144 Journal of Agricultural Research voi. xvi.no. s 



CAUSAL ORGANISM 



There still remains some doubt as to the correctness of the classification 

 of the causal organism. According to the description of Cladochytrium 

 and Physoderma as given by the leading mycologists, the organism 

 evidently belongs in one of these genera. The essential difference between 

 the two genera lies in their method of reproduction. The genus Clado- 

 chytrium may have both thick-walled sporangia, or so-called resting 

 spores, and thin-walled sporangia, or presporangia, while the genus 

 Physoderma is characterized by having only thick-walled sporangia 

 (resting spores). As the species on corn is not know^n to produce the 

 thin-walled sporangia, its thick -walled sporangia definitely place it in 

 the genus Physoderma. Shaw's (<?) description of the species from India 

 was based almost entirely on the resting spores (sporangia). Barrett* 

 found the disease in the State of Illinois in 191 1 and in 191 2 declared the 

 organism identical with the species described by Shaw. Measurements 

 of the sporangia of the fungus in America are practically identical with 

 those given by Shaw {8). Measurements given by Shaw are 18 to 24 

 by 20 to 27 IX, while the waiter finds the sporangia of the organism in this 

 country to measure 18 to 24 by 20 to 30 ju. Therefore, so far as size of 

 sporangia are concerned, the fungus in America is apparently identical 

 with that described by Shaw from India as Physodertna zeae-maydis. 



DESCRIPTION OF THE ORGANISM 



The sporangia of the fungus are smooth, brown, thick -walled, 18 to 

 24 by 20 to 30 IX, slightly flattened on one side where the outline of a 

 definite cap or lid can be seen by careful observation. On germination 

 this trapdoor lid opens, or is carried up by the top of the thin-walled 

 endosporangium, which finally ruptures at the apex and liberates a 

 number of uniciliate zoospores. These zoospores are 3 to 4 by 5 to 7 /x, 

 with a cilium three to four times the length of the spore itself. The 

 zoospores have a comparatively large central oil droplet or nucleus. 

 After their active stage these motile spores come to rest, in most cases 

 lose their cilia, spread slightly in an ameboid fashion, and germinate by 

 putting out fine fibrous hyphae. The mycelium is composed of very 

 fine fibers, about i ix thick, which connect the large vegetative cells which 

 Clinton {3) and Von Minden (7, p. 397-410) term "Sammelzellen." These 

 enlarged cells, which may occur singly or in groups of two or more, 

 produce sporangia directly or send out short fibers which produce terminal 

 sporangia. The fungus is apparently an obligate parasite, and the mycel- 

 ial stage is seen only within the tissues of the host plant. After the 

 sporangia are mature no traces of the mycelium can be seen (Pi. 17, B). 



' Barrett, J. T. op. cit. 



