TOMATO LEAF MOLD 3 



frame plantings here and elsewhere (23, 33), and in some years may appear as 

 early as January on seedlings and plants for the first or spring planting in the 

 greenhouse (23). It is reported to appear first and most seriously in the south 

 and west sides, corresponding to the hottest parts of the greenhouse, and in 

 east and west houses; and least in the north and east sides and in north and south 

 houses (1, 7, 40). The lowest leaves become infected first. The occurrence and 

 the epidemic development of the disease in definite situations and during definite 

 seasons of the year, suggest the close association of the disease with particular 

 environmental factors of light, temperature, and humidity. 



THE FUNGUS 



Careful life-history studies have never revealed more than the conidial stage 

 of the fungus as it occurs on tomato foliage. Sclerotial bodies of the fungus have 

 been noted in infected tomato seed (10), on corn kernels in pure culture (13), 

 and as an overwintering stage which on the advent of spring produced conidia 

 (3); but fruiting bodies of a perfect stage have never been observed (11). In 

 the light of extensive observations and the unsuccessful efforts of other investiga- 

 tors to obtain the perfect stage, it appears that the fungus has never had the 

 ability to produce pleomorphic spores. This fact has simplified the means pos- 

 sessed by the fungus for its survival and spread. 



The conidia possess extraordinary resistance to extremes of environmental 

 conditions which assures the occurrence of the disease from one year to the next. 

 Plants have been infected with conidia from a leaf dried between blotting paper 

 for six months, and after a period of one year some conidia were still viable (13). 

 Spores exposed to a dry atmosphere for eight months were for the most part cap- 

 able of germination; after ten months, only a small percentage was viable; and after 

 twelve months none were viable (11). New spores have been observed to germinate 

 in four hours; spores eight months old, in 24 hours. After being outdoors in a dry 

 condition from January 4 to May I, during which time the lowest recorded tem- 

 perature was — 4° F., spores germinated readily (32). Spores from overwintered 

 leaves produce infection the following spring after passing severe winters on glass- 

 house structures and in the soil (27, 29). 



The writer's experiments have shown that a large percentage of the conidia 

 exposed under various temperature conditions was still viable after nine months. 

 The death of the spores increases slowly with age. The fungus readily survives 

 gaps between tomato crops in the greenhouse, and the source of the spores may be 

 from the greenhouse itself or from the field. 



The fungus has been grown on filter paper (13); on partially dead corn leaves; 

 on detached tomato leaves (32); and on a decoction of tomato and tobacco leaves 

 (11). This would suggest that the fungus exists on dead substrata in the absence 

 of and apart from its susceptible living host, and that it is to be regarded as a 

 facultative parasite. 



The conidia of the fungus are disseminated chiefly by currents of air. The 

 spores are easily dislodged from the leaves when the vines are disturbed in pruning 

 ^nd harvesting and by drafts of air, and are readily spread in the air throughout 

 the greenhouse planting. Tapping or shaking the plants to assist in pollination 

 of the blossoms, which is a usual and de.sirable cultural practice, dislodges clouds 

 of spores into the air. The workers also collect spores on their clothing as they 

 walk between the rows of plants and thus readily disseminate the fungus. 



