This study quantified the frequency of simple versus complex races of Puccinia striiformis Westend. in mixtures of wheat cultivars possessing different race-specific resistance genes. A simple race of a pathogen can infect only one component, and a complex race of the pathogen can infect two or more components of an intraspecific plant mixture. The treatments were designed so that the race that was complex changed depending on the host mixture, thus enabling us to observe the influence of pathogen complexity in different host genetic backgrounds. Six cultivar mixtures and one pure stand of winter wheat were inoculated with three races of P. striiformis (CDL 27, CDL 29, and CDL 41) at two locations for two seasons. Potted plants of three winter wheat cultivars (Paha, Tres, and Tyee) that were each susceptible to one of the three races of the pathogen were used to sample the pathogen during the field epidemics. Disease incidence on the differential cultivars was used to calculate the proportion of the three races in each treatment. The specific cultivars included in the mixtures influenced the frequencies of the three races. Increasing the number of virulent races in a mixture reduced the frequency of the complex race relative to the other two races. When two of the races (races 29 and 41) were complex on the same mixture, location had an effect on which of the races was more frequent. When race 29 was the complex race in the mixture, it was more frequent than when race 41 was the complex race. The results suggest that environmental interactions, genetic background of the pathogen race, host composition, and interaction among pathogen races may be as important in determining race frequencies in mixtures as is stabilizing selection sensu Vanderplank (1968). / Graduation date: 1993

No description available.

Nineteen cultivars and fourteen breeding lines were evaluated for partial resistance to crown rust Puccinia coronata race 264. Multivariate statistical methods such as principal component and cluster analyses were employed to identify significant resistance parameters and to group oat genotypes with similar rust resistance characteristics. This involved two separate investigations consisting of two experiments each, the first experiment conducted under field conditions and the second conducted under growth bench conditions. From both of the investigations a group of oat genotypes with high partial resistance to P. coronata race 264 was obtained. These are OA 712-17, OA 712-33, Glen, Woodstock, QO 220.13, and QO 574.21. These oat genotypes are currently being used as parents in crosses in the Macdonald Campus of McGill University oat breeding program.

The introduction of Quality Protein Maize (QPM), hard endosperm opaque-2 maize, into northern temperate maize growing areas is a desirable breeding objective. In topcrosses with opaque-2 testers, in diallel combination, as inbreds per se, and in inbred disease screening nurseries, some QPM lines performed better than or equal to the best local checks. In general, while agronomic potential is high for some lines and gains from selection are statistically possible, longer days to flowering intervals and higher levels of moisture at harvest than check hybrids indicated a need to improve adaptation for the locations studied. Methodology experiments indicated that detasselling of check hybrids is a suitable experimental method to facilitate the inclusion of normal endosperm local checks into QPM performance tests. The screening for Fusarium graminearum resistance in the seedling stage has not been proven to be a viable alternative to field scale ear inoculation screening procedures. (Abstract shortened by UMI.)

In Lactuca sativa, the dominant gene Tu confers resistance to infection by turnip mosaic virus (TuMV). Tu and Dm5/8, a gene for resistance to Bremia lactucae, are linked in L. sativa. The area surrounding Dm5/8 on the genetic map of L. sativa contains restriction fragment length polymorphism (RFLP) and random amplified polymorphic DNA (RAPD) markers. The orientation of Tu relative to Dm5/8 was not known. Locating Tu would indicate which markers are on the map of lettuce close to Tu. To locate Tu on the L. sativa genetic map, F$ sb3$ families from recombinant F$ sb2$ in the Dm5/8 area of a cross between TuMV-resistant (Cobbham Green) and susceptible (Calmar) cultivars were inoculated with TuMV and phenotyped for Tu by indirect enzyme-linked immunosorbent assay. Polyclonal antibodies for immunodetection were produced using turnip mosaic virus coat protein expressed in E. coli. Phenotypic ratios within F$ sb3$ families were used to determine individual F$ sb2$ genotypes for Tu. With these genotypes, Tu was located on the genetic map of L. sativa relative to data present for Dm5/8 and surrounding markers, between OPM18 and OPY13. Using bulked segregant analysis, bulks created for the Dm5/8 locus were screened for genetic polymorphisms by the RAPD technique. Five new RAPD markers, UBC346, UBC517, UBC563, UBC599, and UBC675 were found linked to Tu after mapping relative to F$ sb2$ genotypes for Tu and other RAPD markers. The resulting three-point mapping information indicates that Tu is flanked by two markers, OPM18/OPL08 and UBC346, at respective genetic distances of 0.4 and 0.7 cM.

The characteristics and inheritance of maize silk resistance to Fusarium graminearum ear rot were investigated. In an in vitro test, genotypic differences in the degradation of detached silk tissue by F. graminearum were correlated to field evaluations of resistance. Susceptibility to infection decreased with silk age. Total phenolics of silk channel silk tissue increased in response to infection in resistant inbreds but decreased in susceptible inbreds. The flavones iso-orientin, iso-vitexin, maysin, luteolin, and apigenin were identified in the silk. No significant genotype by isolate interaction effects were found when 13 inbred lines were inoculated with three F. graminearum isolates. Simple models of quantitative and qualitative inheritance were not adequate to explain the inheritance of resistance. Disease severity ratings were bimodally distributed in the F$ sb1$, F$ sb2$, and backcross generations. In a complete diallel cross among 12 inbred lines, general and specific combining ability effects were significant for both disease incidence and disease severity. A screening of 12 accessions of exotic maize germplasm with resistance to either Aspergillus flavus or Heliothis zeae, identified several possible new sources of resistance to F. graminearum. Visual evaluations of resistance were correlated to deoxynivalenol levels of the ear.

