The most distinguishing feature of the breeding work at the BRA was the importance given to the procedure for exposing plants and tubers to a pathological agent. To develop standardized laboratory methods of infection was a mandatory first step toward the breeding of resistant strains.[84] These methods sought to guarantee not only that selections would be made properly and that plants would be exposed to pathogens, but also that the entire procedure would be streamlined in order to screen a large number of specimens.
The first step undertaken by Müller to explore the resistance of the F-varieties was to infect young shoots of potatoes with fresh-hatched zoospores of the fungus responsible for late blight (Phytophthora infestans) under optimal conditions for the development of the latter. Next he increased efficiency by using seedlings instead of shoots, placing them in pad-stitched boxes. After the seedlings developed their first three or four leaves, he applied a few droplets of a solution containing the Phytophthora zoospores. Susceptible seedlings would die, while immune ones would survive. In a small laboratory with modest resources, Müller was thus able to screen thousands of seedlings in a relatively short period of time.[85]
From 1925 on, the method was intensively used by Müller in his crossings of F-varieties with commercial cultivars. It enabled him to prove that resistance to late blight was inherited independently of crucial economic properties such as yielding and time to maturity.[86] He named the hybrids thus obtained the W-varieties, which he soon was publicizing among German commercial breeders and among fellow public breeders abroad.[87] The W-varieties promised to end one of the chief afflictions of European farmers, responsible not only for the mid-nineteenth-century Irish famine but also in large part for the disastrous food shortages in Germany during World War I.
The memory of the “turnip winter” of 1917 was repeatedly invoked to assert the relevance of the BRA phytopathology work to the Nazi Battle of Production.[88] By the 1930s the losses due to late blight were estimated at a third to a half of the total potato crop.[89] The combination by Müller of standardized inoculation methods with the employment of wild varieties from South America holding the desired resistance genes confirmed the capacity of modern plant breeding to overcome the major challenges faced by European agriculture. Allegedly, it was just a question of time for commercial breeders to start releasing resistant varieties by crossing Müller’s W-varieties with high-yielding European cultivars. In 1934 the Sandnudel made its appearance in the RNS Reichssortenlist as the first commercial variety resistant to Phytophthora infestans, the first enemy of the potato plant.[90] Thanks to the technoscientific organisms coming out of the BRA, Darré’s promises that the national soil would feed the national community didn’t seem crazy reveries. Germans could remain “children of the potato.”
But scientific triumphs are never easy. By 1932 the von Kameke seed company of Streckenthin (Brandenburg) had reported that the W-varieties it was working with had undergone a severe attack of late blight.[91] According to R. Steven Turner, this event marked the beginning of the fall of the heroic age of late blight resistance breeding. Rudolf Schick of the Kaiser Wilhelm Institut für Züchtungsforschung, was quick in criticizing Müller for not having anticipated such possibility. Schick was referring in concrete to the existence of different biotypes of the fungus, which explained the vulnerability of the W-varieties. Contrary to initial hopes, Phytophthora infestans was able to develop new biotypes, a problem plant breeders were very well aware of from their previous experience with the black stem rust of cereals. By 1936, scientists at the Kaiser Wilhelm Institut für Züchtungsforschung had identified eight different biotypes of P. infestans.[92]
