TOMATO LEAF-MOLD 9 



(I), but no such differences were consistently noted and the average of the 

 results of the six tests by each of these two methods of exposure was the 

 same. Sulfuric acid was produced only when moisture was present on the 

 slides but in some instances no acid was produced in the presence of moisture, 

 air conditions being practically the same. Acid media below a pH value of 

 5.7 were consistently toxic to spores, but the toxicity of vaporized sulfur was 

 not due alone to acidity for in some instances complete control of germina- 

 tion occurred in alkaline and neutral media. The production of sulfuric acid 

 did not appear to be influenced by either the exposure or the incubating 

 temperatures, and toxicity was not correlated with any particular tempera- 

 ture. 



Barker, Gimingham and Wiltshire (4) offered as one of the reasons for the 

 toxic action of sulfur the irritant character of finely divided particles which 

 stimulate to an injurious extent cells exposed to their action. One would 

 expect a much greater irritant effect from vaporized sulfur in its change 

 from the fluid to the dry state than from dry sulfur powders. The incrusta- 

 tions of sulfur surrounding the spores after drying of the droplets also would 

 appear to interfere with the absorption of moisture necessary for germina- 

 tion (Plate I, E). Kraemer (19, 20) reported that when sulfur was heated 

 very slowly with free access of air almost one-third of the vaporized sulfur 

 was converted into sulfuric acid with very little or no sulfurous acid. On 

 rapid heating and with very little air the proportion of sulfurous acid was 

 increased while the amount of sulfuric acid formed was very much lessened. 

 He asserted that the increased efficiency of vaporized sulfur over flowers or 

 flour sulfur used as a dust is due to the increased proportion of sulfuric 

 acid produced. Numerous workers have attributed the activity of sulfur to 

 sulfuric acid and recognized that sulfuric acid is fungicidal in very weak 

 dilutions, but Barker, Gimingham and Wiltshire (4) and Vogt (35) found 

 that none of the compounds which might possibly be derived from sulfur, 

 among which are mentioned sulfuric and sulfurous acids and sulfur dioxide, 

 could account for its toxic action. 



Control of Infection of Potted Plants 



Those fungicides considered in the studies on spore toxicity were tested for 

 their value in preventing infection of potted tomato plants. Two plants 8 

 to 12 inches high were used in each test. The potted plants were placed in 

 a glass chamber 34x22x18 inches in size for the dust treatments. Small 

 Feeney hand dusters were used for applying the dusting materials. The 

 pots were placed in inverted and upright positions in the chamber to obtain 

 coverings of dust on the lower and upjjer surfaces alike. The same chamber 

 was charged with vaporized sulfur, and potted plants were confined in the 

 charged atmosphere long enough to show a covering of sulfur. Sprays were 

 applied with a quart capacity atomizer and both leaf surfaces were thor- 

 oughly wetted. The spray was permitted to dry before the plants were 

 inoculated. The plants were inoculated in two ways: (1) sprayed with a 

 water suspension of spores and (2) dusted with spores. 



The water suspension of spores applied after the treatments was allowed 

 to dry previous to incubation. The dry plants were placed in a glass chamber 

 of 175 cu. ft. A saturated atmosphere was obtained and a range of tem- 

 peratures from 80° to 95° F. prevailed. The fungus was incubated for 24 

 to 36 hours. The plants were then removed from the pots and planted in the 

 greenhouse. When the disease was evident on the untreated controls, counts 



