1882.] FUNGI INJURIOUS TO VEGETATION. 175 



the remedy for the grape mildew. " Early and overbearing are 

 prolific sources of mildew," therefore judicious thinning will do 

 much to keep it off. 



The Lettuce Mildew. 



For a number of years the market gardeners in some localities 

 have suffered more or less severely from the attacks of a mildew 

 on their early or forced lettuce. The numerous complaints in the 

 spring of 1880, led me to inquire into the trouble, which had 

 reached such magnitude that one gardener stated that it was taking 

 away from him, and those engaged in the same business around 

 him, the source of their very living. A circular was addressed to 

 many of the leading growers of early lettuce. Secretaries of State 

 Horticultural Societies, and others from whom information could 

 be gained. 



From the responses to these questions it is inferred that the let- 

 tuce disease is at present confined to the Atlantic States, and that 

 it is most prevalent in those localities where lettuce has been 

 grown upon the same ground for a considerable length of time. 

 There is no doubt but that it is a fungus, as a microscopic examin- 

 ation of the specimens abundantly show; in fact the mildew upon 

 the lettuce is not a new thing, it havmg been described a number 

 of years ago. It first manifests itself upon the older and outer 

 leaves as a fine frosty coating, soon causing the leaves to turn dark 

 colored, wilt down, and rot away. 



The lettuce fungus is a near relative of the grape mildew, be- 

 longing, as it does, to the same genus of parasitic pests. In gen- 

 eral structure, methods of growth, and propagation, it is therefore 

 much like that of the mildew of the grape. When the surface of the 

 lettuce leaf is examined with a hand lens, the white substance re- 

 solves itself into a miniature forest of small stems and branches. 

 The tips of the branches are of a peculiar star-like form, and from 

 the radiating points the spores are formed and when ripe are easily 

 detached. 



The portion of the fungus beneath the surface of the lettuce 

 leaf consists of winding threads which, as they pass between the 

 cells of the lettuce tissue, form projections that pass through the 

 cell walls and serve as suckers to absorb the substance of the cells. 

 This is the real nature of the trouble, a multitude — a forest so to 

 speak — of small plants, living upon and drawing away the life of 

 the lettuce. 



