12 MASS. EXPERIMENT STATION BULLETIN 350 



nutrition. Small (29) noted a severe development of leaf mold on manured plants, 

 while on starved plants the infected leaf areas remained small and bore com- 

 paratively little of the fungus. Reusrath (20) cautioned against the use of excess 

 or unbalanced nitrogen and advocated the liberal use of potash as a means of 

 moderating the susceptibility of the tomato to disease. According to Rice (21) 

 superphosphate has an encouraging effect, and potash and nitrogen a retarding 

 influence. 



These opinions cannot be taken too seriously in view of the fact that the chief 

 objective of the grower is a high yield of quality fruit. Plant vigor is a prerequisite 

 to success and obviously is obtained by proper fertilizer balance, soil moisture, 

 temperature, and humidity. The opinions reported above are based largely on 

 sand and peat cultures supplied with different ratios of potash, acid phosphate, 

 and nitrogen; and the effect on yield and quality of the fruit appears to have been 

 of minor consideration. Further, the tomato is susceptible to several physiological 

 disorders such as blossom end rot, growth cracks, pufiiness, hollow stem, blotchy 

 ripening, greenback, edema, etc., which are associated with malnutriton, weather, 

 and soil moisture. The suggestion of Volk (36) that more effort should be directed 

 to provide environmental conditions which render the host less favorable to infec- 

 tion, rather than those which affect the activity or suppression of the parasite, 

 disregards the resulting danger from other abnormalities which may prove more 

 serious than leaf mold. Neither have conditions influencing weak or vigorous 

 growth or the presence or absence of any of the abnormalities mentioned above 

 shown under commercial conditions observed by the writer any apparent influence 

 upon the susceptibility of tomato plants to leaf mold. The proper fertilizer amend- 

 ments to the land, soil moisture, and temperature, to obtain plant vigor are vital 

 to a successful yield, and any contrary practices employed as a means of counter- 

 acting a particular limiting disease factor are unwise. 



Chemical or Induced Immunization 



Efforts have been made to immunize the leaf tissues to infection by introducing 

 chemical substances into the roots. In some experiments reported by Massee 

 (14), tomato plants were immunized to Cladosporium by watering every three 

 days from two to six weeks of age with a solution of copper sulfate (1-7000). 

 After six weeks, the plants were watered every fourth day with a stronger solution 

 (1-6000). None of the plants given the copper sulfate showed leaf mold. 



Norton (18) used similar and weaker solutions of copper sulfate. Injury occurred 

 except at a dilution of 1-10,000 but plants so treated developed Cladosporium 

 in five days. The results from applying chemicals through stems or roots were 

 in general negative. Hasper (11) also reported negative results. In view of these 

 findings and the lack of practical application, no consideration has been given to 

 this phase of the problem in this study. 



Relative Humidity and Temperature 



Volk (36) obtained the highest number of successful inoculations at 95% 

 relative humidity. According to Bewley (4) and Small (26, 27, 28, 31), in- 

 fection is slight or rare at an optimum growing temperature of 72° F. at 70-71% 

 relative humidity, but severe at 80% and higher. At 58% and less despite an 

 optimum growing temperature, or at temperatures less than 59° F. despite a 

 favorable relative humidity, the fungus is suppressed or develops very slowly. 



