TOMATO LEAF MOLD 13 



Rippel (22) declared that average relative humidities are insufficient for a proper 

 analysis of the problem and that values less than 85% for 18 to 20 hours each 

 day are necessary to check or prevent the disease. Walker and Sumner (37) 

 declare that relative humidities around 80% and lower are against infection, 

 while continuous exposure for 48 hours, or for a 2-hour period each day for 14 

 consecutive days, to relative humidities above 80% promotes infection. Alexander 

 (2) found that very little infection occurs when plants are exposed for less than 

 9 hours in a saturated atmosphere at 59°-85° F. In clear weather, in early spring 

 in Ohio, the greenhouse air is saturated usually less than 8 hours daily so that 

 infection does not occur. From late spring to November, high humidities prevail 

 for a long time and infection is epidemic. A significant concept is expressed by 

 Newhall and Wilson (16) and Newhall (17) to the effect that relative humidities 

 of 90-96% prevail at the leaves even though the greenhouse air may be as low as 

 75% and that this is due to leaf transpiration. The coincidence is noted of the 

 disease being most severe when the transpiration rate is the highest or when the 

 average house temperature is above 65°-70° F. 



In greenhouse tomato culture in Massachusetts, the leaf mold disease appears 

 in May, is epidemic during the summer months, and gradually subsides after 

 October if the heating is properly managed. Bad attacks of the disease in August 

 and September cause the fungus to persist throughout the autumn season especi- 

 ally if the weather is mild. The greenhouse mean maximum temperatures rise 

 above 90° F. and prevail there from late June to mid-August, then recede to about 

 65° F. in December (Fig. 2). The outside mean maximum temperatures reach the 

 highest (75°-80° F.) and the lowest (32°-34° F.) levels in the corresponding periods. 

 The influence of high temperatures on leaf transpiration which supplies the mois- 

 ture necessary for spore germination must be conceded. From May to October 

 inclusive, pipe heating is either irregular or discontinued since greenhouse tempera- 

 tures prevail above or close to the minimum temperatures for satisfactory growth 

 (Fig. 3). This situation renders the prevention of high relative humidity in the 

 greenhouse difficult if not impossible during this critical period, even with ventila- 

 tion. After this period, beginning with October, the increasing gap between the 

 inside and outside temperatures, reaching a maximum of almost 45° F., effects 

 a corresponding decrease in the mean relative humidity which is at the lowest 

 level in the months of December, January, and February, corresponding to the 

 widest differences in temperatures. As the season advances, the gap between the 

 outside and inside mean minimum temperatures becomes extremely narrow, being 

 the smallest, or less than 5° F., from June to September inclusive. This tempera- 

 ture differential, directly as it affects the relative humidity, is a major influencing 

 factor governing the behavior and progress of the disease in the greenhouse. 

 The usual failure of the fungus to sporulate on the leaf surface in the winter 

 months is due to the prevailing low relative humidity (27, 36). 



The average mean and mean maximum relative humidity under glass are shown 

 graphically in Fig. 4. These reach peak levels from May to September inclusive, 

 corresponding to the highest monthly mean temperatures. During this period the 

 average mean relative humidity inside is about 80-82%, and the mean maximum 

 100%. At the leaf surface, due to a high transpiration rate under these conditions, 

 the relative humidity is usually near, or at, 100%. The more abundant applica- 

 tion of water to the soil in the warmer 'months, and its influence on the humidity 

 of the greenhouse air, must also be recognized. Since temperatures of 74°-79° F. 

 and relative humidities of 98-100% are optimum for spore germination and infec- 



