B. P. I.— 342. 



APPLE LEAF -SPOT CAUSED BY SPHAE- 

 ROPSIS MALORUM. 



By W. M. Scott, Pathologist, and James B. Rokee, Assistant Pathologist, 

 Investigations of Diseases of Fruits. 



INTRODUCTION. 



The disease of apple leaves known as " brown-spot," " frog-eye," 

 " leaf-blight," or " leaf -spot " is very common throughout the eastern 

 United States. As these names suggest, the disease is characterized 

 by circular or irregular reddish brown spots with slightly raised 

 purplish margins. (PL III, fig. 1.) These spots when first visible 

 to the naked eye are very minute and purple, but rapidly increase in 

 size until they attain a diameter of from one-eighth to one-half inch, 

 while the affected tissue becomes brown and later sometimes gray. 

 The mature spots are usually circular, but after midsummer may be- 

 come more or less irregular or distinctly lobed in outline, a condition 

 apparently brought about by a secondary extension of the disease 

 from two or more points on the margin of the original circular spot. 

 If the infection is bad, a number of spots may coalesce and form 

 large brown patches involving half the leaf or more, but in these 

 dead areas the margins of the individual spots usually remain distinct. 



These spots should not be confused with those produced on apple 

 leaves by the apple-blotch Phyllosticta. The latter are yellowish and 

 minute, never more than one-sixteenth of an inch in diameter, and 

 will not be considered in this paper. 



Leaf-spot is of greater economic importance than is generally sup- 

 posed. It makes its first appearance early in the spring as the leaves 

 are unfolding, and infections take place continuously throughout the 

 growing season. As a result of its attacks trees may become com- 

 pletely defoliated from six weeks to two months before the normal 

 period of leaf fall. If this is repeated for a series of years, the trees 

 become weakened and the life of the whole orchard is materially 

 shortened. Moreover, the fruit from such prematurely defoliated 

 32893— Bui. 121—08 4 47 



