TOMATO LEAF MOLD 5 



of the leaves were removed and examined under the microscope for spore germina- 

 tion, and subsequently placed in moist Petri dishes to determine spore viability. 

 The results in Table 1 show that under optimum temperature conditions spore 

 germination is extremely rare at relative humidity values of 90-95%. Values 

 above 94% are favorable to germination, and a value of 100% is optimum. 

 These results are closely in line with the findings of Newhall (15, 16, 17). 



Table 1. Relation of Relative Humidity to Spore Germination on 



Leaves of Tomato 



*Shor germ tubes 



tGrowth of germ tubes retarded 



Temperature 



The temperature relations of the fungus have received considerable study. 

 Gardner (10) found that the minimum and maximum temperatures for growth 

 and sporulation on potato dextrose agar are 35.6° and 95° F. respectively, and the 

 optimum 68°-75.2° F. Caldis and Coons (5) reported no growth on Shives' 

 dextrose agar at 50° and 86°-89° F. Makemson (13) asserted the minimum, 

 optimum, and maximum temperatures for development of the fungus on the 

 plant to be 48.2°, 68°-77°, and 86° F. respectively. Different opinions are reported 

 concerning the relation of temperature to spore germination (Table 2). On the 

 basis of the writer's study, spore germination occurred over a range of 40°-94° F. 

 with the optimum at 75°-78° F. (Fig. 1). The optimum is considered the same 

 as the most favorable day temperature for growing tomatoes under glass. Between 

 40° and 61° F. germination and the growth rate are slow. As the temperature 

 increases from 79° to 94° F. the growth rate decreases rapidly. 



Hasper (11) found that the spores are killed when frozen in ice at 2° F. for 

 7 days, but when left foi the same period at a minimum temperature of about 

 8° F., a large part of the spores germinated. Spores sown in water at temperatures 

 between 122° and 140° F. were killed. Dry air temperatures of 156°-158° F. for 

 4 hours were lethal, but after 3 hours some germination occurred. Exposure of 

 spores to free winter weather for 3 months destroyed them. According to Spangler 



