Helen M. Alexander

An experimental field study of anther-smut disease of Silene alba caused by Ustilago violacea: genotypic variation and disease incidence

Twenty cloned genotypes of Silene alba differed greatly (0–100%) in the percentage of flowering plants that became diseased by the anther‐smut fungus Ustilago violacea following natural spore dispersal in a two‐year field experiment. Male genotypes with the highest percentage of disease had high rates of flower production; this trait may increase the probability of spore deposition on flowers, a common site of infection. Because of this relationship, male genotypes with the highest percentage of disease also produced the most healthy flowers in the two‐year period. Flowering early in the season was also a predictor of high disease levels for male genotypes in the first year. Variation among female genotypes in disease levels was not correlated with either flower production or phenology, suggesting that the sexes differ in their interaction with the pathogen. Plants of both sexes that remained nonreproductive the first year but flowered the second year could become diseased due to infection of vegetative tissue.

Intraspecific variation in the response to CO2 enrichment in seeds and seedlings of Plantago lanceolata L.

SummaryFour genotypes of P. lanceolata were grown to maturity at combinations of two levels of atmospheric CO2 concentrations and two temperature conditions. Seed weight was determined, and seed germination and seedling growth were measured for the progeny of each genotype under the same environmental conditions. Overall, high CO2 levels decreased seed weight, increased germination percentage and rate, and increased seedling size. Families differed in their response to CO2 enrichment, and to combinations of CO2 and temperature levels for several characters. These results suggest the existence of genetic variability in P. lanceolata in response to CO2 enrichment.

Spread of Anther-Smut Disease (Ustilago Violacea) and Character Correlations in a Genetically Variable Experimental Population of Silene Alba

1 The anther-smut fungus Ustilago violacea produces spores in diseased flowers of the dioecious plant Silene alba; spores are transferred to healthy plants by insect pollinators. 2 To study the ecological and genetic determinants of disease spread and its demographic consequences, an experimental population of S. alba was created that contained both anther-smut inoculated plants (as an inoculum source) and uninoculated plants which were the progeny of crosses between genotypes with high or low disease resistance. 3 Genetic analyses confirmed that variation in resistance is heritable. 4 Total disease incidence increased over the 3-year study, but rates of establishment of new infections decreased and estimates of the latent period increased each year. By the end of the study, the remaining healthy plants were more likely to be from resistant parents, suggesting that changes in genetic structure of the healthy plants may contribute to the decline in new infections over time. 5 Diseased plants were significantly more likely than healthy plants to die during the first winter of the study but there was no difference in the second year. A compilation of several data sets suggests that diseased plants have higher mortality than healthy plants only in winters with overall high plant survivorship. 6 Phenotypic correlations revealed that plants with early flowering dates and high rates of flower production are more likely to become diseased. 7 There was evidence for a genetic correlation between disease resistance and flowering date in males, such that early flowering families were more prone to infection. Such a relationship suggests that resistance could have a fitness cost, since earlier flowering plants are likely to have high fitness in the absence of disease.

Genotypic Variation in Plant Disease Resistance--Physiological Resistance in Relation to Field Disease Transmission

Cloned genotypes of Silene alba differed in the percentage of plants that became diseased following floral and bud inoculation with six isolates of the fungus Ustilago violacea. The three flower and bud inoculation methods used in the study had little effect on the resistance ranking of the genotypes. No differences in virulence were found among the six fungal isolates. Multiple regression analyses revealed that the percentage of diseased plants following inoculation was an important predictor of disease levels of the same genotypes in a field experiment with natural disease transmission. This result validates the use of inoculation methods for large-scale resistance testing in this system (..)

Epidemiology of anther-smut infection of Silene alba (= S. latifolia) caused by Ustilago violacea : patterns of spore deposition in experimental populations

Experimental populations of Silene alba consisting of healthy individuals and individuals infected by the anther-smut fungus Ustilago violacea were used to examine the effect of density and frequency of diseased individuals on spore deposition on healthy flowers. The results showed increasing spore deposition with increasing frequency but not density. Similar trends were observed for pollen deposition on females. The results support the hypothesis that spore transmission is via pollinators that adjust their flight distances to compensate for host density. Frequency-dependent transmission, characteristic of vector borne and venereal diseases, may by an important component of disease transmission of U. violacea in nature.

