, though less precise, measure. Cr2 frequency was rare overall, ranging from 0.004 to 0.008 in the Sierra Nevada to about 0.001 in the central Cascade Range; it was undetectable further north in the Cascades, as well as in the Rocky Mountains and Coast Mountains of the United States and Canada. The diminishing frequency of Cr2 from the southern and central Sierra Nevada northward mirrors that of Cr1 in sugar pine (P. lambertiana) and points to this region as the origin of both genes. We rationalize that this coincidence may have resulted from protection that these genes may have conferred on both species to an endemic pine stem rust congeneric with C. ribicola (C. occidentale) in recent geologic epochs.     

White pine blister rust in north america: past and prognosis

ABSTRACT After a full century in North America, the blister rust epidemic has yet to stabilize, continuing to spread into warmer and drier areas previously considered climatically inhospitable. The disease apparently has no environmental limits wherever white pines and Ribes spp. cohabit and will eventually become pandemic. Although much timber value has been lost, more severe long-term damage is disruption caused to ecosystems by altered patterns of natural succession. During the last half of the century just past, development of genetic resistance superceded other direct control measures-mainly Ribes spp. eradication and antibiotics-which proved ineffective and/or unfeasible in large areas of the white pine range, especially in the West. Several mechanisms of complete (major gene) and partial resistance are common to at least several white pine species. Although North American populations of rust have low genetic variability overall, rust genotypes with specific virulence to major resistance genes exist in some local demes at high frequencies. The challenge will be to package and deploy resistance genes in ways that will dampen sudden increases in rust races of wide virulence. New introductions of blister rust from its gene center in Asia remain the gravest threat to genetic improvement programs.     

Virulence gene distribution and dynamics of the white pine blister rust pathogen in Western north america

ABSTRACT We assayed the distribution and frequency of two genes of the blisterpathogen with specific virulence to major resistance genes in sugar pine and western white pine in inoculum from extensive parts of the hosts' ranges. The genes, vcr1 and vcr2, differentially neutralize the cognate resistance alleles Cr1 and Cr2 of the two respective hosts and are clearly marked by their interaction phenotypes. Basidiospores from each inoculum source were cast over Cr1 and Cr2 host genotypes simultaneously, and interaction phenotypes scored when developed. vcr1 was confined to sites with high concentrations of Cr1 (mostly plantations) where frequencies tended toward fixation. vcr2 showed a similar tendency, except high frequencies were occasionally observed from natural and planted stands of western white pine with very low frequencies of Cr2. Otherwise, no pattern was evident for either allele: frequencies were very erratic from site to site within short distances (<1 to 7 km) of each other and oscillated with high amplitudes at the same sites measured in consecutive years. Intense selection for virulence by Cr alleles occurs locally, but spread of vcr alleles over the landscape is mitigated by remarkably low gene flow. Absence of heterozygotes among single telia inoculum on Cr2 genotypes indicated cytoplasmic inheritance of vcr2, similar to vcr1(previously reported).     

Identification and Characterization of Random Amplified Polymorphic DNA Markers Linked to a Major Gene (Cr2) for Resistance to Cronartium ribicola in Pinus monticola

ABSTRACT DNA markers tightly linked to resistance (R) genes provide a very powerful tool for both marker-assisted selection in plant breeding and positional cloning of R genes. In the present study, a linkage of random amplified polymorphic DNA (RAPD) markers to the single dominant gene (Cr2) for resistance to white pine blister rust fungus (Cronartium ribicola) was investigated in western white pine (Pinus monticola). A mapping population of 128 individual megagametophytes was generated from seeds of a heterozygous resistant tree (Cr2/cr2), and the corresponding seedlings of each megagametophyte were subjected to the test of phenotype segregation by inoculation with C. ribicola. Bulked segregant analysis and haploid segregation analysis identified eight robust RAPD markers linked to Cr2. This constitutes the first Cr2 genetic linkage map spanning 84.7 cM with four markers only 3.2 cM from Cr2. One sequence (U256-1385) of these linked markers was significantly similar to the Ty3/gypsy-like long terminal direct repeats retrotransposons. Another marker, U570-843, had no significant similarity to any entry in either GenBank or the loblolly genomics data bank. As presumed that the average physical distance per centimorgan is about 10 Mb in P. monticola, it is probably unrealistic to use these DNA markers for positional cloning of the Cr2 gene.     

