Summary Armillaria root disease is a contributing factor to oak decline in the Ozark Mountains of central USA. We have identified Armillaria gallica, Armillaria mellea, and Armillaria tabescens in Quercus‐Carya‐Pinus forests of the region. Presence/absence patterns of each Armillaria species as well as all possible Armillaria species combinations were analysed by contingency tables and/or stepwise logistic multiple regressions with principal characteristics of the studied sites and forest stands, both quantitative and qualitative: geographic land‐type association, bedrock type, landform position, slope direction (aspect), soil type and soil surface stone cover, down woody debris, abundance and basal area of woody vegetation and decline mortality by species. Most decline mortality consisted of two red oak species (section Erythrobalanus, Quercus coccinea and Quercus velutina), which also were most sensitive to Armillaria infection. Site characteristics related to the distributions of Armillaria species and decline mortality were also related to the preponderance of Q. coccinea and Q. velutina, regional vegetation history (i.e. conversion of Pinus echinata stands to hardwoods), and the different strategies of territory acquisition and spread of the Armillaria species involved. The presence of A. gallica may reduce the activity of more virulent Armillaria species.

One hundred and fourteen isolates of Macrophomina phaseolina from tissue of hosts in the Asteraceae, Euphorbiaceae, Fabaceae, and Poaceae, and from cultivated and noncultivated soils in Somalia (east Africa) and Arizona were tested for pathogenicity, ability to form pycnidia, and chlorate-utilization phenotype. Hyphal interactions of pairs of isolates from noncultivated soils in Somalia and Arizona were examined to address the possibility of genetic reproductive isolation of two geographically separated populations. Isolates from the Poaceae were less pathogenic and formed pycnidia less frequently compared with isolates from dicot host tissue. Chlorate-utilization phenotype was unrelated to host tissue source. However, isolates that infrequently or never produced pycnidia were more likely to have chlorate-sensitive phenotypes. Apparently successful hyphal fusions were observed for 64.3% of confrontations where one isolate was taken from Arizona and one was from Somalia, implying no barrier to genetic interchange at this initial level of interaction. While M. phaseolina is a heterogeneous species that cannot be partitioned into distinct subspecific groups based upon function, it appears that isolates colonizing the Poaceae are more restricted in pathogenicity than the general population. Key words: chlorate utilization, geographic isolation, host specialization, hyphal interactions, Macrophomina phaseolina.

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Annual census data spanning seventy-five years document mortality and regeneration in a population of saguaro cactus (Carnegiea gigantea) in the Cactus Forest of the Rincon Mountain District of Saguaro National Park near Tucson, AZ. On 6 four-hectare plots, each saguaro was censused and a methodical search for new saguaros was conducted annually each year from 1942 through 2016, with the exception of 1955. Regeneration has been episodic with 828 plants established from 1959 through 1993 compared with 34 plants established between 1942 and 1958 and only three plants established after 1993. The years preceding 1959 and following 1993, include some of the driest decades in centuries in southern Arizona. While woodcutting and cattle grazing are believed to be among the causes of decades of failed regeneration prior to 1958, neither of these factors contributed to the failed regeneration following 1993. The height structure of the population from 1942 to 2016 shifted dramatically from a population dominated by large saguaros (> 5.4 m tall) in the first three decades of the study to a population dominated by small saguaros (< 1.8 m tall) in the most recent two decades. Mortality is shown to be strongly age dependent. In the year following the 2011 catastrophic freeze, 21 of 59 plants older than 80 years died compared with zero deaths in 270 plants between the ages of 29 and 80 years. Saguaros under 40 years old, growing under small shrubs or in the open, have a lower probability of survival than better protected saguaros. Long-term population monitoring is essential to understanding the complex impacts of human and environmental factors on the population dynamics of long-lived species.

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JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. British Ecological Society is collaborating with JSTOR to digitize, preserve and extend access to Journal of Ecology.Summary 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 pathogen's 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.

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