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History of Ecological Sciences, Part 44: Phytopathology during the 1800s

2012; Ecological Society of America; Volume: 93; Issue: 4 Linguagem: Inglês

10.1890/0012-9623-93.4.303

2327-6096

Frank N. Egerton,

Yeasts and Rust Fungi Studies

Click here for all previous articles in the History of the Ecological Sciences series by F. N. Egerton Every ecologist knows the basic history of evolutionary theory—Lamarck's inability to sway many naturalists with his flawed arguments, and Darwin's only gradual success with his masterful presentation. Much less well known is the struggle to establish the germ theory of disease, with many defenders and skeptics for well over a century before it gained acceptance. Although the germ theory of disease is narrower in scope than evolutionary theory, its practical importance is at least as great, and therefore its history merits greater familiarity. We saw in part 29 (Egerton 2008) that both mycology and phytopathology made notable advances during the 1700s—with small steps toward a germ theory—though these advances were not widely known or appreciated. Researchers during the 1800s built upon the work of their predecessors. The science of phytopathology began with investigations of fungi, the earliest known culprits, and fungi were already being accused during the 1700s, though there was a hung jury rather than a clear verdict of guilt (Egerton 2008). If guilt was to be established, it was important for botanists to have a standard system to classify what they knew and make clear what was a new discovery when one occurred. Mycology and phytopathology had to advance together. Although John Needham (1713–1781), whom we met in part 24 (Egerton 2007:147), discovered a "worm" (a nematode now named Anguina tritici) in wheat galls (Needham 1743:640–641), D. J. Raski (1959:386) dates the beginning of nematology to the Histoire naturelle des helminthes ou vers intestinaux 1845 by Felix Dujardin (1801–1860). In 1854–1856, Dujardin's fellow countryman, Casimir-Joseph Davaine (1812–1882) studied the nematode that causes seed-gall disease in wheat, and in 1868 Davaine also discovered a bacterium that caused a plant disease. Not until the late 1800s were there evidences of a virus as a cause of plant disease. Nature is one, but scientists partition it into sciences they can master. Although phytopathology and animal parasitology developed as separate sciences, their concerns overlap in cases of fungal diseases of animals, including humans, and nematode diseases of plants; and both phytopathology and parasitology overlap with bacteriology and virology concerning bacterial and viral diseases. The history of nematode parasitism of plants was outside the scope of both Large's (1940) and Ainsworth's (1981) histories of phytopathology, but nematodes are discussed in textbooks on phytopathology (Heald 1926:831–851, Walker 1969:533–548, Agrios 2005:826–874). With many simultaneous developments occurring, it seems best to discuss studies of fungi first, nematodes and bacteria second and third, and viruses last, in Europe, then in North America. The botanist who began to put the fungal house in order was Christiaan Hendrik Persoon (ca.1761–1836), of Dutch heritage, from Cape Town, South Africa (Donk 1974, Ainsworth 1976:255–258, Magnin-Gonze 2004:170–171). In 1775 he went to Germany, where he received his botanical education, and in 1802 he moved to Paris, where he lived alone, in poverty. He corresponded with many botanists and exchanged specimens with them, exemplified by his correspondence with James E. Smith (Ramsbottom 1934). In 1828 he gave his botanical collections to the Dutch government in exchange for a pension, and when he died he left his remaining collections and his library to the Dutch government. (a) Christiaan Hendrik Persoon in 1796. (b) Elias Magnus Fries. Ainsworth 1976:256, 260. Persoon published his first article on fungi in 1793 (Schmid 1933), introduced his classification system in 1794, and produced his Synopsis methodica fungorum in 1801, which became the foundation of modern mycological systematics. Geoffrey Ainsworth (1976:258) found that "almost all the hundred genera and subgenera he recognized are universally accepted genera of today." Persoon brought together