TOMATO LEAF MOLD 11 



surfaces is handicapped by the lethal influence of strong sunlight on the fungus 

 itself. Temperature and humidity operate more favorably for spore germination 

 on the lower shaded leaf surfaces. When the sun is shining brightly, the tempera- 

 ture is obviously lower on this side of the leaf than on the upper surfaces, and the 

 air is not as dry as that adjacent to the upper surfaces. 



Conditions at the leaf surfaces are vital to the infection process. This is shown 

 by the results of the writer's inoculations of upper and lower surfaces of leaflets 

 with dry spores applied with a camel's-hair brush in mid-August in the green- 

 house, at a time when warm temperatures and strong light conditions prevailed. 

 No germination occurred on the upper lighted leaf surfaces but it was abundant 

 on the lower shaded surfaces. 



Langford (12) noted greater susceptibility to infection with less light and as- 

 serted that the genetic factor for resistance disappeared with the lack of light. 

 The inherent resistance of the Stirling Castle tomato to one strain of Cladosporium 

 fulviini was but slightly expressed during midwinter at Toronto, Canada, and 

 accordingly varieties resistant during the summer season can be susceptible during 

 midwinter. This has not been the case in the writer's experience, nor are any 

 similar instances on record. Infection among susceptible varieties requires very 

 specific conditions of host and environment. Variation in the occurrence of the 

 disease closely follows differences in those fundamental environmental factors 

 which influence stomatal movements and spore germination. Proper incubating 

 conditions for infection are necessary to determine the true reaction of the variety. 

 Varieties and hybrids of Lycopersicum esculetitiim reported to possess resistance 

 have become badly diseased under conditions favorable for infection and have 

 for that reason been abandoned as a source of resistance in this work. Even 

 admitting the possibility of strains of the fungus, the conflicting reports asserting 

 the type of reaction of "esculentum" tomatoes to Cladosporium cannot be taken 

 too seriously. Susceptible varieties, including Stirling Castle, or hybrids of 

 parents showing some resistance in the summer months have manifested little 

 if any disease in midwinter in the greenhouse, and this is due to fundamental 

 influences of temperature and relative humidity at the leaf surfaces, which are 

 more favorable for the fungus at one season than at the other, and not to a host 

 reaction that is genetically plastic to infection with changes in the light experience 

 of the plants. 



Nutrients 



The relation of nutrients to the susceptibility of tomatoes to infection has 

 been given considerable study abroad but there is no consistency in the results. 

 Schaffnit and Volk (24, 25) and Volk (36) found that outstanding resistance was 

 manifested by plants having deficient supplies of phosphoric acid or nitrogen, 

 and that the lack of phosphoric acid devitalized the fungus sufficiently to render it 

 less viable and infective. Superfluity of nitrogen, deficiency of potash, or excessive 

 application of all the constituents rendered the plants very susceptible. Inter- 

 mediate manifestations were shown when the plants were given normal quantities 

 of all the constituents, or an excess of potash and phosphoric acid. Fleischmann 

 (8) found that the development and extension of the parasite was greatly increased 

 on overnourished plants and that deficiencies of important nutrients caused both 

 weak plants and a check in the growth of the fungus. He found that acid phosphate 

 and potash lacked the effect of increasing resistance, and that susceptibility and 

 immunity are varietal manifestations which are not essentially influenced by 



