TOMATO LEAF-MOLD 7 



a .1 per cent strength, was not toxic to spores applied in the water drop to 

 the dry residue (IV), but was toxic by other methods of study. The efifec- 

 tiveness of the organic mercury compounds by all methods of study sug- 

 gests the possibility of their use to control infection in the greenhouse. 



Commercial 40 per cent formaldehyde at dilutions ranging from 1-100 to 

 1-400 (.4 to .1 per cent) was lethal (II, III). Dry spores on slides were 

 placed in dishes moistened with formaldehyde (V) but only 1-100 and stronger 

 mixtures produced a lethal atmosphere. Since the 1-100 mixture is fungici- 

 dal in the air as well as in the soil, its use for soil sterilization is preferable 

 to a 1-400 mixture. Formaldehyde was ineffective by methods of study I 

 and IV, and a strength of 1-1000 was ineffective by every method of study. 

 The data suggest that disinfection of the greenhouse may be obtained by the 

 usual method of drenching the soil with a 1-100 mixture and closing the 

 greenhouse tight after the treatment. 



The toxicity of the fumes of burning sulfur was very striking. Spores on 

 slides confined within Petri dishes placed in a treated atmosphere were 

 killed. The data indicate that the burning of sulfur between crops, as is 

 generally practiced by growers, is an effective and desirable means of de- 

 stroying hibernating parts of the fungus. 



Other than naphthalene, formaldehyde and burning sulfur, none of the 

 materials listed in Table 1 were of any fungicidal value as fumigants. 



The residues of potassium sulfide, copper sulfate, sulfuric acid, and organic 

 mercury sprays showed a toxic effect to spores applied dry to the slides (I). 

 By this method of study some solution of the residue in moisture deposited 

 on the surface of the slides occurred. This fungicidal effect was shown only 

 by readily soluble materials such as noted. 



Vaporized Sulfur. 



On heating sulfur, amorphous particles of sulfur are discharged into the 

 air, float about, and finally are deposited on exposed surfaces. These amor- 

 phous particles of sulfur are fluid and spread over the surface on which they 

 fall. The sulfur droplets are very sticky. This is considered to be due to 

 the fluid property of the sulfur which in this condition is able to rid the 

 surface of air films. Barker and Wallace (5) report that sulfur distributed 

 through the atmosphere by its vaporization not only furnishes a much more 

 complete superficial covering than can be secured by dusting or spraying, 

 but also possesses a remarkable power of adhesion, withstanding the action 

 of heavy rains or sprays of water of considerable force. To compare the 

 relative adhesiveness of vaporized sulfur and pure dusting sulfur, ten glass 

 slides, each of an area of 3 square inches, were weighed with a chemical 

 balance then exposed to an atmosphere treated with vaporized sulfur and 

 again weighed. Similarly ten weighed slides were dusted with a pure dust- 

 ing sulfur and reweighed. Both sets of slides were exposed to a rain of .36 

 inches then air dried in the laboratory and again weighed to determine the 

 amount of loss from washing. (Experiment A.) 



The experiment was repeated (Experiment B), but the amount of rain 

 recorded was .04 inches. The relative adhesiveness of the two sulfurs is 

 indicated quantitatively in Table 2. The data show that washing caused an 

 appreciable loss of dusting sulfur but that its effect upon vaporized sulfur 

 was negligible. 



The dry particles of amorphous sulfur measure 1 to 5 microns in diameter. 

 The largest, which are aggregates of smaller droplets, reach a diameter of 24 

 microns. The distance between the particles varies according to the density of 