By 1935, Müller was already acknowledging in the pages of Der Züchter (The Breeder), the main journal of German academic breeders, that “the W-varieties do not constitute a permanent solution” to late blight.[93] Nevertheless, Müller was confident enough to conclude that same article with a grandiloquent statement boasting about his knowledge of the enemy and “the methods with which to wrestle him to the ground.” And apparently the work with the W-varieties had earned Müller enough of a reputation to convince the leadership of the BRA to establish, in 1936, a new branch in Eichhof (Pomerania) dependent of his own Genetics and Breeding Section of the Botany Department. In September 1935, in his plea to the BRA’s director that 110,000 Reichsmarks be spent at the new branch for the acquisition of experimental fields and the building of greenhouses and laboratories, Müller explicitly linked the identification of new strains of Phytophthora infestans with increasing demands on the breeding work.[94] Different pathogen strains implied the multiplication of the inoculation work, which justified the testing of about 12,000 potatoes to begin with. The additional difficulties didn’t sway him from suggesting to the BRA’s director that sooner or later the best W-varieties produced by the BRA’s new department would make their way to the German fields.[95] Confirming Müller’s promises, in 1940 three new Phytophthora-resistant commercial varieties—named Erika, Robusta, and Frühnudel—were added to the Reichssortenlist, followed by Aquila in 1942.[96] The fungus was not to play the same decisive role in World War II that it had played in the previous conflict.[97]
What may have constituted a serious blow to Müller’s research was thus turned into a justification for more resources: more strains of the fungus demanded more testing of breeding lines, leading to new greenhouses, laboratories, and experimental plots. More than that, it led Müller and his team at the BRA to delve deeper into the mechanisms of resistance reactions in potatoes.[98] What hindered the development of Phytophtora infection was first explored by Müller in 1931, when he scrutinized potato specimens through cytological-physiological methods, reaching the conclusion that infected potatoes released some kind of toxic substance to the fungus.[99] Having previously established the existence of a dominant Mendelian factor (the R-gene) responsible for the resistance of the W-varieties, Müller was now interested in its mode of action. Close observation of infected materials of different varieties suggested that the alleles present in the W-varieties induced a predisposition for the host tissues to “acquire” a local immunity from infection when in contact with the hyphae of P. infestans. Comparison of susceptible and resistant potato varieties revealed that in both varieties host cells were destroyed after contacting the protoplasm of the parasite. If morphological and physiological analyses revealed the same final effect, the difference in degree of resistance depended on the rapidity of the reaction. Further penetration of the hyphae was made impossible by the rapid reaction of the host tissue. In other words, Müller established a relation between the degree of resistance and the velocity of the “defense necrosis” reaction of the host tissue. The R-gene controlled the speed of the reaction without changing its basic nature. Much of the work of Müller’s team at the BRA during the Nazi years dealt directly with the exploration of such mechanism.[100]
84
This view was clearly expressed by the leader of the institution till 1933. See Otto Appel, “Pflanzenpathologie und Pflanzenzüchtung,”
86
K. O. Müller and Lothar Behr, “Mechanism of
87
In 1927 Müller sent a few specimens to Washington, where they formed the basis of the National Potato Breeding Program of the USDA created in 1929 (Turner, “After the famine,” p. 353).
90
K. O. Müller, “Über die Herkunft der W-Sorten, ihre Entwicklungsgeschichte und ihre bisherige Nutzung in der praktischen Kartoffelzüchtung,”
91
K. O. Müller, “Über den augenblicklichen Stand unserer Kenntnisse zur biologischen Spezialisierung des Krautfäuleereegers der Kartoffel (
92
William Rudorf, Paul Schaper, Hans Ross, Marie-Louise Baerecke, and Margarete Torka, “The breeding of resistant varieties of potatoes,”
94
K. O. Müller to the Director of the BRA, “Erwerbung einer reichseigenen Außenstelle zur Durchführung züchtungswissenschaftlicher Untersuchungen bei der Kartoffel“ (14/9/1935), Bundesarchiv, R/3602, p. 3.
96
K. O. Müller, “Die Erfolge der Züchtung phytophthoraresistenter Kartoffelsorten,”
98
Müller’s section also included G. Meyer, M. Klinkowski, and H. Börger. The Kaiser Wilhelm Institute for Plant Breeding, besides determining different biotypes of
99
K. O. Müller, “Über die Entwicklung von
100
E. Riehm, “Biologische Reichsanstalt für Land- und Forstwirtschaft: Wissenschaftlicher Jahresbericht 1939,”