94 



MANUAL OF TREE DISEASES 



dance from year to year. In central Iowa and eastern Nebraska, 

 this disease was so abundant in 1885 that it was difficult to 

 find leaves not affected. The next two summers scarcely any 

 of the rust could be found. In 1888 it was again abundant 

 in the same region. An epiphj-totic of this 

 disease was reported in 1887 around Washing- 

 ton, D. C, with very little of the rust in 

 that region the next year. No recent reports 

 of such outbreaks have been published. 



Symptoms. 



This rust causes swellings which are irregu- 

 lar or more or less globose. They appear on 

 the petioles of the leaves and on the twigs 

 (Fig. 9). Swollen areas are also formed on 

 the leaves which are much distorted. Soon 

 after the swellings are formed they are covered 

 by numerous blister-like protrusions, which 

 break open, leaving cup-shaped areas filled 

 ^ Fig 9.— Ash-rust ^^j^ yellowish powder (Fig. 9). This stage 



Fruiting-bodies of "' • n , i i 



on of the rust is called the cluster-cup stage. 

 The yellow powdery material is composed of 

 the spores (seciospores) of the fungus. The distortion of the 

 petioles and leaves, covered with the yellow cluster-cups, make 

 this disease conspicuous. 



Cause. 



Ash-rust is caused by Puccinia fraxinata. This fungus 

 requires two kinds of host plants to complete its life history. 

 The spores produced in the cluster-cups on the ash do not 

 reinfect the ash, but must find lodgment on the marsh or 

 cord-grasses (Spartina) in order to continue their develop- 

 ment. On the grass plant, spores are produced which infect 

 the ash the next spring. 



causal fungus 

 swollen petiole. 



