Jeanne D. Mihail

THE POPULATION DYNAMICS OF ANNUAL PLANTS AND SOIL-BORNE FUNGAL PATHOGENS

1 Soil-borne fungi are a major group of economically important plant pathogens, yet they have rarely been studied in the context of host-pathogen population biology. We develop general models of annual hosts and soil-borne fungal pathogens to explore the conditions for host-pathogen coexistence in both agricultural and natural plant populations. We use empirical data from the literature to parameterize and simulate dynamics with these models. 2 Initially we consider a simple system in which host density is assumed to be constant, as would be appropriate for agricultural systems. Model analysis shows that initial increase of the pathogen population requires that host (crop) density be above a threshold; this threshold decreases with increases in the pathogens over-winter survival rate and ability to grow saprophytically. 3 A more complex model, in which both host and pathogen populations can vary, is needed for natural populations. Results from this model show that stable coexistence is possible even when the pathogen has a positive intrinsic growth rate (and therefore it is also possible for the pathogen to persist in the complete absence of the host). 4 Model parameter estimates were obtained from the empirical literature for two common and important soil pathogens: Phytophthora spp. and Fusarium oxysporum; these pathogens differ in several life-history features. Computer simulation showed that for Fusarium, there were substantial ranges for which coexistence or loss of the pathogen were predicted, while for Phytophthora most parameter estimates resulted in complete extinction when linear disease transmission was assumed; under the assumption of exponential disease transmission, predicted dynamics were most likely to lead to host persistence. 5 For both pathogens, within biologically realistic regions of parameter space, small changes in parameter values could lead to qualitatively different outcomes, including deterministic chaos, suggesting that long-term dynamics may be difficult to predict.

Seedling disease in an annual legume: consequences for seedling mortality, plant size, and population seed production

Abstract The effect of seed and seedling mortality on plant population dynamics depends on the degree to which the growth and reproduction of surviving individuals can compensate for the deaths that occur. To explore this issue, we sowed seeds of the annual Kummerowia stipulacea at three densities in sunken pots in the field, which contained either field soil, microwaved field soil, or microwaved field soil augmented with oospores of three Pythium species. High sowing density reduced seedling establishment and seedling size, but these effects were independent of the soil treatment. In the oospore-augmented soil, seed and seedling survival was low. The surviving plants were initially smaller but, at maturity, average plant size was greatest in the oospore-augmented soil, compared to the other treatments. Total population seed production was unaffected by soil treatment, suggesting that the effect of disease was limited to the seedling stage, with surviving plants released from intraspecific competition. To test the hypothesis that the surviving plants in the oospore-augmented soil were more disease-resistant, seeds from each of the sowing density-soil type treatments were sown in a growth chamber inoculation study. No evidence for selection for resistance was found. A second inoculation experiment revealed that oospore inoculum reduced plant numbers and mass regardless of whether field or microwaved soil was used, suggesting that results from the field experiment were not dependent on the use of microwaved soil. The findings of this study indicate that the ecological effects of disease on individual plants and on plant populations are not necessarily equivalent.

Identification, distribution and comparative pathogenicity of Pythium spp associated with alfalfa seedlings

Abstract Root system colonization of alfalfa seedlings by Pythium spp was investigated during the first 4 wk after seedling emergence at two field locations in Missouri over 2 years. Pythium spp were isolated from most roots, but isolation was not necessarily associated with necrosis, lesions or overt evidence of disease. P. sylvaticum, P. irregulare, P. ultimum and P. torulosum were the most prevalent species isolated, comprising up to 65% of all Pythium spp isolated at a given site. Changes in the species composition of the Pythium community associated with alfalfa seedlings were monitored in relation to seedling age, root development stage and various environmental conditions. Root systems were classified by root orders using the morphometric system of analysis to define root segments of similar physiological age and function. P. ultimum, P. irregulare and P. sylvaticum caused severe pre- and post-emergence damping-off, necrosis and stunting of root and shoot growth in greenhouse pathogenicity tests. Several other species, including P. dissotocum, P. acanthicum, P. torulosum and P. rostratum reduced root system length, but elicited no other symptoms. Variations in the frequency of isolation of different species were most closely associated with environmental conditions, particularly temperature and moisture. P. sylvaticum was prevalent at higher temperatures and drier conditions, and several other species preferred cooler, wetter conditions. The stage of root development, as described by the morphometric system, and seedling age had no observable effects on the frequency of colonization or the composition of the community of Pythium spp colonizing the roots.

Temporal dynamics of ectomycorrhizal community composition on root systems of oak seedlings infected with Burgundy truffle.

