DANGEROUS INTERNATIONAL FOREST TREE DISEASES 83 



Hosts: Leguminosae; the brooming virus has been transmitted only 



by tissue grafts and only ta Robinia pseudoacacia L. Similar 



brooming symptoms have been observed in a few instances on 



honey locust, Gleditsia triacanthos L. 

 Literature : 

 Grant, T. J., and Hartley, Carl. A witches ? -broom on black locust 



and a similar disease on honey locust. Plant Dis. Rptr. 22 : 28-31. 



1938. 

 , Stout, D. C, and Readey, J. C. A systemic brooming, a virus 



disease of black locust. Jour. Forestry 40 : 253-260. 1942. 

 Holmes, F. O. Handbook of phytopathogenic viruses. Rockefeller 



Inst, for Med. Res., 221 pp. 1941. (Processed.) 



Phloem Necrosis of Elm 



R. U. Swingle 



Agricultural Research Service, U.S. Department of Agriculture, 



Delaware, Ohio 



Morsus ulmi Holmes. An epiphytotic virus disease causing root 

 rot, decline, and death of Ulmus americana and Ulmus alata in the 

 United States. First symptoms are root rot and light-green, sparse 

 foliage that becomes yellow; soon afterward followed by complete 

 defoliation and death of the tree. The inner phloem tissue of large 

 roots, stem, and some branches become discolored before their death: 

 the discolored tissue is confined to the phloem and has a faint odor 

 of wintergreen, or methyl salicylate. It is at first yellow, but the 

 color changes to raw sienna and then to brown or black. The faint 

 wintergreen odor can be detected most readily by confining some 

 discolored phloem tissue in a stoppered vial for a few minutes. The 

 raw sienna color and wintergreen odor of the inner phloem tissue 

 are the specific symptoms of phloem necrosis that differentiate it 

 from other known elm diseases. 



The only known insect vector of the phloem necrosis virus is the 

 leafhopper Scaphoid ens luteolus Van D. The virus may be trans- 

 mitted by budding or grafting, but mechanical inoculations with 

 phloem and leaf extracts have not been successful. Infected trees 

 do not usually develop typical disease symptoms until a year or more 

 after inoculation. After initial disease symptoms appear trees usually 

 die within 12 to 18 months, but some may die within 3 to 4 weeks. 



Intercontinental spread may be possible through shipment of in- 

 fected trees or scion wood, but the hazard of establishing the virus in 

 new areas would depend on the occurrence of a suitable insect vector. 

 The host range of the elm phloem necrosis virus is not well defined, 

 and the possibility of importing plant species in which the virus may 

 be latent exists. 



Range: The central and plains States of the United States, bounded 



approximately X by 30 and 42 degrees latitude and W 80 and 



100 degrees longitude. 

 Hosts: Ulmaceae — 



Ulmm americana L. 



TJlmms alata Michx. 



