Fig. 9. Potato tubers infected by Phytophthora infestans.
Reprinted from Schumann.
6. Gene-for-gene hypothesis. The pathogenic range in a given race of Melampsora lini in flax is determined by pathogenic factors specific for each resistant factor in the host according to Flor in 1942. Avirulence is dominant in crosses between races. This work led to the formulation of a gene-for-gene theory by Flor and its application to other host-pathogen combinations in a search for universal applicability. The gene-for-gene concept has been useful in identifying the roles of hybridization, mutation, heterokaryosis, and somatic hybridization in pathogenic variation in parasitic fungi. Interorganismal genetics as described by Loegering in 1978 grew out of Flor's work. Current research delves into molecular characterization in gene-for-gene systems.
7. Organic fungicides. The organic fungicide era was ushered in during 1934 with the introduction of the dithiocarbamates, especially thiram, by Tisdale and Williams of E.I. duPont de Nemours & Company. Iron, lead, and zinc dimethyl derivatives of the carbamates promised the most as fungicides and injured plants the least as reported by Goldsworthy and associates in 1943. Then came the systemic fungicides such as the oxathiins in 1966, and the benzamidazoles in 1966-1968, which raised a new problem of induced resistance.
8. Fungistasis. A fungistatic factor appears to be widespread in soil according to Dobbs and Hinson in 1953. This factor is masked by glucose or by nutrients liberated from microorganisms in soil. This research was further refined by Lingappa and Lockwood in 1963 and later by other plant pathologists and soil microbiologists, who sought to understand fungistasis in relation to soilborne disease. Studies on suppressive soils in relation to root disease incidence grew out of this work on fungistasis and pertained to work on the biocontrol of root disease. Understanding of the ecology of root-infecting organisms was enhanced by research on fungistasis. 9. Soilborne disease and biological control. Principles of root infection by fungi were developed by Garrett in 1956 in the first of two volumes. His bringing together the findings and conclusions of earlier workers led to his publication on the biology of root-infecting fungi, a monumental work that influenced and stimulated further research on root pathology and the ecology of soil microorganisms and provided a terminology and ecological principles useful in the understanding of biological control of plant pathogens. Hiltner in 1905 had already defined the rhizosphere. Weindling in 1932 was the first to use a known antibiotic-producing antagonist in plant disease control, in reporting parasitism and antagonism of Trichoderma viride to Rhizoctonia solani. The first commercially successful application of biological control by pitting an organism against a plant pathogen was done by New and Kerr in 1972 in which they dipped seedlings of peach into cell suspensions of Agrobacterium radiobacter to control crown gall. These investigations on root pathology, soil microbial ecology, and biocontrol applied principles defined by Garrett in his publications.
Fig. 10. Crown Gall (Agrobacterium tumefaciens). Reprinted from Schumann.
10. Gene location in tumor-inducing plasmids.A major change in research on host-parasite interaction took place about 1975 in the physiology and biochemistry of disease when molecular biology and molecular genetics were applied in host-parasite interactions. To determine a possible role of a given molecule in pathogenesis, a gene is cloned and then bits disrupt activity by transformation. The greatest effect of molecular genetics on plant pathology has been through Agrobacterium tumefaciens. The first major breakthrough was reported in 1975 when it was established that genes controlling crown gall induction were located in a plasmid, the tumor-inducing plasmid (pTi), by Van Larebke et al. (1975) and Watson et al. (1975). Transposons are discrete sequences of DNA incapable of replication and can insert into replications such as plasmids and chromosomes. Transformation strategies depend on stable introduction of foreign DNA stably into the genome of a cell followed by regeneration of that cell into a whole plant. Virulence by fungi is influenced by dsRNA genetic elements as shown in transmissible hypovirulence in Cryphonectria parasitica (Nuss and Koltin, 1990).
What have we learned about scientists in the past century?In the past century considerable changes occurred in the relationships of research workers to universities and institutions. After the appearance of Sputnik, in 1957, there was a major change in funding for research in the United States and in some other nations. Instead of depending on financial support from government, experiment stations, or institutions, investigators had to become entrepreneurs and prepare grant proposals to seek their own funds for doing research. Sometimes, that focused their efforts on what was likely to get funded and precluded research in which the risk of success was unpredictable. In addition, the science of plant pathology, like many other scientific disciplines, became more fragmented as subdisciplines emerged. This fragmentation of the discipline led also to fragmentation of societies and journals, and even to the publication of papers. Whereas, earlier publications represented considerable research, later papers approached a one-experiment, one-paper philosophy, with a consequent proliferation of publications. Then, fax machines and computers became more actively used in research programs, for communication as well as for data processing. Thus specialists at one institution were more likely to communicate with their counterparts throughout the world, than with their colleagues down the hall. These factors contributed to a loss of collegiality and communication among faculty at a given institution, less loyalty to a department or institution, and some erosion of the department as a major force in maintaining a specific discipline. Another major change was the increase in the number of women among scientists. Women brought new talent and new ways of doing things to enrich research programs but this did not alter the factors listed above. Although some departments suffered some as a force in the discipline, other departments maintained their traditional strengths by recruiting specific faculty instead of using the search committee approach. With the advent of molecular biology, there has come to be a blurring of the fields of science and contributions to plant pathology are not always made by plant pathologists.
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Figures 1,3,6,7, and 8 reprinted from The Formal Years of Plant Pathology in the United States by Campbell, Peterson, and Griffith. Copyright 1999 The American Phytopathological Association.
Figures 2,4,5,9, and 10 reprinted from Plant Diseases: Their Biological and Social Impact by Gail Schumann. Copyright 1991 The American Phytopathological Association.