68 PHYSIOLOGICAL RELATIONS 



blights mentioned seem to profit materially from severe alterna- 

 tions or contrasts of weather conditions. Moreover, if heavy dews 

 prevail during a warm summer these fungi spread irrespective of 

 precipitation. In such cases it is apparent that injured or drying 

 portions of the plant are at least the first seats of disease. It seems 

 to be true that many crop diseases are commonly most important 

 under conditions of moisture insufficient for most vigorous crop 

 production. 



Light. The chief role of light in plant economy is connected 

 with the formation of sugar and starch, from which, in large part, 

 the other organic products are ultimately derived. Light, how- 

 ever, calls forth a variety of responses in every green plant, and 

 it may play a direct or indirect role in the relation with parasitic 

 fungi. It has long been observed that celery under lath or cloth 

 screens, that is, half shade, is largely free from the early blight. 

 The leaf spot or so-called rust on the strawberry may be similarly 

 controlled through reduction of the light factor with the increased 

 humidity and diminished evaporation generally incident thereupon. 

 It is also reported that the tent cloth is effective against asparagus 

 rust. Ginseng growers in New York and Missouri are employing 

 the lath screen advantageously in the prevention of a serious blight 

 of this plant, due to a fungus which is believed to gain entrance 

 most readily at the margin of the leaf, possibly following a tend- 

 ency to sun scald in that area. Screening, however, is not advised 

 fgr strawberries, and it would be available in the home garden, in 

 general, only where such a device may be at will readily interposed 

 or removed. In opposition to these beneficial effects of half shade, 

 we have also abundant observations showing that certain powdery 

 mildews are far more effective as parasites under just such con- 

 ditions as above enumerated. I have seen wheat under partial 

 shade badly infested with the powdery mildew, which in the 

 central West, at least, is seldom, if ever, seen in the open. Time 

 and again, in that same region, one may observe that in the case 

 of well-watered lawns the mildew of blue grass abounds in a circle 

 rather sharply limited by the heavier shadow areas of trees. Simi- 

 larly, in the drier West the grape mildew is, as a rule, found mostly 

 on Vitis vinifcra stock, and in shaded places. The fungus soon 

 disappears from leaves to which strong sunlight is admitted. 



