APPLE POWDERY MILDEW AND ITS CONTROL. q 
rials from that particular portion of the leaf or twig surface upon 
which it is growing. 3 
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 
Fic. 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 isestablished. 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. 