Truffles, the hypogeous ascocarps of the ectomycorrhizal genus Tuber, are currently cultivated in orchards to partially offset declining wild production in Europe. Truffle cultivation begins with inoculation of seedling root systems in the greenhouse. Once the mycorrhizal relationship is well established, colonized seedlings are transplanted to a suitable field site. In the USA, little is known about the effect of indigenous ectomycorrhizal species on colonization of host trees by the European Burgundy truffle fungus (T. aestivum). Here we identify the ectomycorrhizal community composition on inoculated seedlings grown in the greenhouse in three types of potting media. We subsequently monitored the ectomycorrhizal community composition for two years after seedlings were transplanted into a field site that had been prepared for truffle cultivation by lime applications. We found that the infection rates of contaminant ectomycorrhizal species present in the greenhouse declined to low levels in this field site. We also found that after two years in the field, both T. aestivum colonization levels and indigenous ectomycorrhizal species richness and abundance increased, indicating that indigenous species, in the short term, did not displace T. aestivum.

Fractal geometry of diffuse mycelia and rhizomorphs of Armillaria species

Fractal geometry was used to quantify the variation in branching patterns of sibling basidiospore-derived diffuse mycelia of Armillaria gallica and rhizomorph systems of A. gallica and A. ostoyae in laboratory culture. The fractal dimension (D) of 11 A. gallica diffuse mycelia ranged from 1·05 to 1·20 at 74 h after basidiospore germination, and from 1·43 to 1·57 at 116 h after germination. The coefficient of variation (CV) for the regression coefficients of D on time was 14·8%, whereas the CV for D of all 11 mycelia at the fourth measured time step was only 2·9%, indicating significant convergence towards a given branching pattern for a particular set of environmental conditions. The rhizomorph systems of diploid A. gallica thalli branched more profusely than those of diploid A. ostoyae . D values among replicate rhizomorph systems for strains representing three A. gallica genets were similar, whereas rhizomorph systems of eight of the 12 A. ostoyae thalli, also representing three genets, either failed to develop or did not branch sufficiently to permit calculation of D. D values of rhizomorph systems were generally temporally invariate for both species. The CV of D for replicate rhizomorph systems at the fourth measured time step was between 1·6% and 3·9% for A. gallica genets and 2·4% for the A. ostoyae genet; D was significantly lower for the A. ostoyae genet than for the three A. gallica genets. A. gallica is only weakly pathogenic and can colonize a taxonomically broad range of food bases saprotrophically. A. ostoyae is more pathogenic and colonizes a narrower range of substrata. The profuse rhizomorph branching pattern of A. gallica is consistent with a foraging strategy in which acceptable food bases may be encountered at any distance, and which favours broad and divisive distribution of potential inoculum. The sparse branching pattern of A. ostoyae rhizomorph systems is less divisive, consistent with the conservation of nutrients necessary to maintain rhizomorph inoculum potential more efficiently with increasing distance from a foodbase.

Foraging behaviour of Armillaria rhizomorph systems

The foraging behaviour of Armillaria rhizomorph systems is poorly understood owing to their cryptic position within the soil. We investigated foraging in a homogeneous environment (i.e. agar), finding that rhizomorph systems of the more parasitic species, A. mellea, A. ostoyae, and A. tabescens, lacked melanin and the approximately cylindrical cord-like form observed in the field. In contrast, rhizomorph systems of the more saprotrophic species, A. calvescens, A. gallica, and A. sinapina, developed radially resembling those in the field. For the three saprotrophic Armillaria species, the number of rhizomorph tips, total rhizomorph length and total rhizomorph surface area were significantly positively correlated with increasing rhizomorph system diameter and elapsed time in two developmental tests. However, the fractal dimension (D), used as a measure of foraging intensity, was temporally invariable, suggesting that one component of foraging behaviour is innate. In a heterogeneous environment (i.e. sand) and in the absence of a potential nutrient source, we observed that rhizomorph systems of A. gallica most often developed asymmetrically. While rhizomorph foraging was unresponsive to the lateral placement of an uncolonised stem segment, we were able to demonstrate directional growth toward an uncolonised Quercus velutina stem segment placed above or below the colonised source stem segment. When neighboring rhizomorph systems were conspecific genets of A. gallica, we observed that the growth of one rhizomorph system was directed toward zones unoccupied by its neighbour. However, the foraging intensity of the neighbouring genets, as measured by fractal dimension (D), was unaffected by the proximity of a neighbour. When neighbouring rhizomorph systems represented different species (A. gallica and A. mellea), A. gallica rhizomorph systems produced more total length and more foraging tips but concentrated their rhizomorph production away from the neighbouring A. mellea genet. In contrast, A. mellea rhizomorph systems produced significantly more foraging tips per unit length, both overall and in the zone of confrontation with the neighbouring A. gallica genet. Our observations are consistent with field observations of territoriality among Armillaria genets, and provide evidence that rhizomorph systems of more parasitic Armillaria spp. are able to compete effectively with the larger rhizomorph systems of more saprotrophic Armillaria species.

