252 



FARM FRIENDS AND FARM FOES 



which the water reaches the leaves are clogged by the 

 growth of the parasitic mycelium. 



The careful studies of several botanists show that the 

 fungus which causes the disease is commonly present in the 

 soil of cotton fields, where it is able to live long periods 

 as a saprophyte getting the materials of growth from 

 decaying matter in the soil. When cotton is planted, how- 

 ever, it commonly becomes a parasite by invading the 

 tissues of the plants. The same or a very similar fungus 

 causes a wilt disease in watermelons and in the cowpea. 



The only successful method of preventing this disease 

 now known is that of planting varieties that can resist 

 its attack. Much progress has been made in the develop- 

 ment of such varieties, and there is reason to believe that 

 still greater progress will be made in the near future. 



THE POWDERY MILDEWS 



From midsummer until autumn one can find curious 

 whitish patches upon the leaves of a great variety of trees, 

 shrubs, and herbaceous plants. In some cases there are 

 but small spots of white upon the green background of the 

 leaf blade, while in others the whole surface may be whit- 

 ened and more or less powdery in appearance. As autumn 

 approaches tiny brownish or blackish spots appear upon 

 the whitened part. Such whitened leaves are very com- 

 mon upon lilacs, roses, willows, red maples, hardy phloxes, 

 and many other plants. They are especially likely to 

 be found on leaves growing in the shade. Such leaves 

 are probably affected by some form of those parasitic 

 fungi called Powdery Mildews. These are among the most 

 characteristic of the ascospore fungi, and form a very dis- 

 tinctive family. 