Association of a novel Pinus monticola chitinase gene (PmCh4B) with quantitative resistance to Cronartium ribicola

Multiple families of pathogenesis-related (PR) proteins are believed to contribute to plant quantitative resistance to various pathogens. Along with other host PR proteins, PR3 chitinase is one protein component participating in genetic resistance of western white pine (Pinus monticola) to the white pine blister rust (WPBR) pathogen (Cronartium ribicola). In the present study, we characterized a novel P. monticola class IV chitinase gene (PmCh4B) and further analyzed its nucleotide variations in the open-pollinated seed families of diverse geographical distribution and variable levels of quantitative resistance to C. ribicola infection. PmCh4B showed high haplotype diversity (Hd=0.94) and nucleotide diversity (π=0.00965), similar to those of other conifer genes related to environmental stresses. A low level of intragenic linkage disequilibrium (LD) (but most of the levels with statistical significance) was found within a distance of ≈800 bp. Based on PmCh4B haplotype frequency, moderate to high levels of population structure were observed among P. monticola seed families currently used in breeding programs for WPBR resistance (average FST=0.163, P<0.001). Association analysis revealed that allelic variants and multiple single-nucleotide polymorphisms of PmCh4B were significantly associated with quantitative levels of P. monticola resistance against C. ribicola. This work represents the first association study for quantitative resistance in western white pine pathosystem and provides a potential for marker-assisted selection in white pine breeding.     

The CC-NBS-LRR Subfamily in Pinus monticola: Targeted Identification, Gene Expression, and Genetic Linkage with Resistance to Cronartium ribicola

ABSTRACT To investigate disease resistance gene analogs (RGAs) encoding coiled-coil-nucelotide-binding site-leucine-rich repeats (CC-NBS-LRR) proteins in western white pine, degenerate primers targeting the conserved motifs in the NBS domain were designed to amplify RGAs from genomic DNA and cDNA. Sixty-one distinct RGAs were identified with identities to well-known R proteins of the CC-NBS-LRR subfamily. These RGAs exhibited variation of putative amino acid sequences from 13% to 98%, representing a complex CC-NBS-LRR subfamily. A phylogenetic tree constructed from the amino acid sequence alignment revealed that these 61 RGAs were grouped with other CC-NBS-LRR members from angiosperms, and could be further divided into six classes with an identity threshold of 68%. To map RGAs, RGA polymorphisms and a modified amplified fragment length polymorphism (AFLP) method with incorporated sequences from the NBS domain were used to reveal NBS or NBS-AFLP markers. RGA polymorphism study revealed that three off the identified RGAs were not linked to the Cr2 gene imparting resistance to white pine blister rust. However, the AFLP strategy, using bulk segregant analysis (BSA) and haploid segregation analysis, identified 11 NBS-AFLP markers localized in the Cr2 linkage, the closest two to the gene being 0.41 cM and 1.22 cM away at either side. Eight of these markers showed significant amino acid sequence homologies with RGAs.     

Molecular epidemiology of white pine blister rust: recombination and spatial distribution

ABSTRACT Multilocus haplotypes (MLHs) were derived for the spermogonial (monokaryotic haploid) stage of Cronartium ribicola, the causal agent of white pine blister rust. Six random amplified polymorphic DNA loci and three single-strand conformational polymorphism markers were analyzed for 246 rust samples collected from two heavily infected white pine plantations. All cankers sampled were spatially located within the plantations. The hypothesis that spores are not locally disseminated was supported by the absence of any spatial clustering in the distribution of the MLHs. A large number of MLHs was found at both sites and the haplotypic diversity was close to the maximum (one) in both populations. All measures of recombination were not different from expectations under a scenario of sexual recombination. Genetic differentiation between the two sites was very low (theta = 0.023), yet it was significantly different from zero (P < 0.01). This analysis is in agreement with a scenario of extensive sexual recombination followed by some long-distance dispersal.     

