58 MASSACHUSETTS HORTICULTURAL SOCIETY. 



fruits, grape rot and many other diseases when the conditions are 

 continually moist. There seems to be no doubt that such conditions 

 of continuous moisture affect not only the spread of the fungus, but 

 also the relations of the host plant. The growers of western New 

 York claim that during a moist season the grape rot is worst W'here 

 the vines are \agorous. This may mean, of course, that heavy 

 leafage prevents air circulation or ventilation, to such an extent 

 that about vigorous vines the conditions are more moist, in w'hich 

 case the relation of vigor to disease is indirect, being really governed 

 by the amount of moisture. 



It is well known that in New England the potato Ph}i;ophthora 

 is unusually destructive during moist weather. AVard thinks that 

 not only the amount of external moisture but the water and soluble 

 food materials in the plant together determine the intensity of an 

 epidemic. He cites Klihn's observation to the effect that there 

 are two stages of growth when the plant is most susceptible. One 

 of these is when all portions of the plant are young and tender. 

 The second period is at about the time when the growth of vegeta- 

 tive parts is complete, and w^hen tuber formation begins w^ith more 

 vigor. At this time he would postulate a more rapid movement of 

 soluble food material tow'ard the tubers and consequently greater 

 sensitiveness. 



The Role of 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 

 the light factor. It is also reported that the tent cloth during a 

 relatively dry summer is effective against asparagus rust. In this 

 latitude screening is not advised for strawberries, and it would be 

 available in the home garden only where such a device may be at 

 will readily interposed or removed. Ginseng growers in New York 