Experiments were conducted to assess the variation for resistance to Fusarium graminearum ear rot among selfed lines from the Zapalote Chico corn population. It was already known that Zapalote Chico possesses some resistance to corn earworm (Heliothis zeae) and variation in Fusarium graminearum ear rot symptom severity had been observed within the population. During two years of experiments, 47 S4- and S5-generation lines were inoculated and ear rot severity was assessed. In addition, eight pigmentation traits and eight morphological traits were recorded. Lines showed significant differences for all recorded traits. Associations were not strong among morphological traits, but were fairly strong among pigmentation traits. Ear rot symptom severity did not seem to be associated with any other trait. The source population and inbred lines exhibited broad ranges of variation and similar bimodal distributions of ear rot symptom severity ratings. Despite significant differences among lines within experiments none of the lines showed distinct and consistent resistance over two years of experiments. It seems that Zapalote Chico lines do not have environmental stability and sufficient potential to be considered worthwhile source of resistance for ear rot.

Due to the rapid destruction of native orchid habitats it has become necessary to house many endangered orchid species in greenhouse environments where enhanced spread of viral disease occurs due to the close contact between plants. This research was concerned with the construction of a library of genes whose expression is induced in response to viral challenge. In uncovering the genes that are activated during plant-pathogen interactions, it may be possible to manipulate these pathways to develop virus resistant orchids. Furthermore, this research will contribute additional information for the existing framework of plant-pathogen interactions of all plant species.In order to construct a library of genes expressed in response to viral infection, suppression subtractive hybridization was performed using the PCR-Select cDNA Subtraction Kit (CLONTECH, Palo Alto, CA) on Sophrolaeliacattleya Ginny Champion 'Riverbend' clones. RNA was isolated from plants that had been inoculated with the Odontoglossum ringspot virus (ORSV) and from control plants that had not been inoculated with ORSV. Following reverse transcription-PCR (RT-PCR) to obtain cDNA, cDNAs of the tester population (those cDNAs containing differentially expressed messages in response to ORSV) and the driver population (reference cDNAs from uninfected plants) were obtained. The two different cDNA populations are mixed together and hybridized. The sequences common to both populations were subtracted, leaving only the differentially expressed sequences available for PCR amplification.A library containing these genes was constructed, and one clone, chosen at random, was sequenced. Based on homology comparisons to known genes, we have cloned a gene that may contain a nucleotide binding site similar to that of the tobacco N gene, important for plant resistance to pathogens. In the near future, this clone will be used to construct probes for use in northern analysis to determine the timing and localization of the products of this gene. This information will aid in characterizing the function of the orchid N-gene and identifying other members of this signal cascade. In addition, many other clones await sequencing and similar characterization. / Department of Biology

Potato is one of the most important crops grown in Canada and all over the world. Late blight caused by P. infestans is one of the major diseases of potato and is mainly managed by fungicides application. The extensive use of fungicides not only causes adverse effects on the environment but also accelerates the development of resistance in this pathogen. Horizontal resistance is considered as the best choice to control P. infestans as it is durable over years. Breeding for durable resistance requires evaluation of hundreds of breeding lines in greenhouses and in the field. This is usually done by testing several epidemiological parameters such as infection efficiency, lesion size, latent period, and area under disease progress curve (AUDPC). These methods are time-consuming and expensive. The present study reports standardization of metabolic profiling protocols and exploration of metabolic profiling based on GC/MS as an additional tool to discriminate resistance in potato against late blight. Potato cultivars varying in horizontal resistance against late blight have been inoculated with water or the pathogen and more than 100 metabolites have been tentatively identified by GC/MS. Univariate analysis has been used to identify several pathogenesis related (PR) and defense related (DR) metabolites that have potential for application as resistance biomarker metabolites. Multivariate analysis of the abundances of metabolites (the mass spectral (MS) ion trap detector outputs were obtained using Saturn Lab Software Version 5.52 and these abundances are positively proportional to the concentration of mass ions of metabolites) in cultivars were mainly used to identify pathogenesis and resistance functions. Following pathogen inoculation, several metabolites such as amino acids, organic acids, fatty acids and sugars, were significantly increased in abundances, especially in the resistant cultivar. Other metabolites such as phenylalanine, tyrosine, shikimic acid and malonic acid detected here are well known for their direct participation in the shikimic acid, the phenylpropanoid, and the malonic acid metabolic pathways. These pathways lead to the production of several defense metabolites including antimicrobial compounds including phenolics, flavonoids and phytoalexins. The metabolic profiling technology developed here has the potential application for screening of potato breeding lines for horizontal resistance against late blight.

Lettuce mosaic virus (LMV) is an economically important pathogen with worldwide distribution. LMV infection in L. sativa can cause significant yield losses. Resistance to LMV in L. sativa is conferred by the recessive gene mo. We attempted to position the mo gene on the L. sativa map. The ultimate goal is a better understanding of plant-virus interactions. To do so, Random Amplified Polymorphic DNA (RAPD) markers were screened in the near isogenic lines (NILs) Vanguard and Vanguard 75. These NILs differ in the presence of the mo gene in Vanguard 75. Polymorphic markers were screened for linkage to mo in two F$ sb2$ populations segregating for resistance to LMV. The F$ sb2$ populations used were derived from 2 crosses, the first one between the L. sativa cultivars Dwarf 2 (resistant to LMV via the presence of mo) and Saffier and the second one between two breeding lines 87-25M-1 (momo) and 87-1090M-1 (MoMo). In order to develop a highly stringent antibody detection system to phenotype plants infected with LMV, a plasmid construct was developed which overproduces LMV coat protein. This construct will be used in the future to produce enough recombinant LMV coat protein for antibody production. To further characterize mo, a selection of cultivars resistant and susceptible to LMV according to the literature were subjected to various temperature changes to determine the environmental influences on virus movement.