APPLE POWDERY MILDEW AND ITS CONTROL. 7 



rials from that particular portion of the leaf or twig surface upon 

 which it is growing. 



The very characteristic powdery appearance of the mildew (Pi. 

 VI, fig. 2) is due, as stated above, to enormous numbers of summer 

 spores. These are produced in chains (fig. 2) from certain branches 

 of the mycelium, and their function is to start new mildew infec- 

 tions during the summer. The chains readily break up into indi- 

 vidual spores, which are very light and are easily carried about 



Fig. 2. — Peach mildew growing on the surface of a peach leaf. Apple powdery mildew 

 has much the same appearance. At s a summer spore is shown germinating. Highly 

 magnified. (After Tulasne.) 



by the wind. When a spore lodges in a suitable place, such as the 

 under side of the young leaf, it quickly germinates, if the moisture 

 conditions are suitable, and sends out a small, threadlike germ 

 tube (fig. 2, s), which is the beginning of a new mycelium, and by 

 this means a new infection is established. These new mildewed areas 

 immediately begin to form and throw off conidia in large numbers. 

 The individual leaves are susceptible to infection during their entire 

 growing period and up to the time when they become fully matured. 