Virulence Phenotypes of a Worldwide Collection of Puccinia triticina from Durum Wheat

ABSTRACT A total of 78 isolates of Puccinia triticina from durum wheat from Argentina, Chile, Ethiopia, France, Mexico, Spain and the United States and 10 representative isolates of P. triticina from common wheat from the United States were tested for virulence phenotypes on seedling plants of 35 near-isogenic lines of Thatcher wheat. Isolates with virulence on lines with leaf rust resistance genes Lr10, Lr14b, Lr20, Lr22a, Lr23, Lr33, Lr34, Lr41, and Lr44 represented the most frequent phenotype. Cluster analysis showed that P. triticina from durum wheat from South America, North America, and Europe had an average similarity in virulence of 90%, whereas isolates from Ethiopia were <70% similar to the other leaf rust isolates collected from durum wheat. Of the 11 isolates from Ethiopia, 7 were avirulent to Thatcher and all near-isogenic lines of Thatcher. The isolates from common wheat had an average similarity in virulence of 60% to all leaf rust isolates from durum wheat. P. triticina from durum wheat was avirulent to many Lr genes frequently found in common wheat. It is possible that P. triticina currently found on durum wheat worldwide had a single origin, and then spread to cultivated durum wheat in North America, South America, and Europe, whereas P. triticina from Ethiopia evolved on landraces of durum wheat genetically distinct from the cultivated durum lines grown in Europe and the Americas.     

Genetic Diversity in Poplar Leaf Rust (Melampsora medusae f. sp. deltoidae) in the Zones of Host Sympatry and Allopatry

ABSTRACT Poplar leaf rust caused by Melampsora medusae f. sp. deltoidae is a widespread disease in North America, where epidemics occur within zones of sympatry and allopatry of telial hosts (Populus spp.) and aecial hosts (Larix spp.). To test the hypothesis that epidemics originate in the zone of sympatry where the rust can complete its life cycle, populations in sympatry and allopatry were analyzed with single-strand conformational polymorphism for codominant detection of alleles directly from uredinia. More alleles were detected in rust populations in the zone of host sympatry than in allopatry. Almost all alleles found in the zone of allopatry were a subset of the allelic diversity present in the zone of host sympatry. Distance analyses clustered populations according to geographic origin, but not sampling year or type of stand (plantation or natural stands). Large differences in allelic and genotypic frequency were observed between years in allopatry but not in sympatry, suggesting new colonizations in allopatric populations. Our results point to a dynamic and complex pattern of inoculum dissemination in polar leaf rust. The hypothesis most consistent with our results is that populations in sympatry represent a source of inoculum for epidemics, with some annual recolonization in allopatry, possibly via intermediate population jumps.     

Fine-level genetic structure of white pine blister rust populations

ABSTRACT The fine-level genetic structure of the white pine blister rust agent, Cronartium ribicola, was investigated by sampling multiple monokaryotic spermogonia directly on cankers in four eastern Canadian white pine (Pinus strobus) plantations and assessing genetic variability, using random amplified polymorphic DNA (RAPD) markers. Ninety-eight percent of the cankers surveyed contained a single DNA haplotype, suggesting spermogonia within cankers are the result of clonal reproduction. A single canker contained two haplotypes that were divided between the upper and lower parts of the canker, suggesting it represented two confluent cankers. In contrast, genotypic diversity was high among cankers. Thirty-seven haplotypes were found among forty-three cankers sampled, and an analysis of molecular variance indicated that 93% (P < 0.001) of the total genetic diversity was attributable to sampling of different cankers, strongly suggesting that multiple infections do not take place in the white pine blister rust pathosystem, i.e., a canker is the result of infection by a single genotype. This result is in contrast with the high level of genetic diversity previously reported among dikaryotic aecidia within cankers and is consistent with the hypothesis that variability in the aecidial stage is the result of outcrossing between resident spermogonia and alien spermatia. The genetic structure of the spermogonial stage, which is the vegetative extension of infection by basidiospores and, therefore, the indirect result of meiosis, was consistent with random mating; the observed genotypic diversity was not significantly different (P > 0.05) from the genotypic diversity expected under the assumption of panmixis. The results indicate that monokaryotic cankers can be genotyped by sampling a single unopened spermogonia per canker.     