The effects of moisture and oxygen availability on rhizomorph generation by Armillaria tabescens in comparison with A. gallica and A. mellea

Compared with other Armillaria species, natural melanized rhizomorphs of A. tabescens are rarely observed. Growing in autoclaved Vitis stem segments, A. tabescens isolates from the Ozark Mountains in the central USA formed fully melanized rhizomorphs, thinner and shorter than those observed for other Armillaria species under field conditions, and only under conditions of both high oxygen availability, ζ2 ug cm −2 min −1 and moisture near saturation. Conducive conditions were used to compare the rhizomorph generation capacities of A. tabescens, A. gallica , and A. mellea , which have overlapping host and geographic ranges in central North America. While the rhizomorphs of A. tabescens were significantly shorter than those of the other two species, A. gallica and A. tabescens produced similar numbers of rhizomorph initials. Finally, we demonstrated the ability of melanized A. tabescens rhizomorphs to span woody food bases and thereby establish viable infections by penetration of intact bark of Vitis stem segments. We hypothesize that A. tabescens rhizomorphs form under conditions of periodic saturation which promote rapid water movement through naturally occurring lacunae in the soil. Thus, the role of A. tabescens rhizomorphs in the spread of the fungus should be re-evaluated.

The fractal dimension of young colonies of Macrophomina phaseolina produced from microsclerotia

Concepts of fractal geometry have been used to describe the branching patterns of a variety of or? ganisms ranging from microorganisms to trees. The fractal dimension is a measure of branching density. The fractal dimension was used to investigate the hy? phal branching patterns of young colonies of the soil- borne phytopathogen Macrophomina phaseolina. The temporal development of branching density was ex? amined by assembling composite photomicrographs of 17 entire thalli (representing three test isolates) at 2-hour time intervals. These photomicrographic im? ages were digitized, and the fractal dimension was de? termined using a box-count algorithm. Analysis of 90 images revealed patterns of thallus development which were remarkably radially asymmetric. The fractal di? mension ranged from 1.21 to 1.84, indicating a wide range of branching density. The temporal develop? ment of branching density was investigated by regres? sion of fractal dimension on time. Branching density increased with time at rates characteristic of each iso? late. Further, the magnitude of fractal dimension to? gether with the rate of temporal increase of the fractal dimension permitted clear distinction among the three test isolates of M. phaseolina. These results suggest that quantitative description of branching density using the fractal dimension might become a novel supplement to the morphological criteria traditionally used in fun? gal taxonomy.

Colonization of Pedunculate oak by the Burgundy truffle fungus is greater with natural than with pelletized lime

European black truffles can be profitable agroforestry crops outside their native ranges. Truffle fungi grow symbiotically as ectomycorrhizae on the roots of host trees, notably hazels and oaks. Conditions in the central USA appear conducive to cultivation of the Burgundy truffle (Tuber aestivum Vitt. syn. T. uncinatum Chatin), but research is needed to determine effects of management practices on truffle establishment and fruiting. In a greenhouse study we tested the effect of lime type, inoculation technique, and two truffle sources on Pedunculate oak (Quercus robur L.) growth and mycorrhizal colonization. We found that the type of lime used to raise potting mix pH can differentially affect the growth rate of root systems inoculated with different selections of Burgundy truffle inoculum. Seedlings inoculated with one selection of the truffle and grown in potting mixes amended with natural crushed dolomitic limestone developed larger root systems with more truffle mycorrhizae compared with potting mix amended with high-calcium pelletized quick-release lime. Seedlings inoculated with a second truffle selection were not affected by lime source and developed root systems as large as those developed with the first truffle source grown with natural lime. Supplemental root dip inoculation did not improve levels of colonization beyond those accomplished by potting mix infestation with truffle ascospores. Use of a hygroscopic polymer to maintain ascospore suspension in the inoculum slurry used to infest the potting mix had no effect on root system development or mycorrhiza formation.

Bioluminescence patterns among North American Armillaria species

Bioluminescence is widely recognized among white-spored species of Basidiomycota. Most reports of fungal bioluminescence are based upon visual light perception. When instruments such as photomultipliers have been used to measure fungal luminescence, more taxa have been discovered to produce light, albeit at a range of magnitudes. The present studies were undertaken to determine the prevalence of bioluminescence among North American Armillaria species. Consistent, constitutive bioluminescence was detected for the first time for mycelia of Armillaria calvescens, Armillaria cepistipes, Armillaria gemina, Armillaria nabsnona, and Armillaria sinapina and confirmed for mycelia of Armillaria gallica, Armillaria mellea, Armillaria ostoyae, and Armillaria tabescens. Emission spectra of mycelia representing all species had maximum intensity in the range 515-525 nm confirming that emitted light was the result of bioluminescence rather than chemiluminescence. Time series analysis of 1000 consecutive luminescence measurements revealed a highly significant departure from random variation. Mycelial luminescence of eight species exhibited significant, stable shifts in magnitude in response to a series of mechanical disturbance treatments, providing one mechanism for generating observed luminescence variation.