the rusts and smuts in the Dermatocarpi, and he named Fontana's small parasitic plants, brown and black rust on wheat, Uredo linearis and Puccinia graminis (Fig. 2). However, Persoon accepted the prevailing belief that these species could arise from abnormal sap or tissue in the host plant, and his description of P. graminis was vague (Ainsworth 1981:34, 43). Black rust Puccinia graminis, figures 57–59 (upper left corner). Drawn by John Edward Sowerby (1825–1870). Cooke 1865: Plate 4, facing 56. (a). Joseph Banks. (b) Thomas Andrew Knight. Simmonds 1954:following 468. Germination and growth of bunt spores. Prévost 1807, 1939: Plate 1, Figs. 1–32. (a) Agostino Maria Bassi. Bassi 1958: facing title page. (b) David Gruby, age 75. Ainsworth 1976:169. John Ramsbottom (1913:81–84) evaluated Persoon's classification of the Uredinales, which classification is probably a fair sample of Persoon's Synopsis. Persoon began a revision of his Synopsis entitled Mycologia Europea (three volumes, 1822–1826), but left it incomplete, no doubt because a younger rival, Elias Magnus Fries (1794–1878) had begun his authoritative Systema Mycologicum (three volumes, 1821–1829 + 2 supplements, 1830–1832). Fries quite literally followed in the footsteps of Linnaeus: his father was a church vicar from the southern Swedish province of Småland, he studied at the Växjö secondary school, then at the University of Lund, and eventually became Professor of Botany at Uppsala University (Eriksson 1952, 1972, Fries 1952, Ainsworth 1976:259–263). At Lund he studied fungi, but before going there he had independently learned between 300 and 400 species (compared to Linnaeus' listing of 92 species). He found Persoon's classification unsatisfactory, and Fries's Systema Mycologicum "did for mycology what Linné did for phanerogamic taxonomy," with his descriptions being "models of accuracy and conciseness" (Fries 1952:180). His Systema is the starting point for fungal names, just as Linnaeus's names are for higher plants; however, Fries worked without a microscope. He did not confine his studies to fungi, but his other studies are beyond the scope of this discussion. Elias Fries was the first of four generations of Fries botanists. We met Sir Joseph Banks (1743–1820) in part 43 (Egerton 2012:200) as the botanist with whom Thomas Andrew Knight (1759–1838) corresponded, and who published Knight's letters in the Philosophical Transactions of the Royal Society of London. Banks was president of the Royal Society for 42 years, so he was a well-respected member of the scientific community (Foote 1970, Carter 1987, O'Brian 1997, Knight 2004). In 1805, in response to an extensive outbreak of black rust (Puccinia graminis) in 1804 (Ordish 1976:115), Banks published A Short Account of the Cause of the Disease in Corn, called by Farmers the Blight, the Mildew, and the Rust, in which he asked, "Is it not more than possible that the parasitic fungus of the barberry and that of wheat are one and the same species, and that the seed transferred from the barberry to the corn [wheat], is one cause of the disease?" (quoted from Ramsbottom 1913:85). He had read Felice Fontana's Observazioni sopora la ruggine del grano (1767), which was little known in Britain. One would think that Banks also would have read A Botanical Arrangement of All the Vegetables Naturally Growing in Great Britain (edition 1, 1776) by William Withering (1841–1899), that claimed the Berberis shrub "should never be permitted to grow in corn [grain] lands, for the ears of wheat that grow near it never fill, and its influence in this respect has been known to extend as far as 300 or 400 yards across a field" (quoted from Ramsbottom 1913:81). However, Banks' claim that "It has long been admitted by farmers, though scarcely credited by botanists, that wheat in the neighbourhood of a barberry bush seldom escapes the Blight" (quoted from Ramsbottom 1913:85) seems to indicate he had not read Withering's work. Banks' pamphlet had two interesting enlarged color illustrations by Francis Bauer (1758–1840) of the rust imbedded in the wheat tissue (one reproduced in Ordish 1976: plate 3; see also Ainsworth 1969:15). Examining the barberry bush attentively, I found upon its fruit a species of fungus similar in colour to that on the straws of the wheat, but