Annual Review of Phytopathology - Volume 33, 1995

ES Luttrell was a mycologist who spent 42 of his 44 professional years at the University of Georgia, first at the Georgia Experiment Station in Griffin and later at the main campus in Athens. He is best known for his innovative classification scheme for the perithecial ascomycetes, in which orders were based on patterns of ascomal ontogeny and mode of ascus dehiscence. This work established him as an authority on ascocarp development. His later studies on the classification of the Helminthosporium complex, in which he showed that small differences in conidium germination and structure could be correlated with distinct teleomorphs, brought him recognition from plant pathologists as well. These studies were notable for his attention to detail and the quality of his work. His contributions to mycology and plant pathology were numerous and varied and brought him various awards later in his career.

Information derived from nucleic acid analyses either has complemented phylogenetic arguments based on phenetic characters or facilitated choice among competing hypotheses. Despite limited taxon sampling, a picture of the interrelationships of filamentous Ascomyceteas at higher taxonomic levels is developing. Intergeneric relationships within groups that include economically important fungi (e.g. Eurotiales, Hypocreales) are being clarified, and generic circumscriptions redefined. Molecular analyses have supported predictions of links between individual asexual species of groups or asexual species of the Fungi Imperfecti, and groups of Ascomycete genera and species. However, individual asexual species have not been linked unequivocally to individual Ascomycete species. Anamorph names are necessary and should be retained because teleomorphs may not be recognized in vivo nor formed in vitro. In the few cases where phenetic and molecular phylogenies seem irreconcilable, the ribosomal genes may not give the most parsimonious explanation. The taxon name “Plectomycetes” is confused and should be dropped.

In plant pathology, terminological confusion still reigns despite national attempts at standardization. Terminological agreements reached within the crop protection community in The Netherlands are elaborated here and presented as an endeavor toward international consensus. Much of the on-going terminological disconcert derives from differences in outlook between academically oriented biologists (including biologically trained pathologists) and pathologists working in and for agricultural institutions where disease and harm have anthropocentric connotations. The name crop protection science more realistically covers and marks the field dealt with by most plant pathologists, and adoption of the FAO-defined term pest to encompass all biotic factors that are harmful to plants and their products is advocated.

The effect of pests on plants and the interrelationships between pests and plants in dependence upon the environment, topical in resistance breeding, are especially dealt with. A diagrammatic model is used to better describe these relationships and to define the terms that denote the phenomena and mechanisms involved.

Ceratocystis fagacearum (Bretz) Hunt, the oak wilt pathogen, is currently causing massive losses of semievergreen live oaks (Quercus fusiformis Small and Q. virginiana Mill.) in central Texas. Given the relatively limited oak mortality caused by C. fagacearum in the deciduous forests of the North Central, Midwestern, and Mid-Atlantic United States, this Texas epidemic was not anticipated. The intensity of oak wilt in Texas is attributed to a number of factors related to host characteristics and the ability of the pathogen to adapt to limiting environmental conditions. Oak wilt management in semievergreen oaks requires considerable revision of the control techniques previously designed for deciduous oaks. The Texas oak wilt epidemic provides a new perspective from which to evaluate questions concerning oak wilt, including the origins of the pathogen as well as the potential for future losses in unaffected oak forests.

The relationship between disease and yield is most often summarized as a simple empirical model that describes average crop performances is the presence of a pathogen. Such models may be robust and useful for surveys but their use is usually constrained to the specific conditions under which the model was developed. Changes in production system usually invalidate the relationship. The alternative is to base the relationship on an epidemiological analysis of the pathogen population and a physiological concept of host growth and development. This review provides the knowledge and conceptual basis and discusses the limitations to progress in the development of such models. It is shown that a host-based assessment of disease is well suited to yield investigations and to multiple pest constraints, and that disease is logically related to yield via radiation interceptions and radiation use efficiency.

This review focuses on the role of two distinct fitness strategies in the growth, survival, and epidemiology of foliar bacterial pathogens. A tolerance strategy requires the ability to tolerate direct exposure to environmental stresses on leaf surfaces, including UV radiation and low water availability. An avoidance strategy requires the ability to seek and/or exploit sites that are protected from these stresses, including endophytic sites. The ability to employ an avoidance strategy and grow endophytically may directly influence the potential for pathogenesis, since endophytic populations, not epiphytic populations, are likely responsible for disease induction. Furthermore, exchange between these two populations is probably crucial to the epidemiology of foliar pathogens. While foliar pathogens can grow and survive in both exposed and internal sites, indicating that they can employ both fitness strategies, the poor internal growth of most saprophytes suggests that saprophytes depend primarily on a strategy of tolerance. This difference between pathogens and saprophytes has important implications for predicting the population dynamics of leaf-associated bacterial species and for selecting effective biological control agents.