CHERRY DISEASES 175 



mycelial threads grow between the leaf-cells, but send haustoria 

 into the cells, thus obtaining food for the invading pathogene. 

 Soon the cells of the affected portion are killed, and they ex- 

 hibit characteristic color-changes, as already described under 

 Symptoms. 



During the summer asexual spores are developed. The 

 mycelium amasses near the surface of the leaf and there results 

 a fruiting body an acervulus. From the interior of this 

 structure conidia arise in great numbers. They push out on 

 the leaf in large quantities, finally piling up until they are visible 

 to the naked eye as whitish masses. These spores are carried 

 by natural agencies to other leaves, where they germinate, and 

 infection results in a manner similar to that described above. 

 As new growth appears it is exposed to infection throughout the 

 growing-season. When the leaf falls, the fungus is carried with 

 it. Here on the ground, within the leaf-tissues, it passes the 

 winter as apothecia formed from the mycelium in late autumn. 



Control. 



The elimination of the old leaves and protection of the 

 developing leaves during the growing-season constitute the 

 known measures of control of leaf -blight. Plowing under 

 fallen leaves removes a large portion of the source of the inocu- 

 lum. This practice, although highly commendable, is not re- 

 liable in itself as a means of combating this disease. A few 

 leaves always remain on the surface of the soil and are sufficient 

 to cause trouble. 



The healthy foliage may be protected by the application of 

 a sulfur fungicide. For the past few years lime-sulfur solution 

 has been used, but recently it has been shown that sulfur dust 

 is very satisfactory and effective and may soon replace the liquid 

 fungicide. Bordeaux mixture, 5-5-50, is also used. In treat- 

 ing orchard trees the applications should be made as follows : 

 (1) when the fruit is free from the calyx; (2) two weeks later; 

 (3) just after the fruit has been picked ; (4) three weeks later, 



