4 MASS. EXPERIMENT STATION BULLETIN 350 



RELATION OF ENVIRONMENTAL FACTORS TO SPORE GERMINATION 



AND GROWTH 



Relative Humidity 



Makemson (13) reported good fungus growth at 97% relative humidity, 

 excellent at 88%, poor at 75%, and none at 60%, on artificially inoculated plants 

 under jars. In his experiments, these humidity values were maintained fairly 

 well with sulfuric acid, but they were regarded as somewhat low due to the 

 inaccuracies of the polymeter used. The optimum humidity for the fungus and 

 the tomato plant were reported to be the same. Newhall (15, 16, 17) could not 

 germinate the conidia below 96% relative humidity at room temperature, and 

 reported that germination was best at 98 and 99%. In contrast, Rippel (22) 

 found that at 40° and 68° F. spore germination was 100% at 95 and 100% relative 

 humidity; 50 and 60% at 90% relative humidity, and none at 85%. He regarded 

 the fungus as a xerophyte, and asserted that the spores were unable to germinate 

 unless a positive moisture differential existed between the relative humidity of 

 the air and the minimum hydration index of the spore, which he asserted to be 

 85% relative humidity. At an atmospheric relative humidity of 100%, the mois- 

 ture available for swelling spores is maximuni and offers the best chances for 

 germination. 



Studies were undertaken by the writer in an effort to obtain further convincing 

 evidence. Conidia of the fungus were obtained from infected leaves and applied 

 to glass slides with a camel's-hair brush. The tests were conducted under bell 

 jars, and variations in the relative humidity of the air were obtained with different 

 concentrations of sulfuric acid. Small tomato plants were placed with the slides 

 under each jar. A Mason hygrometer was suspended in each jar and the entire 

 equipment was incubated in controlled air temperature chambers. Readings of 

 relative humidity were made frequently during the experiment. The conidia 

 were incubated for periods of 24, 48, and 72 hours, and were thereafter placed in 

 moist Petri dishes to make certain of their \ lability. The temperatures used ranged 

 from 68°-72° F. In all the tests in which the spores were exposed to a relative 

 humidity of 100% excellent germination occurred. 



A film or drop of water is not required for germination, although the spores 

 germinate excellently in this medium. In 20 tests in which the conidia were 

 exposed to relative humidity values between 83 and 89% inclusive, there was 

 no germination. In one series of experiments no germination occurred within a 

 range of values of 90-95%. Germination was noted at 96% relative humidity 

 and it was better at higher values up to 100% at temperatures of 68°-72° F. 

 In another series of experiments in which temperatures of 78°-86° F. prevailed, 

 germination occurred at 95 and 96% relative humidity; it was very poor at 

 94%; and there was only a trace at 90 and 92%. In other instances no germination 

 was noted at 90%. Under the conditions of these experiments and under optimum 

 temperature conditions for germination, a relative humidity of at least 95% is 

 required for spore germination. 



In other experiments, the spores of the fungus were applied to selected areas 

 on the lower leaf surfaces of potted tomato plants. These plants were covered 

 with open-topped bell jars plugged with cotton, in each of which was suspended 

 a Mason hygrometer. The entire assembly was subjected to controlled tempera- 

 tures. The relative humidity values were obtained with sulfuric acid. After an 

 incubation of 72 and 144 hours, pieces of epidermis from the inoculated areas 