its seed vessels were larger, and more spherical. I was, however, much disposed to believe the parasitical plants of the same species, and that the difference in the form and size of the seed vessels arose only from the difference of the nutriment they derived from the wheat and from the acrid juice of the barberry. The distant (control) wheat was healthy until Knight took a diseased barberry branch to that wheat, where he sprinkled the branch with water and brushed it over the healthy wheat, which 10 days later became diseased. Simultaneously, he took some diseased wheat to healthy wheat, moistened the diseased wheat and brushed it over the healthy wheat, but the healthy wheat remained healthy. Ramsbottom (1913:86) suggested that Knight was "the first to try inoculation experiments on heteroecism." Knight did not write a letter to Banks about this experiment until after he had read Bank's pamphlet, on 20 March 1806. In 1806, Banks published a second edition of his pamphlet and appended Knight's letter (Banks 1806:26–36, Ramsbottom 1913:85, Dawson 1958:501). In Philadelphia, a retired physician and immigrant from Bath, England, Anthony Fothergill (1732?–1813), read a paper on 11 November 1806 to the Philadelphia Society for Promoting Agriculture, arguing that grain rust was caused by "clusters of a fungus or parasitical plant…[that] insinuate themselves into the pores of the absorbent vessels of the stem, and deprive the grain of the sap destined for its nourishment" (1808, quoted from Campbell et al.1999:33). In 1818, a Danish schoolteacher, Schoeler, carried out the Knight experiment with the same results (Parris 1968:24). Knight recommended in 1818 the use of flowers of sulfur against pear scab (Lycoperdon cancellatum), and in 1834 he recommended the sprinkling in early spring of peaches with sulfur and lime to control leaf curl. John Roberts recommended in 1821 using soap to cause wetting of sulfur in water to control peach mildew, and in 1824 he reported his successful experimental transmission of peach mildew (Knight 1818, 1842, Roberts 1824, Parris 1968:25–26, Ainsworth 1976:160, 1981:32–33, 110). Phytopathologist George L. McNew divided the history of his profession into five periods, with periods three and four occurring during the 1800s (McNew 1963:166–172). Period three he labeled "The Predisposition Period," during which the prevailing view was that fungi originated in diseased plant tissue. Three authors published treatises on plant pathology and remedies in 1807. Freiherr von Werneck and Filippo Ré represented the consensus in not accepting fungi as the cause of the diseases (Whetzel 1918:32, Parris 1968:23, Walker 1969:20–22). Isaac-Bénédict Prévost (1755–1819) did think fungi caused diseases (Keitt 1939a, Large 1940:76–79, Robinson 1975, Ainsworth 1981:30–32). He was the son of a teacher-pastor in Geneva, but at age 22 he became a tutor in Montauban, France, where he remained for the rest of his life. His interests included mathematics, physics, chemistry, biology, and philosophy. In 1797 a member of the Society of Montauban read a memoir on the carie or charbon (bunt or smut) disease of wheat, and the society asked other members to study the problem. Prévost read previous studies on the problem, including those by Duhamel du Monceau (1728) and Mathieu du Tillet (1755), discussed in part 29 (Egerton 2008), and especially Henri Alexandre Tessier's Traité des maladies des grains (Paris, 1783), which attributed rust to stoppage of transpiration, caused by mists. Tessier repeated some of Tillet's experiments and agreed that bunt was contagious (Parris 1968:19). Tessier also stressed the importance of soaking wheat seeds in lime water before planting, which greatly diminished the loss of wheat from bunt in France (Large 1940:76). The owner of a nearby estate made available to Prévost land for large-scale experiments. George Keitt (1939b) provided an excellent guide to and evaluation of Prévost's achievement along with his English translation of the Mémoire. Prévost could not discover everything he wished, but he was remarkably thorough and clear in the presentation of his findings. Tillet had demonstrated in 1755 that bunt (carie, charbon) of wheat was spread by an infectious agent (Egerton 2008:237), but had not demonstrated that the