Minnesota Plant Diseases. 205 



fore they are completed the fungus has often estabhshed itself 

 within. It is therefore obvious that wounds in shade trees and 

 orchard trees must be covered with tar, creosote, or some other 

 substance which will prevent infection. Many wounds are 

 caused unnecessarily and special care should be exercised in 

 preventing as far as possible wounding of plants. Frost, light- 

 ning and storms cause many unavoidable wounds and such 

 should as far as possible be protected. Pruning is also a neces- 

 sity and need not be injurious if the wounds are likewise pro- 

 tected. It has been recommended that the pruning of trees be 

 done in winter or autumn. The tar coating is only efficient if 

 partially absorbed by the wounded surface and this soaking-in 

 occurs only when the tar is appHed in autumn or winter. In 

 spring or summer the tar does not enter so freely and may leave 

 air spaces through which the fungus may enter. It should be 

 realized, however, that the bark of trees is a necessary protec- 

 tion and cannot be carelessly injured without serious results. 

 Insects cause wounds in plants and preventive methods may be 

 employed to avoid these injuries, such as tar-ringing, 



The importance of localities. Certain plants are known to suf- 

 fer more from disease in one locality than they do in others. 

 This may be due to various causes. The dampness may favor 

 fungus growths, in which case dryer situations would be favora- 

 ble. Potato-blight frequently prospers in such moist localities. 

 Again, plants should not be placed in a region in which a dan- 

 gerous disease is known to be prevalent, or at least until the dis- 

 ease has been eradicated. Such has been demonstrated in flax 

 wilt. The disease germs often lurk in the soil for several years. 

 In such plants as are subject to rusts, e. g., cereals, care must 

 be exercised in the surroundings for such diseases may pass a 

 part of their life on other plants and from these may infect the 

 cereal. This is also true of apples and pears where the fungus 

 also dwells on species of juniper. Other fungi may live on wild 

 plants related to the crop plant. Of course it is not always 

 practicable to select localities, but the importance of this feature 

 should be kept in mind. 



Rotation of crops and "pure cultures." The rotation of crops 

 has several advantageous features. When wheat is rotated 

 with clovers it is well known that the latter replenish the worn 