Relative Contribution of Seed-Transmitted Inoculum to Foliar Populations of Phaeosphaeria nodorum

ABSTRACT A marked-isolate, release-recapture experiment was conducted to assess the relative contributions of seed-transmitted (released isolates) versus all other inocula to foliar and grain populations of Phaeosphaeria nodorum in winter wheat rotated with nonsusceptible crops in New York and Georgia, United States. Seed infected with two distinct groups of marked isolates of P. nodorum containing rare alleles (identified by amplified fragment length polymorphisms [AFLPs]) and balanced for mating type were planted in experimental field plots in two locations in each state. Recapture was done by isolating P. nodorum from leaves showing necrotic lesions at spring tillering and flowering stages, and mature grains from spikes showing glume blotch. Isolates from these samples were genotyped by AFLPs and categorized as released or nonreleased to infer sources of inoculum. Both infected seed and other sources of the pathogen contributed significant primary inocula to populations recovered from leaves and harvested grain. Seed-transmitted genotypes accounted for a total of 57% of all isolates recovered from inoculated plots, with a range of 15 to 90% of the populations of P. nodorum collected over the season in individual, inoculated plots at the four locations. Plants in the noninoculated control plots also became diseased and 95% or more of the isolates recovered from these plots were nonreleased genotypes. Although other potential sources of P. nodorum within and adjacent to experimental plots were not ruled out, nonreleased genotypes likely were derived from immigrant ascospores potentially from sources at a considerable distance from the plots. Our results suggest that, although reduction of seedborne inoculum of P. nodorum may delay foliar epidemics, this strategy by itself is unlikely to result in high levels of control in eastern North America because of the additional contribution from alternative sources of inoculum.     

Gene cloning of a thaumatin-like (PR-5) protein of western white pine (Pinus monticola D. Don) and expression studies of members of the PR-5 group

To understand the molecular defence response of western white pine to the blister rust pathogen Cronartium ribicola, we have endeavoured to isolate and characterise pathogenesis-related (PR) proteins from western white pine. A full-length cDNA (Pin mTLP) was isolated from a cDNA library constructed from inoculated foliage. BLASTX search results indicated that the sequence shared a high degree of identity with the thaumatin-like proteins (TLPs) of the PR-5 family, and in particular with those of the ‘small-TLP’ subgroup mainly identified in monocots. The mature protein predicted from the cDNA sequence has a molecular mass of 16 kDa and pI of 4.1. Southern blot analysis showed that Pin mTLP is a single-copy gene member of a multi-gene family.Two-dimensional Western blot analysis of total western white pine protein extracts probed with a Douglas-fir anti-TLP antibody detected a major and a minor protein spot of 23 and 17 kDa, respectively. Both 23 and 17-kDa proteins were identified as TLPs by Nanospray MS/MS analysis. The 23-kDa protein was shown to accumulate in canker margins in the bark of infected seedlings indicating that it is locally induced in response to fungal invasion. Both local wounding and methyl jasmonate treatments induced expression of the protein, whereas salicylic acid treatment did not. These results suggest that the 23-kDa TLP plays a role in the molecular defence response of western white pine to wounding and pathogen attack.     

Rust resistance in Salix to Melampsora larici-epitea

A total of 174 Salix (willow) clones belonging to 57 species and 14 interspecific hybrids were inoculated with seven pathotypes of Melampsora larici-epitea using the leaf disc method. Infection types were scored based on the uredinial pustule area data and the inoculum density. A close correlation ( R ² = 0·82) was found between the average pustule area and the average number of spores produced. Most of the willows were also assessed in the field for rust in 1999. Most willow clones belonging to the species native to western Europe were infected by the rust. In inoculation experiments, uredinia developed on 46 S. viminalis clones, out of a total of 47. In the field, all the S. viminalis clones were infected by rust. Within the subgenus Vetrix , eight out of the 17 willow species that originated from North and South America produced rust pustules in inoculation experiments. Of these, S. pellita was most susceptible. Salix irrorata and S. lasiolepsis var. bracelinae produced well developed pustules after inoculation but no rust infections were detected in the field. In both leaf disc tests and field assessments, no rust infections were found on S. candida , S. cordata , S. drummondiana , S. eriocephala , S. hookeriana , S. houghtonii , S. humilis , S. rigida var. mackenziana and S. syrticola . Of 12 species of subgenus Vetrix native to northeast Asia and Japan, only S. kochiana was susceptible both in inoculation tests and in the field. Salix rossica produced no symptoms in leaf disc tests but showed low levels of infection in the field. The maximum infection type scores in leaf disc tests were highly significantly correlated with field disease severity scores (Spearman rank correlation coefficient was 0·76, P  < 1 × 10⁻¹⁰).     

