70 MANUAL OF FRUIT DISEASES 



the return of dry air they are promptly carried to the apple. 

 It is clear, then, that following a short rain it is but a question 

 of a few hours before the fungus is actually inside the apple 

 leaf. 



The relation of the development of G. Juniperi-virginiancp 

 to the weather is very striking, and is a question of great im- 

 portance as well as of great interest. From what has been said 

 it is obvious that the weather factors concerned are moisture, 

 temperature and wind. Moisture is necessary for sporidial 

 production. Winds and warmth aid in evaporation and in 

 sporidial liberation. Moisture again is necessary for germina- 

 tion of sporidia, seciospores and teliospores. Temperature is 

 likewise a limiting factor in this connection. Winds appear 

 to be the only agents of inoculation. It is not necessary that 

 the winds be strong, since the spores are very light and float 

 easily in air currents. 



Control. 



The simplest and most effective method of handling apple- 

 rust consists in the eradication of the red cedars. This is 

 practiced by larger growers where the disease has become 

 important. This work must be done thoroughly. All red 

 cedars within a radius of at least one mile about the apple 

 orchard must be destroyed. The cost of this work has been 

 found to be less than forty-eight cents an acre in West Virginia. 

 It has been found that galls may produce teliospores and 

 sporidia two months after the trees are cut. It is therefore 

 advisable to burn any red cedars which are cut later than 

 March first. The removal of galls from the red cedars is a 

 commendable practice only when the cedars are more desired 

 than the apples. Such may be the case in public parks and 

 on private estates. The galls should be removed in advance of 

 sporidial production, that is, before March first, to make sure 

 of effectiveness. The work must be done every year. Spray- 

 ing the cedars likewise is to be done only where the cedars are 