agent was a fungus. Prévost was able to induce the bunt dust to germinate in water and illustrated its growth, thus demonstrating the dust was fungal spores. He illustrated its progressive development on the first of his three plates. To those believing a fungus arose within the wheat plant, he replied that he only observed branches from the spore growing from the surface into the seedling (1939:34–35, paragraphs 44–46). Fruiting bodies grew within ears of wheat and spores were disseminated by the wind. Prévost scattered spores on soil where wheat seed was planted and concluded that it was only at germination, or soon after, that infection occurred (Prévost 1939:38, paragraph 57). He discovered that copper dust suspended in water, or liquid copper compounds, inhibited spores from germinating (Prévost 1939:72–82, paragraphs 138–177, Ainsworth 1976:160–161, 1981:124–126), and arsenic was reported in England to have the same effect (Prévost 1939:83, paragraph 178). The Society of Montauban sent a published copy of Prévost's Mémoire to the Institute [de France?] in Paris for evaluation. Tessier reported for a review committee favorably, but no actions were taken. Mendel was by no means the only author of a clear publication that failed to gain the recognition deserved upon publication. Lavoisier, Lyell, and Darwin achieved their scientific revolutions in part because of prominence in their scientific communities. Prévost and Mendel were amateurs on the periphery of their scientific communities, who were neglected, rather than being either challenged or widely recognized. Nevertheless, Prévost gave the first experimental proof of the pathogenicity of a microorganism, and he first described motile spores (zoospores) in fungi (Ainsworth 1976:62, 147). In Italy, Agostino Maria Bassi (1773–1856), like Prévost, was on the periphery of science, but his important parasitical work did attract the attention it deserved, because he promoted it (Belloni 1961:20–26, Robinson 1970, Ainsworth 1976:163–168). He grew up near Pavia and attended its university. He studied law there to please his parents, but his interests were in science, and he studied under Antonio Scarpa, anatomist, Alessandro Volta, physicist, and Giovanni Rasori (1766–1836), pathologist. Rasori (portrait in Belloni 1961:22) wrote a book defending the contagium vivum theory that appeared posthumously in 1837. Both Rasori and Bassi were influenced by the writings of Lazzaro Spallanzani (1729–1799), who discredited the theory of spontaneous generation and accepted the conclusions of Fontana and Targioni-Tozzetti that small parasitic plants can cause diseases in vascular plants (Egerton 2008:235, 239). The silk industry could flourish in northern Italy and in France, if it could eliminate an epidemic disease, muscardine (calcinaccio). In 1807 Bassi decided to investigate; he grew silkworms, assuming the disease (Bassi 1958:4) …arose spontaneously in the silk worm and was due to some difference in the atmosphere, the food, or the method of breeding, or rather to the various fumes emanating from the fermenting litter… He varied environmental conditions for different groups but failed to induce the disease in his caterpillars. When he obtained diseased caterpillars, he found the disease spread at temperatures of 7°–30°R, but not above 38°R (Bassi 1958:8). The disease could be transmitted to caterpillars of other species. It was contagious, because it always began with one or a few caterpillars, whereas an environmental or physiological disease could affect all individuals about the same time. Bodies of dead caterpillars became covered with "a patina or efflorescence like flakes of pure snow" (Bassi 1958:7). In 1835, Bassi announced that muscardine was caused by a fungus, and he added a footnote (Bassi 1958:10, note on 15) The eminent and meritorious compilers of the celebrated Giornale Fisico-Chimico, Professors Configliachi and Brugnatelli, were the first to put forward the hypothesis that the mark disease is produced in the silk worm by the development of a species of fungus, basing it on the fungus-like smell that comes from worms which have died of that disease; and although, in that journal, they appealed to breeders to undertake experiments to test this supposition, these last, perhaps regarding it