TOMATO LEAF MOLD 5 



Impressive instances of the association of epidemics of late blight rot on tomato 

 due to Phytophthora infestans (Mont.) De By., and timber rot on cucumber due 

 to Sclerotinia libertiana Fckl. with sprinkler watering systems in greenhouses 

 have also been observed. The practice is to be condemned. It offers none but a 

 labor-saving advantage over the slower method of bottom-watering and truly 

 is an example of false economy. 



In contrast to overhead irrigation, the watering of the beds through lines of 

 buried tile has been recommended (13, 16, 23, 34). Most greenhouse ranges in 

 Massachusetts devoted to cucumber and tomato culture are underlaid with tile 

 pipes for steam sterilizing which might be adapted for irrigation purposes. Sub- 

 terrean watering, according to reports, exerts little if any influence on the humidity 

 of the greenhouse air and the surface soil remains dry. This is an important factor 

 in dealing with the disease. Ball (7) invited attention to subterranean watering 

 through tile lines in sweet pea culture and the merit of the idea in controlling 

 foliage diseases. Good control of tomato leaf mold has been reported where the 

 tomatoes are grown in terraces or ridges and the water applied to intervening 

 grooves or furrows (34, 47). In this manner only a small area of the ground is 

 wetted. 



Watering should be done on bright da>s when the houses can be aerated with 

 drier outside air, although in times of prolonged dull or wet weather this rule 

 cannot always be followed. The practice of watering in the morning on bright 

 days makes it easier to control damp conditions at night with heat and ventila- 

 tion. The greenhouse air at night is more apt to become saturated with moisture 

 and the plants covered with "sweat" if the watering is done late in the day. 

 Mulches of straw or manure on the beds absorb and hold water. Their use 

 emphasizes even more the importance of morning watering and ventilation. 



Excessive applications of water and afternoon watering on warm bright days 

 cause the release of water from the leaves and stems, especially if the air in the 

 night following is cool and moist (Fig. 4). Transpiration is suddenh- checked 

 while the absorption of water through the roots continues. The exudation of 

 water or guttation creates moisture conditions on the foliage favorable for disease. 

 These drops or beads of water become larger and finally run or drop off the plants. 

 If the watering is moderate in amount and is done in the morning in bright weather 

 and if \'entilation is continued at night, guttation can be avoided. 



Disregard of these simple rules governing the watering of greenhouse tomatoes 

 can be an important factor contributing to the incidence and progress of tomato 

 leaf mold. 



TEMPERATURE AND HUMIDITY CONSIDERATIONS 



The management of the greenhouse air to prevent atmospheric conditions 

 favorable to the prevalence of tomato leaf mold is b>- no means simple. It may 

 contribute to the success of other worthx' control measures, or it may undo all of 

 the good accruing from them. 



Small (42, 45) emphasized the importance of keeping the relative humidit>- of 

 the greenhouse air below 75 percent, but this is impossible in greenhouses of any 

 design in Massachusetts. Rippel (40) found that a relative humidity below 85 

 per cent for 18 to 20 hours each day is not impossible in greenhouses in Germany 

 and this, according to his studies, is necessary to prevent infection. Abnormal 

 atmospheres in the greenhouse are the result of inefficient ventilating and in- 

 adequate ventilating area. Both, therefore, are vitally concerned in the problem 

 of controlling tomato leaf mold. 



