366 GROWTH OF PLANTS 



It seems probable that the phytotoxicity of sulfur fungicides, especially 

 lime sulfur, is also due to the production of H2S, for in the work on the 

 toxicity of H2S to green plants ^^ discussed in Chapter 5 it was found that 

 the symptoms of lime sulfur injury were identical with those of H2S. 

 Studies on the toxicity of other industrial gases to fungi ^^' ^^ and to green 

 plants are also described in detail in Chapter 5. 



Particle size. Various commercial sulphur dusts were found to differ in 

 their toxicity toward conidia of Sderotinia fructicola.^'^ The difference could 

 not be ascribed to the rate of formation of H2S nor to the acidity of the 

 water extracts; it is due to the size of the sulfur particles, and the toxicity 

 increases with a decrease in the size of particles. When straight unmodified 

 sulfur dusts, particles of which differ significantly in their mean diameter, 

 were compared on an equal weight basis, there were significant differences 

 in toxicity. However, when these dusts were compared on the basis of an 

 equal number of particles per unit area, there was no difference in toxicity, 

 as is graphically demonstrated in Fig. 143. 



A convenient method for determining the mean particle size of sulfur 

 dusts was developed. This involves counting the number of particles fur- 

 nished by a knoun weight of sulfur dusted on a surface of kno\vn area. 



The tenacity of sulfur dusts to glass slides, following laboratory "rain" 

 tests, was found to depend on the degree of fineness of the dust, the smaller 

 particles being the most tenacious. With a given dust, an increase in the 

 amount applied results in a decrease in the percentage adhering. It was 

 found that in general dusts adhere better to leaves than to glass slides, and 

 that the tenacity improves with the roughness and hairiness of the leaf 

 surface. 



A useful measure of the dusting qualities of sulfur dusts was found in 

 the "angle of slope," that is, the angle between the side and base of a cone 

 of dust carefully built up to the maximum height attainable; the smaller 

 the angle the better the dusting quality. 



Thus the most important single factor determining the toxicity of sulfur 

 applied as a dust is the number of particles furnished by a unit weight of 

 the material. The greatest number of particles is furnished by the dust of 

 smallest mean particle diameter; this is also the most tenacious and will 

 give a denser distribution for a given weight. This relation between particle 

 size and toxicity seems to hold \vithin wide limits from relatively coarse 

 particles down to those of colloidal dimensions. However, the dust having 

 the highest toxicity does not necessarily have the best dusting qualities. 



Fungicidal Action of Copper 

 Since the classical discovery of Bordeaux mixture by Millardet in 1882, 

 the copper fungicides have become our most effective group. About 

 100,000,000 pounds of copper sulfate are consumed annually in the control 

 of plant diseases in the United States. Although in recent times numerous 

 substitutes have been developed, Bordeaux mixture remains by far the 