as the figment of a heated imagination, neglected it, whereas, had they taken the trouble to examine it, they might easily have found, by experiment, that these able men were not mistaken and had really hit on the truth. Although Bassi blamed the silkworm growers for their indifference to the professors' suggestion, one may wonder how many growers ever read this physico-chemical journal or even knew how to undertake an experiment. Neither Bassi nor his English editors gave a more precise reference to the professor's comments than is in this footnote. Nor does Bassi indicate whether his investigation was undertaken in response to their comments, but if he had made his discovery without having read their suggestion, he perhaps would have said so. In another footnote, he stated (Bassi 1958:17) If this parasitic plant is observed with the great microscope of the illustrious De Amici, which magnifies the object more than thirty million times (sic.), it will be possible to see all its minutest ramifications in it, and perhaps its reproductory organs as well. He explained that the "disease-bearing dust" was spread by air, water, and dogs, cats, rats, mice, flies, and contaminated food for silkworms (Bassi 1958:23). The English translation of Bassi's treatise is limited to part 1, on theory (1835). In 1836 Bassi published part 2, on ways to combat the disease. These included disinfecting or burning containers and materials used in raising them (Ainsworth and Yarrow 1958:x–xi). Bassi realized that his discoveries would be controversial, and he requested permission to perform his experiments for faculty members at the University of Pavia. They agreed, and afterwards provided a testimonial signed by nine professors, including Pietro Configliachi and Brugnatelli, mentioned in the footnote quoted above. Bassi then published their testimonial in his preface to part 1 (Bassi 1958:1–2). Giuseppe Balsamo-Crivelli named the fungus Bassi discovered Botrytis Bassiana (now Beauveria bassiana), and an abridged French translation of Bassi's part 1 was published in Paris in 1836. Both botanist Jean-Francois-Camille Montagne (1784–1866) and entomologist Jean-Victor Audouin (1797–1841) investigated Bassi's claims for the Académie des Sciences in Paris and published confirmations, with a plate of illustrations of the fungus (Audouin 1836, Montagne 1836, Belloni 1961:25–28; Audouin's illustration is reproduced by Ainsworth 1976:167). This was the first known fungal parasite of an animal; Prévost had already demonstrated a fungal parasite of plants. In the early 1840s, two Jewish physicians published evidence of fungal disease in humans. David Gruby (1810–1898) was a Hungarian who obtained his doctorate in Vienna in 1840 and immigrated to Paris, where he practiced and published (Kisch 1954:193–226, Ainsworth 1976:169–171). Although the claim that he was one of the most brilliant biologists of the 1800s (Zakon and Benedek 1944:155) seems an exaggeration, his discoveries were important for both medicine and phytopathology. His six important scientific papers (1841–1844), all rather brief, are translated from French into English (Zakon and Benedek 1944:157–168). In 1841 he published his microscopic observations showing that favus was caused by a fungus; in 1842 he indicated that ringworm of the beard was caused by an ectothrix trichophytosis. He named neither of these fungi, but in 1843 he identified the cause of human microsporosis and named the fungus Microsporum audouinii to honor Victor Audouin. Even though his discoveries were easily repeated by any competent microscopist, the Paris medical community remained skeptical of his conclusions (Zakon and Benedek 1944:156). He also discovered several microscopic invertebrate parasites during the early 1840s (Foster 1965:115–116), before devoting the rest of his life to a very successful medical practice. Robert Remak (1815–1865) was Polish and immigrated to Berlin at age 18. In 1837 he discovered the fungal nature of favic crusts but did not publish this until 1845. He was then a physician in a clinic run by Professor J. L. Schönlein, and Remak named the fungus Achorion schoenleinii, honoring Schönlein (Kisch 1954:227–296, Ainsworth 1976:168–169). Following in Gilbert White's footsteps as an English clergyman–naturalist, Miles