May 6, 1918 Caukev of Poplars and Willows 337 



infected trees, and the high winds common in the semiarid regions of the 

 Southwest could easily lift into the air the spore-laden particles of dirt 

 and carry them long distances, thereby infecting other trees. Birds 

 and insects may also play a minor role in carrying the pycnospores. In 

 the towns and cities of the Southwest where this disease is most prevalent, 

 the only bird present to any extent is the English sparrow. 



The shipment of nursery stock infected with this disease may explain 

 its presence in many isolated places where there is no natural growth 

 of its host to normally harbor and transmit the disease. An instance 

 of this kind was seen at a home in the plains country in eastern New 

 Mexico. Two species of poplar {Populus deltoides and P. italica) had 

 been planted from two different nurseries. Sixty-five per cent of the 

 young trees obtained from one of these nurseries were either dead or were 

 seriously attacked by the disease within four years, and the trees from 

 the second nursery were in good condition, except here and there a tree 

 was being infected from the diseased material from the other nursery. 

 In this instance the disease was apparently introduced in the nursery 

 stock. 



CONDITIONS UNDER WHICH CYTOSPORA CHRYSOSPERMA BECOMES 

 A SERIOUS PARASITE 



There are four general conditions under which this disease usually does 

 much damage in the Southwest: (i) On trees which are growing at the 

 outer limits of their range and are therefore in more or less unfavorable 

 environment; (2) On trees planted in streets, lawns, and cemeteries, 

 where they have been weakened from neglect and lack of sufficient 

 water; (3) On trees which have been severely pruned, as in pollarding; 

 (4) On cuttings in propagating beds, where the usual method of propa- 

 gation is used. 



When poplars are growing at the extreme limits of their range or are 

 planted in the treeless regions of semiarid countries, Cytospora chrysos- 

 perma becomes a serious parasite. The aspen at the lower limits of its 

 range is often attacked and the smaller trees are killed outright by this 

 disease, while the larger trees are sometimes seriously injured. The 

 instance given under the dissemination of this disease through nursery 

 stock in eastern New Mexico (p. 337) where 65 per cent of the trees 

 were killed shows how virulent this fungus can become when once estab- 

 lished on plants growing in treeless regions. The following is another 

 instance of the same kind from western Kansas. 



Near Syracuse, in Hamilton County, many of the Carolina poplar trees 

 {Populus deltoides) planted as shade trees are being killed by this disease.^ 

 Syracuse is located in the western portion of Kansas where trees are very 

 scarce and where the rainfall is very light. In such a region trees are 



' The data concerning the presence of this disease at Syracuse were kindly furnished by Prof. L. E- 

 Melchers, of the Kansas Agricultural College Experiment Station. 



