[Chap. XLV PLANT DISEASES 575 



killed. Fire-blight cankers are caused by bacteria. The nailhead canker 

 on stems of apple trees is caused by a fungus. Similar dead areas may be 

 caused by low temperatures and by sun scald. 



10. Yellowing or chlorosis of leaves is caused by viruses, by some 

 fungi and bacteria, by low temperatures, or by a deficiency of certain 

 salts. 



Control of plant diseases. In recent years it has become increasingly 

 evident that really efficient measures for controlling plant diseases must 

 depend on an accurate and detailed knowledge of the life history of the 

 pathogen. The acquisition of this knowledge is sometimes a laborious 

 and technical procedure. Special training and well-equipped laboratories 

 are essential to progress in this field. 



When the life cycle of the causal organism is known, experience has 

 shown that there is usually some point in the cycle that is subject to 

 attack by methods of control. Spores or young hyphae outside the plant 

 are often easily destroyed by chemicals or heat; but it may be impossible 

 to eliminate an internal mycelium that develops from them without in- 

 juring the host. For example, a spore or hypha of the apple scab fungus 

 may be destroyed by a suitable fungicide on an apple leaf before the 

 first hypha has grown into the leaf. After the hvpha is once inside the 

 leaf it is difficult to kill it without destroying the leaf. Similarly it is much 

 more economical to try to eradicate the black stem rust of wheat by re- 

 moving barberry plants than by attempting to sprav the wheat. 



Numerous plant diseases are known, and many control measures have 

 been devised and applied with varying success. Even when a partially 

 efiicient control measure has been worked out, it has been found by 

 bitter experience that the application is not a rule-of -thumb matter. No 

 single procedure will give the same results in all kinds of weather, with 

 all varieties of plants, or at all times of the year. In other words, the suc- 

 cessful application of such treatments as sprays, dusts, and hot water 

 depends on the condition both of the host and of the parasite; and such 

 measures must be used with keen discrimination if the results are to be 

 eflFective. 



With these general considerations in mind, representative diseases 

 caused by fungi, bacteria, slime molds, nematodes, and viruses will now 

 be considered. 



Apple scab. In the United States as a whole, scab is the most destruc- 

 tive of apple diseases. The olive-brov^ni areas of "scab" are quite notice- 