Joseph Berkeley (1803–1889) had a childhood interest in nature that led eventually to his becoming the foremost British mycologist of the 1800s (Massee 1913, Whetzel 1918:55–57, Ramsbottom 1948, Ainsworth 1969:14, Taylor 1970, Stafleu and Cowan 1976–1988, I:192–195, Desmond 1977:60, Buczacki 1991, Elliott 2004a). He attended Cambridge University, 1821–1825, but left two years before John Stevens Henslow became its professor of botany. As an undergraduate, he collected algae, mosses, and mollusks. Berkeley's earliest published papers were on mollusks, and he only began to specialize on fungi in 1832, when William Dalton Hooker invited him to write the fungal volume of The English Flora (Berkeley 1836). Berkeley's introduction to plant pathology occurred in 1845, when the potato murrain, that had been discovered in Liège, Belgium in 1842, and occurred in the eastern United States in 1843 (Stevens 1933, Campbell et al.1999:38–39) and in Western Europe in 1844, then spread to Britain and Ireland (Berkeley 1948:14–17, Bourke 1964:14–17, Bourke 1969). When its seriousness in Ireland became evident, British Prime Minister Sir Robert Peel formed a scientific commission to study the matter (Large 1940:26–27, Woodham-Smith 1962:44–47), under London Botany Professor John Lindley (1799–1865; Stearn 1999) and London Chemistry Professor Lyon Playfair (1818–1898), who had received his Ph.D. under Liebig (Large 1940:26–27, Woodham-Smith 1962:44–47). The commission went to Ireland but did not find a clear cause of the disease. (a) Miles Joseph Berkeley. Ainsworth 1976:155. (b) Anton de Bary. Jost 1930:facing 1. Montagne (mentioned above) found a fungus associated with blighted potatoes, which he named Botrytis infestans (now Phytophthora infestans) on 30 August 1845 (portrait in Virville 1954:198; Ainsworth 1976:154–155, 1981:54–55). He was uncertain about whether the fungus caused the murrain, but sent his comments and drawings to his regular correspondent, Berkeley (Lamy 1989), who, at first (1845) shared Montagne's uncertainty (Walker 1969:23). However, Dr. C. F. A. Morren, head of a school of agriculture in Liège, believed the fungus caused the murrain (1845), and he conducted crude inoculation experiments to support the claim (Walker 1969:23). Berkeley agreed with Morren before publishing his own study on the disease (1846). Berkeley also published Montagne's drawings (one of which is in Fig. 7). Berkeley presented the case for the disease having an environmental cause and cited the authorities who supported this theory (1948:23–28), and then he argued the case for "The decay is the consequence of the presence of the mould, and not the mould of the decay" (Berkeley 1948:28). He ended his article (Berkeley 1948:35–37) with a Latin and English description of Botrytis infestans Mont. and related species (many of which, like infestans, are no longer placed in that genus). Large (1940:32) comments that Berkeley's article swept away many but not all objections. Berkeley did not observe spore germination, nor did he document the actual infection of the potato plants. (a) Miles Joseph Berkeley. Ainsworth 1976:155. (b) Anton de Bary. Jost 1930:facing 1. Berkeley became both mycologist and plant pathologist, and he entered the fray in 1847 over the cause of a new grapevine disease, discovered by a gardener, Tucker, in 1845 near Margate, England. Tucker reported that he controlled this powdery mildew with a mixture of sulfur and lime in cold water. Berkeley studied the mildew, named it Oidium Tuckeri, and illustrated it with hyphae growing within the leaves, as he had drawn B. infestans. The French physician–mycologist Joseph-Henri Lévillé (1796–1870, portrait in Virille 1954:218) did not accept the claim that O. Tuckeri caused the downy mildew, and he found no hyphae within the leaf tissue, only on the leaf surface (Large 1940:44–45). Berkeley translated Lévillé's discussion and added his own response (Lévillé and Berkeley 1851). Lévillé was right about O. Tuckeri not having any detectable hypha within the leaf tissue, but wrong in thinking that morbid tissue gave rise to the fungus. In 1851, Dr. Zanardine, in Venice, argued that the surface fungus, O. Tuckeri, was indeed parasitic, because it had little suckers he called "fulcra" (now "haustoria") that obtained sustenance