With the ultimate goal of producing resistant varieties to include in the RNS catalog, thorough tissue analyses were made of extensive breeding materials to determine the sensitivity of different varieties in function of the number of cell layers penetrated by the fungus. The various stages of the defense necrosis were distinguished, the last two being characterized by cell collapse and by heavy infiltration of the fungus.[101] Above, I insisted on the importance of the streamlining of the inoculation procedure in order to have a fast and reliable method of establishing the degree of resistance of hundreds of thousands of seedlings. With the growing number of different fungus strains identified (eight of them by 1936), the case for parallel investigations on the biology and cultivation of the fungus was even stronger. Isolation of P. infestans biotypes and safe separation through cultures on artificial substrates were required so that conclusions about the relation between host tissue and fungus growth could be drawn from the results of controlled experiments. Breeding resistant potatoes demanded not only the transformation of the tuber into a suitable laboratory object but also the careful standardization of its parasites—in this case, the P. infestans fungus.
In 1935, in the same paper in which he conceded that W-varieties constituted not a definite answer to late blight, Müller was already suggesting that resistant plants could act as “inductors” of new biotypes of P. Infestans, with the virulence of the latter influenced by passages through host plants of different grades of resistance.[102] In fact, as Müller would later demonstrate, the necrosis depended less on the absolute reaction rate and more on the relation between the hyphal growth rate and the rapidity of the reaction.[103] Because not only P. infestans but also other microorganisms are unable to exist in the necrotic host tissue, Müller postulated that this localized cell death must be accompanied by the production or activation of a substance he called phytoalexin (‘alexein’ meaning “defense” in Greek). In subsequent years, biochemists would explore this fungicide in depth in order to replace Müller’s functional definition with a structural one.[104] According to Turner, until the 1970s the Phytophthora infestans associated with Solanum tuberosum was the major model organism of plant pathology for studying the general response mechanism of plants against pathogens.
Viruses
The different combinations of potato varieties, pathogen strains, and inoculation methods constituted generative experimental systems that led to new epistemic objects and the possibility of incorporating new techniques—to use Hans-Jörg Rheinberger’s phraseology.[105] Not only did potatoes have an obvious economic and social relevance in the German context; they also were research objects able to sustain the growing of a vast community of scientists and instruments at the Biologische Reichsanstalt für Land- und Forstwirtschaft. Each research team at the BRA developed a particular experimental system. The team working on plant breeding and genetics (Dienstelle für Pflanzenzüchtung und Vererbungslehre), headed by Karl Otto Müller, concentrated on the W-varieties and Phytophthora; the team working on agriculture botany (Landwitschaftliche Botanik) concentrated on wart disease.[106] The interest in Solanum demissum that Müller had developed in the course of his work on Phytophthora resistance led to the use of that wild species in the Colorado Beetle work, and the methods used in wart research led to the classification standards developed in the varietal registry section. In other words, the different experimental systems that structured the organizational chart of the BRA were built on resources previously developed by other sections of the BRA.
The virus group (called the physiologic botany section of the botany department in 1934, and renamed the plant virus pathology section in 1943) was led by Erich Köhler, who had begun his career at the BRA in 1921 doing research on wart disease and then had turned to plant virology in 1932.[107] Köhler was celebrated for having introducing into Germany the notion—already in vogue in the Netherlands and the United States—that viruses were the pathogenic agents responsible for potato degeneration (Kartoffelabbaus), in contrast to ecological theories arguing that potato leaf roll was caused by environmental factors such as water imbalance.[108] In fact, the cause of potato degeneration was, from very early on, a major issue at the BRA, Otto Appel himself having published a leaflet on leaf roll differentiating it from the crinkle of the potato and making no mention of viruses. The concern about degeneration is not surprising. The yields of contaminated fields, according to the BRA numbers, were reduced by between 25 and 50 percent in the first year, and by more than 60 percent in subsequent years.[109]
In accordance with its usual approach, the BRA proposed a method to identify infected seeds so as to guarantee the distribution among German farmers of healthy material from commercial breeders. More important than maintaining old varieties in a virus-free state was to ensure that new seedlings that were hoped to be resistant to late blight would not be exposed to viruses in their early stages of multiplication. For this Köhler proposed the Stecklingsprobe (a variation of the Tuber Index Method developed by American scientists at Cornell University), which entailed removing and growing a seed piece from each individually numbered tuber selected from a stock.[110] The plants grown from these seeds were then observed for the presence of viruses. When the presence of a virus was confirmed, the diseased tuber was discarded. Only tubers corresponding to healthy plants were reproduced. Two central elements of the method are now familiar: careful recording of each tuber and the respective plant and allocation of greenhouse space for the quick growth of potato plants shortly after the harvesting of potatoes.
The main challenge presented by the method was detecting the presence of viruses quickly. If in some cases viruses manifested themselves through obvious leaf symptoms in an infected potato plant, latent viruses were made visible by rubbing tobacco plants with sap from the tuber seedlings.[111] From 1932 on, Köhler tinkered at the BRA with different combinations of viruses, potato varieties, and test plants, aiming to come up with an effective way of identifying viruses so as to guarantee the health of the certified stock seeds to be used by peasants.[112] Different test plants were tried for different types of viruses.[113] In addition to tobacco and pepper plants, the wild variety Solanum demissum was also found to be an effective test plant for the main types of viruses.[114]
101
Karl Otto Müller, G. Meyer, and M. Klinkowski, “Physiologisch-genetische Untersuchungen über die Resistenz der Kartoffel gegenüber
105
Hans-Jörg Rheinberger,
106
This is, of course, a simplified version of the BRA’s organizational chart. The Plant Breeding and Genetics section of the Botany Department, for example, was not concerned only with potatoes; it also put a lot of effort into studies of plant fodders. But a reading of annual reports supports makes obvious the overwhelming weight of potato-related work. Each section is explicitly identified with a particular experimental system by the BRA’s director on pp. 16–17 of
107
W. Laux, “Contributions to the development of phytomedicine in Berlin,” in
108
E. Riehm, introduction to “Virustagung der Biologischen Reichsanstalt für Land- und Forstwirtschaft am 23 januar 1943,”
110
E. Köhler, “Der Nachweis von Virusinfektionen am Kartoffelpflanzgut mit der Stecklingsprobe,”
111
E. Köhler, “Der Virusnachweis an Kartoffeln. Eine Anleitung für Züchter und Kartoffelbegutachter,”
112
See the reports of Köhler’s activities from 1932 to 1942 in Bundesarchiv, R/3602/2395 Tätigkeitsberichte, Naturwissenschaftliche Abteilung II, Dienstelle IIIb.
114
H. Semsroth, “