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PRINCIPLES OF FARM PRACTICE 



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Mildew 



still smaller bodies. These small bodies are spores. If some 

 of these spores are scattered over the surface of a piece of 

 damp bread, each will start to grow and a new growth of 

 mold will soon appear. 



Powdery mildews, which frequently are found on the leaves of 

 the lilac, rose, and other plants, are parasitic fungi. Small pro- 

 jections, called haustoria, extend from the threads and secure 



food from the living 

 cells of the leaf . The 

 threads are on the 

 surface of the leaf, 

 giving it the appear- 

 ance of being covered 

 with fine down or 

 cobweb. Among 

 these threads on the 

 surface of the leaf, 

 spores are to be found, 

 some uncovered and 

 others enclosed in 

 spore cases. Some of 

 these spores are carried, when ripe, by the wind or by other 

 means to uninfected leaves where they develop into new 

 fungi. 



Means of control of plant diseases. There are several 

 general methods of control of plant diseases. 



One method is based upon the destruction of spores which 

 may infect healthy plants. This is accomplished when seed 

 is disinfected by means of a chemical compound such as 

 formaldehyde or corrosive sublimate, or the spores killed by 

 means of heat. For example, seed potatoes may be treated 

 with formaldehyde before planting. Oats may be treated 



A. Chains of summer spores; i and 2 different 

 stages of development. 



B. (i) Spore case containing sacs of winter 

 spores. 



(2) A sac showing winter spores. 

 (Washington Agricultural Experiment Station.) 