from leaves. Skeptics could still argue that the fungus may not have arisen if the leaf was not already moribund, but the consensus was shifting toward Berkeley (Large 1940:44–49). Berkeley published 173 articles on "Vegetable Pathology" in the Gardner's Chronicle, 1854–1857, some of which are reprinted (Berkeley 1948:41–108). He also published a steady stream of articles on fungi: 35 articles on 2050 British fungi, 1837–1885; 7 of these articles are only his and 28 were coauthored by C. E. Broome; all are now reprinted (Berkeley and Broome 1967). In addition, he was one of three European mycologists who described fungi brought back from elsewhere in the world, the other two being Montagne in France and Elias Magnus Fries (1794–1878) at Uppsala University, Sweden, whose Systema mycologicum (three volumes, 1821–1832) is recognized as one of the two foundations of modern fungal nomenclature, along with Persoon's Synopsis (Fries 1950:61–63 + portrait on frontispiece, Eriksson 1972, Ainsworth 1976:273). Berkeley published his descriptions of foreign fungi in batches of 10 descriptions per article (or at least 10 in the first article, and various numbers after that, but all called "decades" nevertheless), for 27 articles, 1844–1856, three of which were coauthored, and all of which are now reprinted (Berkeley 1969). Berkeley was a careful observer but was not an experimentalist. (Heinrich) Anton de Bary (1831–1888) was the leading physiological mycologist and a leading phytopathologist of the 1800s and was an outstanding experimentalist (Whetzel 1918:45–47, Jost 1930, Robinson 1971, Sparrow 1978). He was also one of the most respected and revered professors in that century. He was the son of a physician in Frankfurt am Main who raised fruit trees and flowers, and as a teenager Anton joined a local group of amateur naturalists on field trips into the nearby countryside. A physician–botanist at the Senckenberg Medical Institute, Professor Georg Fresenius, introduced him to the study of algae and fungi. After graduating from the Frankfurt Gymnasium (high school) in 1848, de Bary studied medicine at Heidelberg, Marburg, and Berlin, receiving his M.D. degree in 1853. He had published his first scientific paper in 1852, on the water mould Achlya prolifera, from a peat bog near Berlin (illustration in Sparrow 1978:234). In 1853 he published his M.D. dissertation on the sexuality of plants and a book on fungal rusts and smuts. Berkeley had published strong evidence that the potato murrain was caused by a fungus, but without convincing all skeptics. De Bary's Untersuchungen über die Brandpilze und die durch sie verursachten Krankheiten der Pflanzen 1853 undermined the remaining doubts that fungi do cause diseases. McNew's fourth period of phytopathology is "The Etiological Period," which began with de Bary's Untersuchungen (McNew 1963:167–168). De Bary's "phytopathological classic" is now translated into English (De Bary 1969), and one can see why it ended doubts on disease causation. It is divided into three parts: I. Specific observations concerning the form and development of the brand fungi; II. Systematic conclusion; III. Concerning the relationship of the brand fungi to the brand and rust diseases of plants. By describing various life cycles (I), he provided the basis for a new classification (II), which enabled botanists to discuss specific fungi in relation to specific vascular plant hosts. Paradoxically, botanists who believed the fungi were products of disease rather than the cause—he provided extensive citations in his notes (De Bary 1969:64–67)—still thought of the fungi as distinct species. In contrast, he listed only 10 botanists who were convinced that fungi were the cause of disease (De Bary 1969:68). De Bary pointed out that one can observe the germination of spores and show their capacity to infect, the hereditary nature of the brand fungi, and their relation to stomata of plants they infect. He also provided eight plates showing fungi in relation to their hosts. Any remaining skeptics would have to show how the leading authority on these fungi was wrong. None tried it. Fertilization and overwintering stage of Cystopus candidus Lev. fungus causing white rust of cabbage and mustard. De Bary 1863, from Large 1940:102. He returned to Frankfurt to