206 Agricultural Experiment Station, Ithaca, N. Y. 



The time of the appearance of early blight may, therefore, de- 

 pend upon questions of plant physiology fully as much as upon 

 the season, or even more so. Since fully developed plants, re- 

 gardless of variety, are more subject to disease than the younger 

 and more vigorously growing ones, it would seem reasonable to 

 conclude that conditions which would cause the plants to ripen 

 prematurely, or that will check the normal growth, will at the 

 same time favor the appearance of the early blight. Facts tend 

 to support this view of the case. 



It has been my observation that plants grown upon dry soils, 

 those which are naturally warm and " quick," are more subject 

 to the disease than those grown in moister places in the same 

 field. That is, the early blight appeared first upon the high 

 and dry knolls, and it is here also that the tubers mature the ear- 

 liest. Seasons of protracted drought, therefore, might be sup- 

 posed to have a similar effect, and the testimony of all observers 

 bears out the supposition. It is in dry weather that the early 

 blight progresses most rapidly, the late blight requiring a moist 

 atmosphere for its best development. The falling of rain upon a 

 field in which the tops are gradually yielding to the invasion of 

 early blight has a tendency to freshen the plants and apparently 

 to give them a new lease of life. Water seems to be the one 

 thing most needed. Upon lower land the conditions are different, 

 and, as a rule, such lands suffer less from drought, and the potato 

 crops less from the early blight. 



A curious exception to the above may here be noted. I have 

 many times seen potatoes growing under trees in dry fields where 

 all the potato plants were suffering severely from the early 

 blight except those protected by the foliage of the trees. As a 

 rule the thicker the foliage upon the tree, and the nearer the 

 branches came to the ground, the less was the injury from blight 

 to the potatoes below. This may be explained by supposing that 

 the spores of the fungus (assuming it to be the sole exciting cause 

 of the trouble) are unable to reach the plants, a scarcely warrant- 

 able belief; or, that the spores which do succeed in reaching the 

 potato foliage are unable to germinate on account of lack of moist- 

 ure. There are several arguments forming the second supposi- 

 tion, for, with the exception of the more or less complete absence 