Genetic Variation and Virulence on Lr26 in Puccinia triticina

ABSTRACT The genetic relationships between isolates of Puccinia triticina virulent on wheat with the Lr26 resistance gene were studied. The diversity within and between isolates of P. triticina from Israel, Europe, and the United States was determined by virulence on near-isogenic Thatcher lines and by random amplified polymorphic DNA. According to the molecular markers, isolates that were virulent on Lr26 had diversity levels similar to those of Lr26 nonpathogenic isolates. Distances between subpopulations of isolates virulent and avirulent on Lr26 varied and were unrelated to the Lr26 virulence phenotype. Cluster analysis suggested four groups, three of which were closely associated with the geographical origin of the isolates-Israel, the United States, and Europe. All four groups included both Lr26 virulent and avirulent pathotypes. The results showed that Lr26 virulent rust pathotypes are as genetically dissimilar as the rest of the population. The cluster analysis showed that the rust population in Israel includes at least two different subpopulations, both of which contain Lr26 virulent and Lr26 avirulent isolates.     

Genetic Structure of Cronartium ribicola Populations in Eastern Canada

ABSTRACT The genetic structure of populations of Cronartium ribicola was studied by sampling nine populations from five provinces in eastern Canada and generating DNA profiles using nine random amplified polymorphic DNA markers. Most of the total gene diversity (H(t) = 0.386) was present within populations (H(w) = 0.370), resulting in a low level of genetic differentiation among populations in northeastern North America (F(st) = 0.062). A hierarchical analysis of genetic structure using an analysis of molecular variance (AMOVA) revealed no statistically significant genetic differentiation among provinces or among regions. Yet, genetic differentiation among populations within regions or provinces was small (AMOVA phi(st) = 0.078) but statistically significant (P < 0.001) and was several orders of magnitude larger than differentiation among provinces. This is consistent with a scenario of subpopulations within a metapopulation, in which random drift following migration and new colonization are major evolutionary forces. A phenetic analysis using genetic distances revealed no apparent correlation between genetic distance and the province of origin of the populations. The hypothesis of isolation-by-distance in the eastern populations of C. ribicola was rejected by computing Mantel correlation coefficients between genetic and geographic distance matrices (P > 0.05). These results show that eastern Canadian provinces are part of the same white pine blister rust epidemiological unit. Nursery distribution systems are controlled provincially, with virtually no seedling movement among provinces; therefore, infected nursery material may not play an important role in the dissemination of this disease. Long-distance spore dispersal across provincial boundaries appears to be an epidemiologically important factor for this pathogen.     

Histochemical and chemical evidence for lignin accumulation during the expression of resistance to leaf rust fungi in wheat

Levels of individual phenolic acids were examined in primary leaves of wheat (Triticum aestivum) after inoculation with avirulent and virulent strains of the leaf rust fungus (Puccinia recondita f. sp. tritici) at stages when previous work had shown fungal and host cells to be affected by expression of the Lr20 or Lr28 alleles for resistance. The predominant phenolic acid, ferulic acid, as well as p-coumaric and syringic acids were detected in primary leaves in both unbound and bound forms. They were not detected in germinating urediniospores of either rust strain. Levels of unbound phenolic acids changed little in response to infection. In Lr28-bearing leaves inoculated with an avirulent strain, increased concentrations of bound phenolic acids and three other unidentified compounds were observed about 4 h after many single or small groups of cells had undergone hypersensitive collapse. In an Lr20-bearing cultivar, levels of bound phenolic acids fell in leaves inoculated with either a virulent or avirulent rust strain. Coniferyl aldehyde and coniferyl alcohol were not detected in healthy or inoculated leaves of either wheat cultivar. Attempts to affect expression of resistance by application of inhibitors of phenylalanine ammonia-lyase were not successful and both wheat cultivars remained resistant to avirulent rust strains. The bound phenolic acids which accumulate in cells undergoing a hypersensitive response may play a role in resistance of Lr28-bearing wheat to the leaf rust fungus.     

Components of partial resistance in the slash pine-fusiform rust pathosystem

ABSTRACT Progeny of rust-resistant, open-pollinated slash pine families exhibited components of partial resistance in greenhouse tests. Nine-month-old seedlings of some resistant families had (i) a greater frequency of short galls (相似文献