TOMATO LEAF MOLD 9 



to Small (29), leaflets of various ages are equally susceptible, and this conclusion 

 is substantiated by the writer's inoculation experiments. According to Makemson 

 (13), the germ tube may immediately enter a stoma but usually considerable 

 growth in length takes place before penetration. He noted cases of penetration 

 36 hours after inoculation. Williams (39) found that penetration had not occurred 

 within 39 hours and concluded that a long period elapsed before penetration takes 

 place. Hasper (11) observed that penetration usually required 4 to 8 days, while 

 10 to 12 days were required in other instances. 



The writer's studies point to the importance of stomatal numbers and diameter 

 in infection. It was observed that under greenhouse conditions the stomatal 

 openings on the lower leaf surfaces are larger and the stomata much more numer- 

 ous (11), and open over much longer periods each day of 24 hours, than on the 

 upper leaf surfaces. The presence of infection only on the lower leaf surfaces in 

 the beginning of epidemics is evidence of the significant relation of stomatal 

 openings and numbers to infection. It has been asserted that plants in the open 

 have fewer stomata than plants in the greenhouse (6) and that this is one factor 

 accounting for the less frequent occurrence of the disease in the open. No differ- 

 ences in stomatal numbers were apparent from the writer's examinations of 

 greenhouse and outdoor plants of the same variety. 



Soil Moisture 



Small (27, 31) noted that turgid plants become infected more readily than 

 drooping plants, and observed that spores on drooping foliage failed to germinate. 

 The higher humidity around turgid leaves permitted germination and infection. 

 The belief prevails that artificial forcing conditions influenced by high tempera- 

 tures, excessive moisture, and fertilization, greatly increase the susceptibility to 

 disease, while more natural conditions in the open are less inducive to infection 

 (14, 19). Volk (36) found no difference in the susceptibility of the leaves of plants 

 growing in soils with moisture between 50 and 80% of the water-holding capacity. 

 On wilting plants growing in a soil containing 25-30% moisture the incubation 

 period was prolonged, fructifications were more abundant, and the viability of 

 the fungus was extended. 



Both turgid and wilting tomato plants were inoculated by the writer on August 

 16. On August 20 germination on the turgid foliage was considerable, the germ 

 tubes were up to 80 microns in length, some capped with appressoria as noted by 

 Makemson (13), and some tubes already lodged in the stomatal cavities. No 

 branching of the tubes was evident. The tips of many of the tubes were lying 

 directly over the closed stomatal cavities. In contrast to this condition, wilting 

 plants inoculated at the same time and maintained in a wilting condition showed 

 no evidence of spore germination by August 22, and when examined frequently 

 the stomata on both leaf surfaces were tightly closed. On September 7 the foliage 

 of the turgid and wilting plants was examined for infection (Table 5). The dif- 

 ference in the number of leaflets available for counting was due to the vast differ- 

 ence in growth associated with these e.xtreme soil moisture conditions. 



In another experiment the lower leaf surface of turgid and wilting plants grown 

 ,in ground beds in the greenhouse was inoculated on August 18. Examinations of 

 foliage 48 hours after inoculation revealed no spore germination on wilted plants 

 growing in dry soil, but an abundance of it on turgid plants growing in moist soil 

 in the same greenhouse; and the same condition existed on August 22. A deficiency 

 of moisture sufficient to induce a flaccid condition of the leaf causes the 