Dispersal and Disease Gradients of Anther‐Smut Infection of Silene Alba at Different Life Stages

Given the increased recognition of the importance of spatial aggregation of infected individuals on disease spread, we used a field experiment to examine spore dispersal and disease transmission at increasing distances from an inoculum source in a well-studied system, the herbaceous plant Silene alba, infected by the anther-smut fungus Ustilago violacea. Two different life stages (vegetative and floral) were examined. We measured spore deposition on flowers and, as an analog of dispersal to vegetative plants, in small vials on the ground. Spore and disease gradients (number of spores or proportion of plants infected as a function of distance) were best fit with the classic power law, Y = aD-b. The slope parameter, b, was similar for spore dispersal gradients of vegetative (b = -2.38) and flowering plants (b = -1.91). However, at every distance measured, the proportion of flowers with spores was higher than the proportion of ground vials with spores, indicating overall higher relative spore dispersal to flowering plants. We also determined that disease gradients for flowering plants were more shallow (b = -0.07) than spore dispersal gradients, and vegetative infection was more spatially restricted than floral infection (no infection beyond 3.2 m for vegetative plants while floral infection was detected up to 11.2 m). The relationship between spore dispersal and disease incidence was linear for vegetative plants and curvilinear for flowering plants, with limiting returns on disease after a low threshold number of spores deposited. Overall, our study suggests that the limited dispersal of spores is likely to be important in the aggregation of diseased plants in nature; such aggregation of inoculum in turn can increase or decrease disease spread depending on how much inoculum is needed to cause infection.

TIR-NBS-LRR genes are rare in monocots: evidence from diverse monocot orders

BackgroundPlant resistance (R) gene products recognize pathogen effector molecules. Many R genes code for proteins containing nucleotide binding site (NBS) and C-terminal leucine-rich repeat (LRR) domains. NBS-LRR proteins can be divided into two groups, TIR-NBS-LRR and non-TIR-NBS-LRR, based on the structure of the N-terminal domain. Although both classes are clearly present in gymnosperms and eudicots, only non-TIR sequences have been found consistently in monocots. Since most studies in monocots have been limited to agriculturally important grasses, it is difficult to draw conclusions. The purpose of our study was to look for evidence of these sequences in additional monocot orders.FindingsUsing degenerate PCR, we amplified NBS sequences from four monocot species (C. blanda, D. marginata, S. trifasciata, and Spathiphyllum sp.), a gymnosperm (C. revoluta) and a eudicot (C. canephora). We successfully amplified TIR-NBS-LRR sequences from dicot and gymnosperm DNA, but not from monocot DNA. Using databases, we obtained NBS sequences from additional monocots, magnoliids and basal angiosperms. TIR-type sequences were not present in monocot or magnoliid sequences, but were present in the basal angiosperms. Phylogenetic analysis supported a single TIR clade and multiple non-TIR clades.ConclusionWe were unable to find monocot TIR-NBS-LRR sequences by PCR amplification or database searches. In contrast to previous studies, our results represent five monocot orders (Poales, Zingiberales, Arecales, Asparagales, and Alismatales). Our results establish the presence of TIR-NBS-LRR sequences in basal angiosperms and suggest that although these sequences were present in early land plants, they have been reduced significantly in monocots and magnoliids.

Anther-smut infection of Silene alba caused by Ustilago violacea: factors determining fungal reproduction.

SummaryThe anther-smut fungus Ustilago violacea sporulates in flowers of the dioecious host plant Silene alba. Growth chamber comparisons of healthy and diseased plants, with the genetic background of host and pathogen controlled, revealed that fungal infection increases the number of flowers produced per plant and decreases the size of individual flowers. There were few consistent effects of plant genotype or fungal isolate on diseased flower traits, but differences between the plant sexes were apparent. Stimulation of flower production is proportionally greater in females than males: thus, although healthy male plants produce many more flowers than healthy females, sexual differences in diseased flower number are reduced. Sexual differences in diseased flower size also exist, with male flowers smaller than females. A field inoculation study confirmed dimorphism in diseased flower size and demonstrated that spore production per flower was greater for males than females for all flower size classes.

Epidemiology of anther-smut infection of Silene alba caused by Ustilago violacea: patterns of spore deposition and disease incidence.

Spore dispersal and disease incidence in the S. alba-U. violacea system were examined in natural and experimental populations in Virginia, U.S.A.

The interaction between plant competition and disease

Abstract It is well documented that pathogens can affect the survival, reproduction, and growth of individual plants. Drawing together insights from diverse studies in ecology and agriculture, we evaluate the evidence for pathogens affecting competitive interactions between plants of both the same and different species. Our objective is to explore the potential ecological and evolutionary consequences of such interactions. First, we address how disease interacts with intraspecific competition and present a simple graphical model suggesting that diverse outcomes should be expected. We conclude that the presence of pathogens may have either large or minimal effects on population dynamics depending on many factors including the density-dependent compensatory ability of healthy plants and spatial patterns of infection. Second, we consider how disease can alter competitive abilities of genotypes, and thus may affect the genetic composition of populations. These genetic processes feed back on population dynamics given trade-offs between disease resistance and other fitness components. Third, we examine how the effect of disease on interspecific plant interactions may have potentially far-reaching effects on community composition. A host-specific pathogen, for example, may alter a competitive hierarchy that exists between host and non-host species. Generalist pathogens can also induce indirect competitive interactions between host species. We conclude by highlighting lacunae in our current understanding and suggest that future studies should (1) examine a broader taxonomic range of pathogens since work to date has largely focused on fungal pathogens; (2) increase the use of field competition studies; (3) follow interactions for multiple generations; (4) characterize density-dependent processes; and (5) quantify pathogen, as well as plant, population and community dynamics.