Mayi5, I9IS Seedling Diseases of Sugar Beets i6i 



This probably occurred while the leaves were still within the seed coat. 

 The fungus w^as found to be capable of attacking the beet after it was 

 5 or 6 weeks old. Peters's statement (33, p. 228) that it is able to infect 

 the side roots during the entire vegetative period is probably correct. 

 When the taproot is once attacked by P. debaryanum, the ultimate 

 death of the plant seems to be assured. Fortunately the soil relations 

 in early seeding time are usually not sufficiently favorable to the rapid 

 development of the fungus to make it an aggressive parasite under 

 average field conditions. This fungus does not develop well in cold 

 soil, but does its most serious work under seed-bed and greenhouse 

 conditions or in the fields which have been seeded very late when the 

 soil temperatures have begun to rise. 



UNDESCRIBED SPECIES INJURIOUS TO SUGAR BEETS 



In the author's preliminary note (12) it was reported that Aphanomyces 

 laevis De By. had been found as a damping-off fungus of sugar beets in 

 America, but subsequent detailed morphological studies of the fungus 

 as it developed in artificial culture and on beet seedlings have shown 

 that it differs in some important respects from the published descriptions 

 of De Bary and others. A . laevis was first reported in a parasitic relation 

 by Peters (31) in 1906. He found the fungus as a damping-oflf parasite of 

 considerable importance upon sugar beets in Germany. Barrett (4) has 

 reported its occurrence in America as the cause of a disease of radishes. 

 The first cultures of the fungus temporarily mistaken for A. laevis 

 were secured from damped-off beet seedlings grown in soil which had pre- 

 viously produced the black-root disease of the radish, like those shown 

 in Plate XXIV, figure 2. It was later obtained from soils at Madison, 

 and from Kenosha, Wis., as well as from seedlings damping-ofif in soil 

 which had been infected with fragments of a diseased radish obtained 

 from Illinois. The causal relation of the organism to the radish disease 

 as well as to damping-off of sugar-beet seedhngs was confirmed repeatedly 

 by inoculation experiments, and it was at first thought possible that the 

 discrepancies between this fungus and published descriptions of A . laevis 

 might be the result of response to the changed environmental conditions 

 of culture or to variations within the species, since it is well known from 

 the study of many investigators that the Saprolegniaceae are exceedingly 

 variable. Through the courtesy of the Kaiserliche Biologische Anstalt 

 at Dahlem, Germany, and Dr. Leo Peters, of that institution, the author 

 was permitted to isolate Aphanomyces from the experimental fields of 

 the Anstalt. An organism was secured from damped-off seedlings, which 

 Dr. Peters identified as the organism with which he had worked and which 

 conformed in every respect to De Bary's description of .4. laevis. It was 

 secured in pure culture, and its pathogenicity to beet seedlings was con- 

 firmed by inoculation experiments. Unfortunately the culture was lost 



