22 James G. Horsfall 



Krakover (1917:284) by examining the "infection drop" directly 

 under the microscope found that penetration occurs directly through 

 the cuticle after about fourteen hours. Although entrance through 

 stomata occurs, it is uncommon. 



At first it was thought that the concentric zones might be cicatrices 

 laid down by the suscept in successive attempts to halt the invasion of 

 the pathogene, but sections through the lesions show, however, neither 

 cicatrice (Cunningham, 1928) nor thickenings of the cell" walls which 

 might hinder the advance of the hyphae into adjacent living tissues. In 

 fact, no good explanation of the concentric zones can be offered at this 

 time. The cells within the holonecrotic area are collapsed and browned 

 with disintegrated chloroplasts. 



One may assume from the work of Krakover in which he inoculated 

 plants from infested soil that the fungus was living saprophytically in 

 that soil. The vigorous growth of the fungus in artificial culture indicates 

 that it is not unamenable to a saprophytic existence. 



Secondary cycles occur throughout the season, and are chiefly responsible 

 for the distinctive development of the disease. 



- Control 



Milburn (1915:72) upon finding the fungus in the seed recommends 

 seed treatment. To what extent this may be effective and practicable 

 remains to be determined. 



Because of the absence of the disease from the experimental areas, 

 two-seasons field experiments with sulfur dust have given no indications 

 of value. On the other hand, a large amount of data regarding the effect 

 of sulfur and copper dusts upon the germination of conidia on glass slides 

 has been accumulated in the laboratory. 2 The effect of the fungicides in 

 inhibiting infection in the greenhouse has been studied as well, but to a 

 less extent. 



Laboratory experiments 



Effect of sulfur. The results of several experiments on the toxicity of 

 sulfur to spores of M. sarcinaeforme are presented in table 6. 



This table shows that elemental sulfur in these forms is almost, if not 

 utterly, incapable of inhibiting germination of the spores. In order to 

 see if soluble sulfur, on the other hand, would inhibit germination, a test 



2 The technic for the laboratory testing of fungicides reported in this paper is essentially that long used 

 in the Department of Plant Pathology at Cornell University, and described recently by McCallan (1930). 



The writer's methods have differed from those of McCallan in a few minor details. Duplicate elides 

 for each treatment were used in each experiment. Each slide contained four separated drops of a spore 

 suspension diluted approximately to 50 spores to each drop, as examined by the low power of the micro- 

 scope. In determining the percentage of germination, all germinated and ungerminated spores were 

 counted separately in one field of the low power of the microscope. Invariably the materials were dusted 

 on dry slides on which the spore suspensions were placed subsequently. Bank's colloidal sulfur dust is a 

 patented product prepared by fusing finely ground sulfur with bentonite, a colloidal clay, and regrinding. 

 Kolodust is a trade name applied to a dilution of Bank's colloidal sulfur in 300-mesh dusting sulfur. 



