TOMATO LEAF MOLD 21 



and free circulation of outside air of real low relative humidity produces the 

 greatest evaporating effect at the leaf surface. Between these extremes of max- 

 imum heating in the winter and maximum evaporation from ventilation in the 

 driest summer weather, favorable conditions for infection commonly occur in the 

 greenhouse which, even in spite of heat or natural and forced ventilation or both, 

 and within the limits of good plant growth, are often beyond any reasonable or 

 practical means to counteract. 



Small (26) noted the most progressive development of the disease in July and 

 August, the warmest months, in England and when air saturation occurred in 

 the houses at night. The evidence fully substantiates the view that seasonal 

 variations in the se\erity of leaf mold are related fundamentally to temperature 

 (16, 17, 27), other conditions being favorable for good growth of the tomato plant. 



SUMMARY 



In greenhouse tomato culture in Massachusetts, the leaf mold disease is usually 

 epidemic from June to October. It occurs in field plantings near greenhouses 

 and in areas of uneven or sheltered topography and poor atmospheric drainage. 

 The fungus is a facultative parasite. 



Only the conidial stage of the fungus is known. The spores withstand severe 

 winters and remain viable under the most adverse conditions for 9 to 12 months. 

 Conidia are disseminated by air currents and are readily dislodged by shaking 

 or disturbing the plants. 



A relative humidity of 100% or precipitated moisture and temperatures of 

 75°-78° F. are optimum for germination. A relative humidity of 95-96% is mini- 

 mum and 40° F. and 94° F. are the minimum and maximum temperatures for 

 spore germination. Conidia lose their viability when exposed to a temperature 

 of 115°-116° F. for 2 hours. Longer exposures at slightly lower temperatures are 

 also lethal. The exposure of infected plants for 3 hours to 118°-123° F. does not 

 afTect germination of the fungus appreciably and is injurious to the tomato plant. 



Light retards growth and suppresses spore germination. Strong light is lethal 

 to the spores. 



Infection is through the stomata, usually or entirely in the lower sides of the 

 leaves where the stomata are most numerous and where conditions within the 

 leaf and light, temperature, and relative humidity at the leaf surface operate 

 more favorably for spore germination than at the upper leaf surfaces. 



Open stomata and transpiration are associated with light and turgid growth. 

 Flaccidity of the foliage, brought on by a moisture deficiency in the leaves, and 

 darkness operate to close the stomata and to hinder infection. 



Conditions at the leaf surface are fundamental to the infection process, and 

 variations in the occurrence of the disease in the greenhouse at different times of 

 the year are due to fundamental environmental factors operating to favor or 

 prevent spore germination on the leaf. 



Plant vigor is a necessary asset to high yields and quality, and the result of 

 good growing conditions. Efforts at modifying the nutrition of the plant to in- 

 fluence the reaction of the host in the direction of resistance are not practical or 

 commercially successful. Immunization of the host with chemical solutions applied 

 to the soil has not proved safe or successful. 



The disease is epidemic during the warmer months from June to October in- 

 clusive, when maximum greenhouse temperatures of 80° to 92° F. prevail; when 



