CONTROL OF TOMATO LEAF MOLD IN GREENHOUSES 



By E. F. Guba.i Research Professor of Botany 



INTRODUCTION 



Tomato leaf mold is essentially a foliage disease of greenhouse tomatoes. 

 Outdoor tomato plantings become diseased in weather marked by periods of high 

 temperatures and stagnant, moist air, and particularly in locations lacking good 

 air drainage and near greenhouses contaminated with the fungus. The disease 

 is never of general occurrence and usually is of no importance on field tomatoes 

 in Massachusetts. 



The disease is epidemic in the greenhouse in September and October on the 

 fall or second planting of the year and before much harvest of fruit. On the 

 spring or first planting of the year the disease often becomes epidemic in June 

 after the last trusses have set fruit but when it is too late for the disease to do 

 much noticeable damage to the yield. The measured loss may, however, be con- 

 siderable. 



The seriousness of the disease in the greenhouse is associated with high tem- 

 peratures and high relative humidities prevailing during the warm months of 

 the year. These conditions are readily obtained in a confined atmosphere of 

 growing plants. The differences occurring between inside and outside atmos- 

 pheric conditions during the year have a significant bearing on the development 

 of the disease. 



In greenhouse tomato culture in Massachusetts the monthly mean ma.Kimum 

 temperatures are usually about 10° to 16° F. higher from May to October inclusive 

 (corresponding approximately to the off-heating season) and usually about 20° 

 to 40° F. higher from November to April inclusive (the heating season), than the 

 corresponding outside monthly mean ma.ximum temperatures. The monthly 

 mean minimum temperatures in the greenhouse from May to October are usually 

 about 2° to 12° F. higher than the corresponding outside monthly' mean minimum 

 temperatures. This difference increases to about 40° to 42° F. during December to 

 February inclusive and gradually diminishes again as the season advances. This 

 increase and decrease in the temperature difference as the outside temperatures 

 diverge in the fall from and converge in the spring upon the inside minimum 

 growing temperature of 60° F. significantly influence the humidity of the green- 

 house air and the prevalence of the disease. In the period from May to October 

 inclusive, boiler heating is unsteady or lacking; and in May, June, September, and 

 October, practice allows the greenhouse temperatures to drop more or less to 50° F. 

 In the off-heating season the monthly mean maximum relative humidity is 100 

 percent andthe monthly average mean relative humidity is about 78 to 82 percent 

 in contrast to a monthly mean maximum relative humidity of 80 to 94 percent and 

 a monthly average mean relative humidity of 70 to 80 percent in the heating season, 

 the lowest relative humidity coinciding with the coldest months. The seriousness 

 of the disease, therefore, is as.sociated with the off-heating or warmest months 

 of the year when the relative humidity and temperature of the greenhouse air 

 are at their maximum. Since the disease is not of any significance and indeed is 

 usually absent in field plantings in this climate because of the weather peculiar 



iThe writer is gratefully indebted to Dr. W. S. Ritchie and Dr. E. B Holland of the Departr 

 of Chemistry for the chemical analyses reported in Tables \^. 15. 16. and IS. and for cheniic.-; 

 terpretations relative to these studies. 



